EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: AmeliaBuns on January 20, 2026, 10:03:41 pm
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Hello.
I was trying to probe my circuit powered by my garbo Amazon power supply.
when doing so, I noticed some very very light buzzing/humming from my PSU when my ground clip is attached. I also once saw a slight spark when removing the clip. It made me worried so I stopped and investigated.
I found 4.8Vppk on both terminals of my power supply relative to ground. To check the impedance/current, I tried creating a shunt using a 20k and 2.2k resistor.
The 20k resistor collapsed the 4.8Vpp to 1.38Vpp and the 2.2k collapsed the 4.8Vppk to 0.740Vpp. This is obviously a leakage voltage from either the transformer or other components, maybe the Y capacitor is letting 60hz pass through too. My calculations tell me it'd be 96uA if I'm correct. (50,000ohm~ leakage impedance)
This makes me concerned that the currents flowing to the ground terminal of my scope, although very small could potentially damage it. Is probing isolated circuits like this a good/common practice? Is this simply nothing to worry about and normal? What is the best solution for this? Is it to just make sure all my circuits are earth-references? Is it to buy a better Lab bench power supply (The good ones are sadly upwards of 500$ and lower wattage, an insane price for me)? I've been told that to avoid the need for an expensive (500CAD minimum...) differential probe, isolating the circuit is a common practice. Is this wrong/bad?
My other issue with differential probe is not just the cost, but the noise, 20mv Peak to peak is a great amount of noise, not to mention the very low bandwidths of most common budget probes. I have been debating trying a DIY approach, but I have extreme doubts about being able to create the same quality probe for cheaper.
I knnow that just by existing near electricity, our bodies act as antennas and pass small currents through it, same must already be happning to the scope (Although I assume it'd be in the nano amp range, not micro amps that I calculated, 96uA)
I'm also going to order a GFCI in the future because I can be clumsy/forgetful...
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GFCI is a good idea if you are working on a mains device or have questionable quality equipment. You can either install in the wall or buy a junction box, wire a standard plug to it, and use it as an extension.
uA of current is not going to damage the scope, if you ground 120V with your scope probe, its going to blow something up (http://youtube.com/watch?v=xaELqAo4kkQ) though. Possibly damage the scope, definitely the probe. Probably y cap leakage as you say.
What is the specific PSU?
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GFCI is a good idea if you are working on a mains device or have questionable quality equipment. You can either install in the wall or buy a junction box, wire a standard plug to it, and use it as an extension.
uA of current is not going to damage the scope, if you ground 120V with your scope probe, its going to blow something up (http://youtube.com/watch?v=xaELqAo4kkQ) though. Possibly damage the scope, definitely the probe. Probably y cap leakage as you say.
What is the specific PSU?
Are the power strips with GFCI any good? I live in a rental sadly :/
oh so im paranoid :) I have foun da 50ma or less as a number online but it wasn't based on anything. this is way way lower. the spark does worry me, I wonder if it's forming capacitance with something and then parasitic inductance is causing a spark? do you think a TVS to protective earth would work? as if it's safe, I'd love to still keep doing this to prevent needing a 1000CAD probe (which still isn't fast and it's too noisy)
This is the PSU I have, I have no idea if this leakage is normal or too much or even good. I bought this 3 years ago when I knew little about electronics.
https://www.amazon.ca/dp/B0CBM2D3DW?ref_=ppx_hzod_title_dt_b_fed_asin_title_0_0 (https://www.amazon.ca/dp/B0CBM2D3DW?ref_=ppx_hzod_title_dt_b_fed_asin_title_0_0)
to get even remotely close from a reputable brand, it seems to cost 500CAD. that's with paralleled inputs (which I thought was a horrible idea? maybe if the voltages are very low noise/tightly regulated and current limited well it'll be fine?)
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Looking at the PSU you have, I would check the continuity between the earth pin on the power plug, and the green "GND" post on the front. If it's good (low ohms, less than 1), I would short the -ve (black) and GND (green) binding posts together. If it's not good (more than a couple of ohms), I would get a different PSU, assuming the internal GND connections are not loose (I have seen this).
If you have a capacitance meter, you could remove the Y capacitor and check it's not failing, or swap it for a smaller capacitance Y cap. A smaller capacitance value would mean a lower leakage current. There's also a non-zero chance that the "Y" cap in the PSU isn't Y rated, in which case, replace it with one that is.
EDIT: You might stalk the Buy/Sell/Wanted section of this forum, to see if you can find a decent used PSU in your area.
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GFCI is a good idea if you are working on a mains device or have questionable quality equipment. You can either install in the wall or buy a junction box, wire a standard plug to it, and use it as an extension.
uA of current is not going to damage the scope, if you ground 120V with your scope probe, its going to blow something up (http://youtube.com/watch?v=xaELqAo4kkQ) though. Possibly damage the scope, definitely the probe. Probably y cap leakage as you say.
What is the specific PSU?
Are the power strips with GFCI any good? I live in a rental sadly :/
oh so im paranoid :) I have foun da 50ma or less as a number online but it wasn't based on anything. this is way way lower. the spark does worry me, I wonder if it's forming capacitance with something and then parasitic inductance is causing a spark? do you think a TVS to protective earth would work? as if it's safe, I'd love to still keep doing this to prevent needing a 1000CAD probe (which still isn't fast and it's too noisy)
This is the PSU I have, I have no idea if this leakage is normal or too much or even good. I bought this 3 years ago when I knew little about electronics.
https://www.amazon.ca/dp/B0CBM2D3DW?ref_=ppx_hzod_title_dt_b_fed_asin_title_0_0 (https://www.amazon.ca/dp/B0CBM2D3DW?ref_=ppx_hzod_title_dt_b_fed_asin_title_0_0)
to get even remotely close from a reputable brand, it seems to cost 500CAD. that's with paralleled inputs (which I thought was a horrible idea? maybe if the voltages are very low noise/tightly regulated and current limited well it'll be fine?)
There may well be a reason why it is discontinued. In the meantime, would an isolation transformer between the switching PSU and the 120 help? A band aid, but an isolation transformer (not medical) is a handy thing to have anyway.
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A band aid, but an isolation transformer (not medical) is a handy thing to have anyway.
Those can be dangerous too, if the user doesn't understand what they're doing with it.
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Looking at the PSU you have, I would check the continuity between the earth pin on the power plug, and the green "GND" post on the front. If it's good (low ohms, less than 1), I would short the -ve (black) and GND (green) binding posts together. If it's not good (more than a couple of ohms), I would get a different PSU, assuming the internal GND connections are not loose (I have seen this).
If you have a capacitance meter, you could remove the Y capacitor and check it's not failing, or swap it for a smaller capacitance Y cap. A smaller capacitance value would mean a lower leakage current. There's also a non-zero chance that the "Y" cap in the PSU isn't Y rated, in which case, replace it with one that is.
EDIT: You might stalk the Buy/Sell/Wanted section of this forum, to see if you can find a decent used PSU in your area.
huh there's a buy sell section?
Is this even dangerous? as I was mentioning to another poster, the currents are only 96uA. tho they can add up.
The ground pin is probably good, but it comes as isolated by default? I'm assuming that they wanted to give you the option to earth it or not on your end? Siglent SPD1168X is also floating(you can just short the green and black). If using an isolated power supply as a substitute for a dif probe is an okay practice, I'd much rather keep my lab bench PSU isolated. I'm actually wondering if those siglent ones are even isolated or not.
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GFCI is a good idea if you are working on a mains device or have questionable quality equipment. You can either install in the wall or buy a junction box, wire a standard plug to it, and use it as an extension.
uA of current is not going to damage the scope, if you ground 120V with your scope probe, its going to blow something up (http://youtube.com/watch?v=xaELqAo4kkQ) though. Possibly damage the scope, definitely the probe. Probably y cap leakage as you say.
What is the specific PSU?
Are the power strips with GFCI any good? I live in a rental sadly :/
oh so im paranoid :) I have foun da 50ma or less as a number online but it wasn't based on anything. this is way way lower. the spark does worry me, I wonder if it's forming capacitance with something and then parasitic inductance is causing a spark? do you think a TVS to protective earth would work? as if it's safe, I'd love to still keep doing this to prevent needing a 1000CAD probe (which still isn't fast and it's too noisy)
This is the PSU I have, I have no idea if this leakage is normal or too much or even good. I bought this 3 years ago when I knew little about electronics.
https://www.amazon.ca/dp/B0CBM2D3DW?ref_=ppx_hzod_title_dt_b_fed_asin_title_0_0 (https://www.amazon.ca/dp/B0CBM2D3DW?ref_=ppx_hzod_title_dt_b_fed_asin_title_0_0)
to get even remotely close from a reputable brand, it seems to cost 500CAD. that's with paralleled inputs (which I thought was a horrible idea? maybe if the voltages are very low noise/tightly regulated and current limited well it'll be fine?)
There may well be a reason why it is discontinued. In the meantime, would an isolation transformer between the switching PSU and the 120 help? A band aid, but an isolation transformer (not medical) is a handy thing to have anyway.
I don't own an isolation transformer and tbh i'm a bit scared of using them. I feel too clumsy and distractable to mess with mains, as much as I'd love to. do they transformers cause any issues on the power supply in terms of noise/wave forms? I don't see why they should personally.
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I tore the power supply down! I don't understand ac/dc design so can't say much, but the design is strange...
the markings for the mains and low level is half there and it stops.
the DC/DC side has no ground planes?
The cut-offs at high voltages are only present in some areas.
I did not find a y capacitor on the main circuit, but the small little PSU powering the display has a y capacitor (I think?) that might be the culprit....
I also found 4 EI transformers. I'm wondering why that is. I thought I'd just find one (aside from the small separate PSU on the side). I'm assuming the funky one near the plug is a common's mode choke.
also I don't think the shunt has a Kelvin connection.
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I tore the power supply down! I don't understand ac/dc design so can't say much, but the design is strange...
the markings for the mains and low level is half there and it stops.
the DC/DC side has no ground planes?
The cut-offs at high voltages are only present in some areas.
I did not find a y capacitor on the main circuit, but the small little PSU powering the display has a y capacitor (I think?) that might be the culprit....
I also found 4 EI transformers. I'm wondering why that is. I thought I'd just find one (aside from the small separate PSU on the side). I'm assuming the funky one near the plug is a common's mode choke.
also I don't think the shunt has a Kelvin connection.
Yes a bit weird in using so many SMPS transformers. :-//
Typically a small cap is across the main transformer but I don't see one unless the big brown one is it.
(https://www.eevblog.com/forum/testgear/confusion-around-probing-isolated-circuits-is-this-a-safe-practice/?action=dlattach;attach=2736639)
Looking at the PSU you have, I would check the continuity between the earth pin on the power plug, and the green "GND" post on the front. If it's good (low ohms, less than 1), I would short the -ve (black) and GND (green) binding posts together. If it's not good (more than a couple of ohms), I would get a different PSU, assuming the internal GND connections are not loose (I have seen this).
If you have a capacitance meter, you could remove the Y capacitor and check it's not failing, or swap it for a smaller capacitance Y cap. A smaller capacitance value would mean a lower leakage current. There's also a non-zero chance that the "Y" cap in the PSU isn't Y rated, in which case, replace it with one that is.
EDIT: You might stalk the Buy/Sell/Wanted section of this forum, to see if you can find a decent used PSU in your area.
huh there's a buy sell section?
Is this even dangerous? as I was mentioning to another poster, the currents are only 96uA. tho they can add up.
The ground pin is probably good, but it comes as isolated by default? I'm assuming that they wanted to give you the option to earth it or not on your end? Siglent SPD1168X is also floating(you can just short the green and black). If using an isolated power supply as a substitute for a dif probe is an okay practice, I'd much rather keep my lab bench PSU isolated. I'm actually wondering if those siglent ones are even isolated or not.
Yes and normal for a good linear PSU.
The stepdown transformer gives you full isolation from mains and PE when done correctly.
SPD1168X does offer a front panel PE which you can link to an output when required.....maybe when your DUT has a power rail connected to PE.
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The mains earth connects to the chassis.
The green center jack seems to connect to the chassis. Hard to tell.
C44 and C45 look like Y caps, connecting the two outputs to the chassis for EMI suppression. (Edit: bottom right in center pic, near where the red & black wires attach to the PCB.)
When probing inside it, I'd connect the scope ground to the earth/chassis ground (center jack) since both outputs are floating.
When using it to power a DUT don't expect the negative (black) jack to be grounded, and in fact to be floating. The earth is for EMI suppression and a safety ground for the PSU, nothing else. It's not a common ground.
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Are the power strips with GFCI any good? I live in a rental sadly :/
Didn't know they existed, they look to be a good option, I just made my own as I have the spare outlets laying around.
https://www.homedepot.ca/product/southwire-shock-shield-gfci-tri-tap-outlet-2-ft-12-3-120v-15a/1001013005 (https://www.homedepot.ca/product/southwire-shock-shield-gfci-tri-tap-outlet-2-ft-12-3-120v-15a/1001013005)
oh so im paranoid :) I have foun da 50ma or less as a number online but it wasn't based on anything. this is way way lower. the spark does worry me, I wonder if it's forming capacitance with something and then parasitic inductance is causing a spark? do you think a TVS to protective earth would work? as if it's safe, I'd love to still keep doing this to prevent needing a 1000CAD probe (which still isn't fast and it's too noisy)
A 12V or 24V TVS might be of some benefit to reduce the sparking. Or shorting the earth connector to Gnd.
The risk with that is now your circuit is referenced to earth so clipping the probe gnd to something other than gnd will short it (as shown in daves vid). So TVS or a resistor could be a compromise.
This is the PSU I have, I have no idea if this leakage is normal or too much or even good. I bought this 3 years ago when I knew little about electronics.
https://www.amazon.ca/dp/B0CBM2D3DW?ref_=ppx_hzod_title_dt_b_fed_asin_title_0_0 (https://www.amazon.ca/dp/B0CBM2D3DW?ref_=ppx_hzod_title_dt_b_fed_asin_title_0_0)
to get even remotely close from a reputable brand, it seems to cost 500CAD. that's with paralleled inputs (which I thought was a horrible idea? maybe if the voltages are very low noise/tightly regulated and current limited well it'll be fine?)
Probably normal.
It is difficult to get a high power/current linear PSU. In future you could consider using one noisy switch mode PSU for high currents, and something linear for low current stuff.
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I sadly can’t afford one, maybe one day….
Also the augments seem to also have a big flaw?
On turn on, they short the output together. Unlikely to cause a big issue, but someone had their battery explode because they turned it on while it was connected. A bad practice regardless is this issue but it’s strange and scares me….
500CAD is so much. Those Chinese clones and power supplies are slightly tempting now
I assume linear, If fully linear has less leakage? Although the 60hz leakage would be the exact t same wouldn’t it?
Another option is a DIY PSU using a medical grade power supply. I could also purchase a cheap dc to dc isolated buck converter or make one as a second layer of isolation. That’s probably make it less than 30uA, but 96ua that I measured is already pretty great afaik. The medical ones were rated less than 300ua.
For now, I’ll either stick a TVS or even simpler use a 20k resistor across it to reduce that, while keeping my clumsy self safe from blowing up or damaging my scope ;P
I wonder if 20k is big enough for me to just treat the circuit as fully isolated and probe it differentially? Might need a TVS for that instead.
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A band aid, but an isolation transformer (not medical) is a handy thing to have anyway.
Those can be dangerous too, if the user doesn't understand what they're doing with it.
Josh,
Respectfully, HUH???? :-/O
What piece of test equipment IS safe if the user does not know how to use it? In this case, the issue was using a SMPS and getting leakage when the scope was grounded. More dangerous than removing the cover of any power supply without understanding AC/DC?
Would not putting an Isoxformer between the SMPS and the mains solve the issue at modest cost ($100 for a small one that is not varable) while adding to the toolbox?
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Looking at the PSU you have, I would check the continuity between the earth pin on the power plug, and the green "GND" post on the front. If it's good (low ohms, less than 1), I would short the -ve (black) and GND (green) binding posts together. If it's not good (more than a couple of ohms), I would get a different PSU, assuming the internal GND connections are not loose (I have seen this).
If you have a capacitance meter, you could remove the Y capacitor and check it's not failing, or swap it for a smaller capacitance Y cap. A smaller capacitance value would mean a lower leakage current. There's also a non-zero chance that the "Y" cap in the PSU isn't Y rated, in which case, replace it with one that is.
EDIT: You might stalk the Buy/Sell/Wanted section of this forum, to see if you can find a decent used PSU in your area.
huh there's a buy sell section?
Is this even dangerous? as I was mentioning to another poster, the currents are only 96uA. tho they can add up.
The ground pin is probably good, but it comes as isolated by default? I'm assuming that they wanted to give you the option to earth it or not on your end? Siglent SPD1168X is also floating(you can just short the green and black). If using an isolated power supply as a substitute for a dif probe is an okay practice, I'd much rather keep my lab bench PSU isolated. I'm actually wondering if those siglent ones are even isolated or not.
Yep: https://www.eevblog.com/forum/buysellwanted/ (https://www.eevblog.com/forum/buysellwanted/)
Although the leakage current (probably) isn't dangerous to you (I have no idea if you have a medical condition that increases your sensitivity), it will add noise to your measurements, and every time you connect to your scope, the Y capacitor will dump all its charge through your scope ground, and every time you disconnect, you'll have a tiny arc, which has the potential to damage your scope input. Connecting the PSU -VE to GND on the front panel literally short-circuits this problem, protecting your scope front end, and reducing the noise of your measurements.
In the context of what you are doing, your PSU stops being isolated as soon as you connect your scope probe ground, so you may as well do it at the source, and save the extra stress to the scope.
All PSUs that are even half decent will have the option to connect to ground; my Siglent SPD3303X does, my HP and Agilent ones do, my TTi EX354D does, and the various EP PSUs I use for HV stuff come with -VE linked to GND by default.
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Would not putting an Isoxformer between the SMPS and the mains solve the issue at modest cost ($100 for a small one that is not varable) while adding to the toolbox?
If a device of protection class I (basic isolation + protective earth ground) is connected and operated via an isolating transformer, the transformer replaces the measure "protective earthing" by the measure "protective isolation" that the transformer provides. Therefore, the protective earth (PE) contact of the device on the secondary side must not be connected to the earth of the supplying network or to any other potential with ground reference. Otherwise the additional protective isolation that the transformer provides is defeated.
With the PE on the secondary side grounded, an insulation fault in the connected class I device would remain undetected, because no residual current would be flowing to earth and trip a breaker or RCD, even though you'd still be safe. But a second fault in another place could lead to a large current that is possibly not detected by the breaker or RCD in your installation, therefore this is not allowed according to DIN VDE 0100-410 for regular operation.
But more importantly for lab power supply, if you run it isolated and connect the output to another circuit that has a large potential with respect to local earth, that potential could be transferred to the housing of the power supply, which can be dangerous.
So never put your test equipment on an isolation transformer (unless you know exactly what you are doing). If you want to make isolated measurements on a mains supplied device under test (DUT), always connect the DUT to the isolation transformer. And be aware that connecting the DUT to another potential (e.g. your scope ground) defeats the protective isolation. This is not for regular operation.
A small leakage current between a lab power supply output and PE is not unusual and generally harmless. I measure about 500µArms between PE and the outputs of my Rohde & Schwarz HMP4040 (topology: toroidal mains transformer --> switchmode converter --> filter --> linear post-regulator), and no current for my Agilent E3646 (purely linear).
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There may well be a reason why it is discontinued. In the meantime, would an isolation transformer between the switching PSU and the 120 help? A band aid, but an isolation transformer (not medical) is a handy thing to have anyway.
I own a number of isolation transformers, mainly ONEAC, but I've never owned or used a medical rated one. I'm curious about your views on not using one. I would have thought that they'd have even lower leakage and better shielding between the primary and secondary.
I generally isolate the neutrel of the output from ground. The grounds I leave intact.
I do check all of mine for leakage from time to time with a Simpson 228 and a Simpson 229-2 leakage tester.
Thanks in advance!
WoD
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A band aid, but an isolation transformer (not medical) is a handy thing to have anyway.
Those can be dangerous too, if the user doesn't understand what they're doing with it.
Josh,
Respectfully, HUH???? :-/O
What piece of test equipment IS safe if the user does not know how to use it? In this case, the issue was using a SMPS and getting leakage when the scope was grounded. More dangerous than removing the cover of any power supply without understanding AC/DC?
Would not putting an Isolation transformer between the SMPS and the mains solve the issue at modest cost ($100 for a small one that is not varable) while adding to the toolbox?
to be fare, I'd be cautious recommending an isolation transformer to a beginner too. All tools are dangerous but mains is a bit... "directly" dangerous ya know?
my Philosophy is that awareness protects users, so the recommendation is good with some warnings alongside it. I don't like how some spaces "ban" these topics. it's just more likely for people to fool around on their own without knowledge.
Funny enough, I've seen this effect a lot in the country I grew up in. Most subjects are taboo in my country, and as a result, a lot of people fool around and find out the hard way because they weren't made aware of the dangers or HOW to do it safely :P.
Personally. I'd much prefer to put a DC/DC isolation converter in front of the PSU rather than on the mains side.
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Looking at the PSU you have, I would check the continuity between the earth pin on the power plug, and the green "GND" post on the front. If it's good (low ohms, less than 1), I would short the -ve (black) and GND (green) binding posts together. If it's not good (more than a couple of ohms), I would get a different PSU, assuming the internal GND connections are not loose (I have seen this).
If you have a capacitance meter, you could remove the Y capacitor and check it's not failing, or swap it for a smaller capacitance Y cap. A smaller capacitance value would mean a lower leakage current. There's also a non-zero chance that the "Y" cap in the PSU isn't Y rated, in which case, replace it with one that is.
EDIT: You might stalk the Buy/Sell/Wanted section of this forum, to see if you can find a decent used PSU in your area.
huh there's a buy sell section?
Is this even dangerous? as I was mentioning to another poster, the currents are only 96uA. tho they can add up.
The ground pin is probably good, but it comes as isolated by default? I'm assuming that they wanted to give you the option to earth it or not on your end? Siglent SPD1168X is also floating(you can just short the green and black). If using an isolated power supply as a substitute for a dif probe is an okay practice, I'd much rather keep my lab bench PSU isolated. I'm actually wondering if those siglent ones are even isolated or not.
Yep: https://www.eevblog.com/forum/buysellwanted/ (https://www.eevblog.com/forum/buysellwanted/)
Although the leakage current (probably) isn't dangerous to you (I have no idea if you have a medical condition that increases your sensitivity), it will add noise to your measurements, and every time you connect to your scope, the Y capacitor will dump all its charge through your scope ground, and every time you disconnect, you'll have a tiny arc, which has the potential to damage your scope input. Connecting the PSU -VE to GND on the front panel literally short-circuits this problem, protecting your scope front end, and reducing the noise of your measurements.
In the context of what you are doing, your PSU stops being isolated as soon as you connect your scope probe ground, so you may as well do it at the source, and save the extra stress to the scope.
All PSUs that are even half decent will have the option to connect to ground; my Siglent SPD3303X does, my HP and Agilent ones do, my TTi EX354D does, and the various EP PSUs I use for HV stuff come with -VE linked to GND by default.
oh nooo! I didn't know it's dangerous. hope I didn't ruin my expensive precious scope D: I'm always scared of that. I spent so much on it. (And it doesn't even have 50 ohm inputs!)
Being honest. I'm more worried about my scope. Wounds heal, scopes won't :P
Why does the arc happen btw? would a TVS diode trying the negative/positive terminal to earth help prevent this issue?
I've been trying to use this to my advantage to probe across a resistor as a shunt or to measure something relative to VCC not GND. I guess i'll stop doing that? I'm debating DIY-ing one of those differential probes. although I wonder how well they really work....
I also wish I had a power rail probe. those are so much more expensive, more so than even differential probes :(
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One of the reasons isolation transformers are dangerous is because inexperienced (and sometimes experienced) people will do something stupid with their test setup, at which point, the user may become the path to ground. Ooops.
+1 for the SPD3303X being a great PSU. Fully isolated channels, linear supply. I bought mine used on fb marketplace cheap.
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Ah yes, the classic question when it comes to measurement...
The oscilloscope must remain grounded unless it has isolated inputs.
If the scope is connected to an isolating transformer, dangerously high potentials can build up at the inputs.
If the scope does not have isolated inputs, all inputs have the same potential—as described in the input post, this can lead to very undesirable effects, including destruction of the circuits.
An isolation transformer is only permissible if the test circuit must be decoupled from ground, e.g., to avoid ground-related interference.
In this case, however, it is indispensable.
You are on the safe side if:
- The scope is and remains grounded.
- And differential probes are used for measurements.
These in turn must be grounded(This is taken care of when the scope is grounded), otherwise they will not function properly.
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- The scope is and remains grounded.
- And differential probes are used for measurements.
Exactly. Some idiot on youtube recommended using an isolation transformer so that a standard probe could be used as a diff probe by connecting the ground lead to a different part of the circuit instead of ground. :palm:
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One of the reasons isolation transformers are dangerous is because inexperienced (and sometimes experienced) people will do something stupid with their test setup, at which point, the user may become the path to ground. Ooops.
+1 for the SPD3303X being a great PSU. Fully isolated channels, linear supply. I bought mine used on fb marketplace cheap.
Are you able to measure the leakage current to ground just out of curiosity? I hope I find one of those for sale maybe :P I have so much I want... I have an old weller but it's so weak and it's hard to swap the tips. I've been wanting a geeboon ta305 so bad...
not to mention the probes and a 50 ohm input probe which I probably won't afford LOL
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Ah yes, the classic question when it comes to measurement...
The oscilloscope must remain grounded unless it has isolated inputs.
If the scope is connected to an isolating transformer, dangerously high potentials can build up at the inputs.
If the scope does not have isolated inputs, all inputs have the same potential—as described in the input post, this can lead to very undesirable effects, including destruction of the circuits.
An isolation transformer is only permissible if the test circuit must be decoupled from ground, e.g., to avoid ground-related interference.
In this case, however, it is indispensable.
You are on the safe side if:
- The scope is and remains grounded.
- And differential probes are used for measurements.
These in turn must be grounded(This is taken care of when the scope is grounded), otherwise they will not function properly.
Hmm so what I'm hearing overall is that even if you're circuit is isolated, just don't probe it as is without a differential probe? and that I should ground my circuits unless for some reason, it must not be grounded:? I'm a bit confused. the 60-200ua of current going into the ground of my scope is apparently just fine, the tiny spark is the spooky part. (not sure how that's happening)
for me the only realistic option is to give up. maybe eventually buy a DP700 for times where I don't need high badnwith or low noise (17mv RMS is pretty horrible for power supply measurements for example, and 10mhz is pretty bad for rise time etc)
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- The scope is and remains grounded.
- And differential probes are used for measurements.
Exactly. Some idiot on youtube recommended using an isolation transformer so that a standard probe could be used as a diff probe by connecting the ground lead to a different part of the circuit instead of ground. :palm:
What's wrong with that?
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Exactly. Some idiot on youtube recommended using an isolation transformer so that a standard probe could be used as a diff probe by connecting the ground lead to a different part of the circuit instead of ground. :palm:
What's wrong with that?
One of the reasons isolation transformers are dangerous is because inexperienced (and sometimes experienced) people will do something stupid with their test setup, at which point, the user may become the path to ground. Ooops.
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uA of current is not going to damage the scope, if you ground 120V with your scope probe, its going to blow something up (http://youtube.com/watch?v=xaELqAo4kkQ) though. Possibly damage the scope, definitely the probe. Probably y cap leakage as you say.
Yes, scopes are very rigidly grounded to mains earth, just look at any oscilloscope teardown video. The BNC is often bolted the metal chassis, so it's crazy low impedance. The scope probe ground lead is the thing that vaporise first.
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- The scope is and remains grounded.
- And differential probes are used for measurements.
Exactly. Some idiot on youtube recommended using an isolation transformer so that a standard probe could be used as a diff probe by connecting the ground lead to a different part of the circuit instead of ground. :palm:
What's wrong with that?
Becaause it can be dangerous, especially with say measuring mains circuits (a common reason for doing this). It means that the entire metal chassis of your scope will be at a live potential relative to ground, and if you touch it, ouchy.
Oscilloscopes are mains earth referenced for safety reasons. It's why USB powered scopes like the new Rigols for example, come with a safety earth clip so you can ground it.
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uA of current is not going to damage the scope, if you ground 120V with your scope probe, its going to blow something up (http://youtube.com/watch?v=xaELqAo4kkQ) though. Possibly damage the scope, definitely the probe. Probably y cap leakage as you say.
Yes, scopes are very rigidly grounded to mains earth, just look at any oscilloscope teardown video. The BNC is often bolted the metal chassis, so it's crazy low impedance. The scope probe ground lead is the thing that vaporise first.
Huh that's cool! I didn't realize those scopes are USB powered! It can act as a portable one in a pinch I guess :) Good for very small workspaces like mine. Although in the case of USB powered scopes, I am slightly confused why grounding would help with safety (Although I can see how it'd help with noise/measurement). Unless the USB adapter isn't isolated, the other side is low voltage anyways. I thought transformers are dangerous because the other side is also still high voltage. Most USB charger's I've seen do have horrendous earth leakage tho.
But as for my original question:
The confusion for me has been two things so far. the strange spark I saw once, and what the range or "safe practice" is here. Is it okay to do? I'm confused as why arcs would form at only 100uA.
I keep hearing online about how you can isolate your DUT to avoid spending money on a DIFF probe that costs a ton and has a lot of noise and low bandwidth. Maybe I can get away with it with another layer of isolation but I also love my scope so much and 600-800CAD is a LOT of money to me. Don't want to see my precious investment smoking! Or worse, become inaccurate/malfunction without me knowing! I assume the device has protection built-in but I rather not rely on that :sweat:. I'm kind of a semi-newbie and I don't wanna do anything stupid...
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- The scope is and remains grounded.
- And differential probes are used for measurements.
Exactly. Some idiot on youtube recommended using an isolation transformer so that a standard probe could be used as a diff probe by connecting the ground lead to a different part of the circuit instead of ground. :palm:
What's wrong with that?
Becaause it can be dangerous, especially with say measuring mains circuits (a common reason for doing this). It means that the entire metal chassis of your scope will be at a live potential relative to ground, and if you touch it, ouchy.
Oscilloscopes are mains earth referenced for safety reasons. It's why USB powered scopes like the new Rigols for example, come with a safety earth clip so you can ground it.
Well no, we're talking about a different scenario here, if I understand correctly:
- the scope is grounded, meaning, chassis connected to mains PE;
- DUT is powered via an isolation transformer;
- the probe's ground clip is attached to an arbitrary node in DUT.
Apart from seeing a tiny spark produced from the capacitance between the transformer's windings when the ground clip is attached, what else can go wrong in this setup? Obviously, assuming that we don't try to connect another probe's ground to a node with a different potential in DUT.
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Looking at the PSU you have, I would check the continuity between the earth pin on the power plug, and the green "GND" post on the front. If it's good (low ohms, less than 1), I would short the -ve (black) and GND (green) binding posts together. If it's not good (more than a couple of ohms), I would get a different PSU, assuming the internal GND connections are not loose (I have seen this).
If you have a capacitance meter, you could remove the Y capacitor and check it's not failing, or swap it for a smaller capacitance Y cap. A smaller capacitance value would mean a lower leakage current. There's also a non-zero chance that the "Y" cap in the PSU isn't Y rated, in which case, replace it with one that is.
EDIT: You might stalk the Buy/Sell/Wanted section of this forum, to see if you can find a decent used PSU in your area.
huh there's a buy sell section?
Is this even dangerous? as I was mentioning to another poster, the currents are only 96uA. tho they can add up.
The ground pin is probably good, but it comes as isolated by default? I'm assuming that they wanted to give you the option to earth it or not on your end? Siglent SPD1168X is also floating(you can just short the green and black). If using an isolated power supply as a substitute for a dif probe is an okay practice, I'd much rather keep my lab bench PSU isolated. I'm actually wondering if those siglent ones are even isolated or not.
Yep: https://www.eevblog.com/forum/buysellwanted/ (https://www.eevblog.com/forum/buysellwanted/)
Although the leakage current (probably) isn't dangerous to you (I have no idea if you have a medical condition that increases your sensitivity), it will add noise to your measurements, and every time you connect to your scope, the Y capacitor will dump all its charge through your scope ground, and every time you disconnect, you'll have a tiny arc, which has the potential to damage your scope input. Connecting the PSU -VE to GND on the front panel literally short-circuits this problem, protecting your scope front end, and reducing the noise of your measurements.
In the context of what you are doing, your PSU stops being isolated as soon as you connect your scope probe ground, so you may as well do it at the source, and save the extra stress to the scope.
All PSUs that are even half decent will have the option to connect to ground; my Siglent SPD3303X does, my HP and Agilent ones do, my TTi EX354D does, and the various EP PSUs I use for HV stuff come with -VE linked to GND by default.
oh nooo! I didn't know it's dangerous. hope I didn't ruin my expensive precious scope D: I'm always scared of that. I spent so much on it. (And it doesn't even have 50 ohm inputs!)
Being honest. I'm more worried about my scope. Wounds heal, scopes won't :P
Why does the arc happen btw? would a TVS diode trying the negative/positive terminal to earth help prevent this issue?
I've been trying to use this to my advantage to probe across a resistor as a shunt or to measure something relative to VCC not GND. I guess i'll stop doing that? I'm debating DIY-ing one of those differential probes. although I wonder how well they really work....
I also wish I had a power rail probe. those are so much more expensive, more so than even differential probes :(
The arc happens because as you disconnect the scope ground lead, the point it was connected to is no longer shorted to ground, and the AC current flowing through* the Y capacitor allows the potential to rise to a level that an arc occurs.
*technically the current doesn't flow through the capacitor, but that's a different question.
- The scope is and remains grounded.
- And differential probes are used for measurements.
Exactly. Some idiot on youtube recommended using an isolation transformer so that a standard probe could be used as a diff probe by connecting the ground lead to a different part of the circuit instead of ground. :palm:
What's wrong with that?
Becaause it can be dangerous, especially with say measuring mains circuits (a common reason for doing this). It means that the entire metal chassis of your scope will be at a live potential relative to ground, and if you touch it, ouchy.
Oscilloscopes are mains earth referenced for safety reasons. It's why USB powered scopes like the new Rigols for example, come with a safety earth clip so you can ground it.
Well no, we're talking about a different scenario here, if I understand correctly:
- the scope is grounded, meaning, chassis connected to mains PE;
- DUT is powered via an isolation transformer;
- the probe's ground clip is attached to an arbitrary node in DUT.
Apart from seeing a tiny spark produced from the capacitance between the transformer's windings when the ground clip is attached, what else can go wrong in this setup? Obviously, assuming that we don't try to connect another probe's ground to a node with a different potential in DUT.
The spark isn't going to be from capacitive coupling in the transformer, that's going to be far too small to be noticeable in this way at the voltages we're talking about. It's going to be from the Y capacitor, connected between a live mains conductor (should be the neutral when using a 3-pin plug; with a 2-pin, all bets are off) and the 0V rail of the secondary circuit. It's not unusual to see 1-2mA via this route, though currents in the few hundred uA range are more common.
It's not a great idea to probe this way, because it's far too easy to make a mistake and connect another ground at a different potential.
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- The scope is and remains grounded.
- And differential probes are used for measurements.
Exactly. Some idiot on youtube recommended using an isolation transformer so that a standard probe could be used as a diff probe by connecting the ground lead to a different part of the circuit instead of ground. :palm:
What's wrong with that?
Becaause it can be dangerous, especially with say measuring mains circuits (a common reason for doing this). It means that the entire metal chassis of your scope will be at a live potential relative to ground, and if you touch it, ouchy.
Oscilloscopes are mains earth referenced for safety reasons. It's why USB powered scopes like the new Rigols for example, come with a safety earth clip so you can ground it.
Well no, we're talking about a different scenario here, if I understand correctly:
Correct, I was making a generic point about scope safety because KuFuJosh was also talking generally. "floating the scope" is an age old technique that can be dangerous, even deadly.
If someone doesn't understand it, they they certainly should not be encouraged to use that technique.
Proper battery powered scopes avoid this safety issue because they are supplied with special insulated BNC connectors and ground clip leads, so you can't accidently touch the ground system.
(https://5.imimg.com/data5/SELLER/Default/2021/9/WX/HD/HI/2486614/fluke-scopemeter-190-504-s-portable-oscilloscope.jpg)
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The confusion for me has been two things so far. the strange spark I saw once, and what the range or "safe practice" is here. Is it okay to do? I'm confused as why arcs would form at only 100uA.
I keep hearing online about how you can isolate your DUT to avoid spending money on a DIFF probe that costs a ton and has a lot of noise and low bandwidth. Maybe I can get away with it with another layer of isolation but I also love my scope so much and 600-800CAD is a LOT of money to me. Don't want to see my precious investment smoking! Or worse, become inaccurate/malfunction without me knowing! I assume the device has protection built-in but I rather not rely on that :sweat:. I'm kind of a semi-newbie and I don't wanna do anything stupid...
As others have said, a couple of hundreds of µA leakage current into your scope ground are not going to hurt it, so you are doing okay here.
But: If you measure an isolated low-voltage DC powered DUT fed from your power supply, the leakage current from your power supply will flow through your probe's ground lead to the scope, and though its PE connection back to mains earth. Since the ground lead has a finite impedance, the scope and DUT ground potential can become slightly different. This may affect sensitive measurements and you may see a mains ripple superimposed on the scope trace (but we are talking microamps and milliohms here, so this is generally not a big problem). To avoid this you can try the following:
- Add another low impedance connection between your DUT and scope ground that carries most of the leakage current.
- Ground one output of your power supply, usually the negative one (this assumes that the negative output is you DUT ground).
Another issue, not related to mains leakage, is that the ground lead of your probe has quite a large inductance which can affect the measurement of fast signals with a large bandwidth, such as fast rising edges. For these signals you will want the impedance between the ground ring of your probe and the DUT ground as low as possible, e.g. by using a ground spring.
And again: never isolate your safety class I test equipment (scope and power supply) with an isolation transformer (unless you know exactly what you are doing). Their safety relies on their cases being connected to the PE of your installation.
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- The scope is and remains grounded.
- And differential probes are used for measurements.
Exactly. Some idiot on youtube recommended using an isolation transformer so that a standard probe could be used as a diff probe by connecting the ground lead to a different part of the circuit instead of ground. :palm:
What's wrong with that?
Becaause it can be dangerous, especially with say measuring mains circuits (a common reason for doing this). It means that the entire metal chassis of your scope will be at a live potential relative to ground, and if you touch it, ouchy.
Oscilloscopes are mains earth referenced for safety reasons. It's why USB powered scopes like the new Rigols for example, come with a safety earth clip so you can ground it.
Well no, we're talking about a different scenario here, if I understand correctly:
Correct, I was making a generic point about scope safety because KuFuJosh was also talking generally. "floating the scope" is an age old technique that can be dangerous, even deadly.
If someone doesn't understand it, they they certainly should not be encouraged to use that technique.
Proper battery powered scopes avoid this safety issue because they are supplied with special insulated BNC connectors and ground clip leads, so you can't accidently touch the ground system.
(https://5.imimg.com/data5/SELLER/Default/2021/9/WX/HD/HI/2486614/fluke-scopemeter-190-504-s-portable-oscilloscope.jpg)
I think KungFuJosh was the only one suggesting floating a scope. I explicitly described how to use it between her non-xformered SWMPS (which has been discontinued; likely because of the issue she found) and mains, LEAVING her scope properly grounded.
In this context, user naivete is a canard. Even a soldering iron is dangerous if the user does not know how to use it. Why is RTFM such a popular acronym in this forum? What equipment IS safe if the user does not know how to use it?
For AmeliaBurns, I have a simple, cost effective, safe solution if she is working on circuits below 20V. Buy a Digient Analog Discovery for $300 which provides all the TE she probably needs. In her situation, buying a lab grade PSU may well be shooting a flea with an elephant gun. She is not a professional.
I am surprised no one brought up ensuring the PSU is meets UL or EU safety standards.
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I think KungFuJosh was the only one suggesting floating a scope.
You're not aware we tend to go off on tangents on here?
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I think KungFuJosh was the only one suggesting floating a scope. I explicitly described how to use it between her non-xformered SWMPS (which has been discontinued; likely because of the issue she found) and mains, LEAVING her scope properly grounded.
Floating a safety class I power supply is as dangerous as floating a scope. See reply #15 (https://www.eevblog.com/forum/testgear/confusion-around-probing-isolated-circuits-is-this-a-safe-practice/msg6168341/#msg6168341). And her lab power supply has isolated outputs, the isolation is provided by the transformers you see in the pictures. The case also seems to be properly connected to PE. Of course we don't know if it is compliant to all applicable safety standards. The leakage current she has seen does not, by itself, call its safety into question.
And in Josh's defense, I don't think he said that, but I'll leave it to him to clarify. ;)
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BTW, is there a "go-to" design for a DIY HV differential probe with a reasonable bandwidth and CMRR high enough to be usable for probing generic SMPS'es?
I remember there have been quite a few threads to that effect, is there some design that would stand out in being not overly complex and good enough at the same time? Surely someone tried to build one?
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I think KungFuJosh was the only one suggesting floating a scope.
You're not aware we tend to go off on tangents on here?
I can be soooo naive. :palm:
But seriously, especially when we are dealing with amateurs like myself, we should avoid confusion. I remember how flummoxed I was by the correct, but confusing advice I got when I first started here. I appreciate and support making threads a "matter of record" for future readers, but sometimes we make it so complicated.
What do people think of the Analog Discovery for people working on low voltage circuits who have not yet learned about mains voltage; and may not for a pretty long time.
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BTW, is there a "go-to" design for a DIY HV differential probe with a reasonable bandwidth and CMRR high enough to be usable for probing generic SMPS'es?
I remember there have been quite a few threads to that effect, is there some design that would stand out in being not overly complex and good enough at the same time? Surely someone tried to build one?
https://www.eevblog.com/forum/projects/cheap-hv-differential-oscilloscope-probe/ (https://www.eevblog.com/forum/projects/cheap-hv-differential-oscilloscope-probe/)
https://www.eevblog.com/forum/projects/diy-mains-voltage-differential-probe-for-smps-design/ (https://www.eevblog.com/forum/projects/diy-mains-voltage-differential-probe-for-smps-design/)
https://www.eevblog.com/forum/projects/hv-50mhz-differential-probe-(diy-proyect)/ (https://www.eevblog.com/forum/projects/hv-50mhz-differential-probe-(diy-proyect)/)
https://www.eevblog.com/forum/projects/my-high-voltage-differential-probe-design/ (https://www.eevblog.com/forum/projects/my-high-voltage-differential-probe-design/)
https://www.eevblog.com/forum/beginners/low-cost-diy-differential-probe/ (https://www.eevblog.com/forum/beginners/low-cost-diy-differential-probe/)
https://www.youtube.com/watch?v=GOlgaEK2Hsk (https://www.youtube.com/watch?v=GOlgaEK2Hsk)
https://www.youtube.com/watch?v=GOk1BYKSsOI (https://www.youtube.com/watch?v=GOk1BYKSsOI)
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I think KungFuJosh was the only one suggesting floating a scope. I explicitly described how to use it between her non-xformered SWMPS (which has been discontinued; likely because of the issue she found) and mains, LEAVING her scope properly grounded.
Floating a safety class I power supply is as dangerous as floating a scope. See reply #15 (https://www.eevblog.com/forum/testgear/confusion-around-probing-isolated-circuits-is-this-a-safe-practice/msg6168341/#msg6168341). And her lab power supply has isolated outputs, the isolation is provided by the transformers you see in the pictures. The case also seems to be properly connected to PE. Of course we don't know if it is compliant to all applicable safety standards. The leakage current she has seen does not, by itself, call its safety into question.
And in Josh's defense, I don't think he said that, but I'll leave it to him to clarify. ;)
Thanks for this. I was going to offer to send her a Tenma plug and play iso until you posted this. Thank you for the caution.
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BTW, is there a "go-to" design for a DIY HV differential probe with a reasonable bandwidth and CMRR high enough to be usable for probing generic SMPS'es?
I remember there have been quite a few threads to that effect, is there some design that would stand out in being not overly complex and good enough at the same time? Surely someone tried to build one?
https://www.eevblog.com/forum/projects/cheap-hv-differential-oscilloscope-probe/ (https://www.eevblog.com/forum/projects/cheap-hv-differential-oscilloscope-probe/)
https://www.eevblog.com/forum/projects/diy-mains-voltage-differential-probe-for-smps-design/ (https://www.eevblog.com/forum/projects/diy-mains-voltage-differential-probe-for-smps-design/)
https://www.eevblog.com/forum/projects/hv-50mhz-differential-probe-(diy-proyect)/ (https://www.eevblog.com/forum/projects/hv-50mhz-differential-probe-(diy-proyect)/)
https://www.eevblog.com/forum/projects/my-high-voltage-differential-probe-design/ (https://www.eevblog.com/forum/projects/my-high-voltage-differential-probe-design/)
https://www.eevblog.com/forum/beginners/low-cost-diy-differential-probe/ (https://www.eevblog.com/forum/beginners/low-cost-diy-differential-probe/)
Yes, thanks, I can use the search function :).
The question was whether there's a certain one that's more attractive than the rest. Will still review all of them though.
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I think KungFuJosh was the only one suggesting floating a scope. I explicitly described how to use it between her non-xformered SWMPS (which has been discontinued
No! I stated floating the scope should NOT be done. I was referring to an idiot on youtube who suggested doing that. My primary point was to not mess around dangerously and/or ignorantly.
And in Josh's defense, I don't think he said that, but I'll leave it to him to clarify. ;)
Indeed. Thank you. ;)
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The arc happens because as you disconnect the scope ground lead, the point it was connected to is no longer shorted to ground, and the AC current flowing through* the Y capacitor allows the potential to rise to a level that an arc occurs.
*technically the current doesn't flow through the capacitor, but that's a different question.
Wait, but unconnected, the voltage on the negative/positive terminal barely goes above 4.8Vp. That's barely enough to create an arc. Why would that happen during a disconnect? I thought it's caused by "parasitic inductance" in most devices, but 96ua is so low. Is there a way to avoid this?
But the main reasons seems to be connecting two different ground clips in places you shouldn't? or the sparks.
I usually avoid using more than one probe at a time when doing this. once another probe is connected to my scope, I make sure everything's earthed and only connected to ground.
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I think KungFuJosh was the only one suggesting floating a scope. I explicitly described how to use it between her non-xformered SWMPS (which has been discontinued; likely because of the issue she found) and mains, LEAVING her scope properly grounded.
In this context, user naivete is a canard. Even a soldering iron is dangerous if the user does not know how to use it. Why is RTFM such a popular acronym in this forum? What equipment IS safe if the user does not know how to use it?
For AmeliaBurns, I have a simple, cost effective, safe solution if she is working on circuits below 20V. Buy a Digient Analog Discovery for $300 which provides all the TE she probably needs. In her situation, buying a lab grade PSU may well be shooting a flea with an elephant gun. She is not a professional.
I am surprised no one brought up ensuring the PSU is meets UL or EU safety standards.
Wait my PSU isn't discontinued, it's still up for sale! Plus, don't even the fancy power supplies have some leakage in a similar range to my my unit? How much should I expect out of those? I didn't realize/think the leakage would be all that different. 90-100uA seems to be considered very good for leakage? What is "non-xfornered"? Sorry for all the questions :) I'd be tempted to see if I can modify the components to reduce this etc, but that'd be not a great idea for a beginner honestly. I do feel like I'm a bit too paranoid about mains.
I'm technically a software engineer :) I do embedded too. but I do electronics for fun. Although I have this terrible habit of trying to be "professional" at every hobby I do. so I read all about impedance matching and all the rules even tho I'm just designing a crappy LED Controller and worry about EMI :-D I forced myself to learn all that in just 4 days so my head hurts a bit. Might be why I'm getting so confused, I might need a break. This crazy attitude did get me my last job, but I did get laid of so... :P
I might design my own active power rail probe/differential probe if I'm good enough to trust myself :)
I also am still surprised nobody suggested the DC/DC isolation circuit. It's used in medical equipment apparently where strong isolated is needed for vulnerable patients where apparently even 100uA can hurt them(?). That'd have to be DIY tho.
That test device is interesting, a scope, power supply and logic analyzer all in one? Still too expensive for me at this moment. I also doubt any of those functions are decent for such a compact and relatively cheap device. Although I'm a bit confused how it'd help. Is it because it contains differential inputs already?
Also sadly my current circuit is 24v, although that's only for the MSOFET and switching side, the rest is 3.3v. I do a lot of different things tho so it does vary. Most of my projects are more embedded style, but I do want to eventually do RF and maybe some analogue etc.
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I think KungFuJosh was the only one suggesting floating a scope. I explicitly described how to use it between her non-xformered SWMPS (which has been discontinued; likely because of the issue she found) and mains, LEAVING her scope properly grounded.
Floating a safety class I power supply is as dangerous as floating a scope. See reply #15 (https://www.eevblog.com/forum/testgear/confusion-around-probing-isolated-circuits-is-this-a-safe-practice/msg6168341/#msg6168341). And her lab power supply has isolated outputs, the isolation is provided by the transformers you see in the pictures. The case also seems to be properly connected to PE. Of course we don't know if it is compliant to all applicable safety standards. The leakage current she has seen does not, by itself, call its safety into question.
And in Josh's defense, I don't think he said that, but I'll leave it to him to clarify. ;)
Yeah this is what confuses me. it's not a GREAT power supply, the output is a bit noisy (slightly less than I anticipated for the price/type. although my measuring was vey crappy as it was hard to use a ground spring). but it's not a god awful unit :P and for this specific purpose. isolation is the main thing that matters!
From my understanding, the sparks are the main thing I should remove/worry about in terms of damage as long as I don't attach multiple probes, but I'm not sure what is the best aproch. I still wonder if a TVS at slightly above my voltage (30v) would do much to help? and i'm a bit confused by how such a low voltage and current can do that (only 100uA max, 4.8Vpp AC at 60hz)
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BTW, is there a "go-to" design for a DIY HV differential probe with a reasonable bandwidth and CMRR high enough to be usable for probing generic SMPS'es?
I remember there have been quite a few threads to that effect, is there some design that would stand out in being not overly complex and good enough at the same time? Surely someone tried to build one?
I've seen quiet a few! Although not many of them are properly tested or succeeded well. Some of them don't do some things I've heard is good to do, but it's designed by people who surely know a lot more than I do.
https://resources.altium.com/p/differential-oscilloscope-probe-project (https://resources.altium.com/p/differential-oscilloscope-probe-project)
This one doesn't look... extremely good? https://www.instructables.com/DIY-1GHz-Active-Probe-for-Under-20/ (https://www.instructables.com/DIY-1GHz-Active-Probe-for-Under-20/) it's an active fet tho not differential.
https://xellers.wordpress.com/electronics/1ghz-active-differential-probe/ (https://xellers.wordpress.com/electronics/1ghz-active-differential-probe/) this one didn't really work well up to 1ghz, but it still worked at lower freq? also these days 4 layer PCBs with specific layouts barely cost a few dollars. I can do 50x50mm for 2$ at
https://hackaday.io/project/167197-poor-mans-1ghz-differential-probe (https://hackaday.io/project/167197-poor-mans-1ghz-differential-probe)
JLCPCB!! Rogers is 96$ for a 2 layer 100x50mm tho, 48 for teflon.
I thought about building one of these!
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I think KungFuJosh was the only one suggesting floating a scope. I explicitly described how to use it between her non-xformered SWMPS (which has been discontinued; likely because of the issue she found) and mains, LEAVING her scope properly grounded.
In this context, user naivete is a canard. Even a soldering iron is dangerous if the user does not know how to use it. Why is RTFM such a popular acronym in this forum? What equipment IS safe if the user does not know how to use it?
For AmeliaBurns, I have a simple, cost effective, safe solution if she is working on circuits below 20V. Buy a Digient Analog Discovery for $300 which provides all the TE she probably needs. In her situation, buying a lab grade PSU may well be shooting a flea with an elephant gun. She is not a professional.
I am surprised no one brought up ensuring the PSU is meets UL or EU safety standards.
Wait my PSU isn't discontinued, it's still up for sale! Plus, don't even the fancy power supplies have some leakage in a similar range to my my unit? How much should I expect out of those? I didn't realize/think the leakage would be all that different. 90-100uA seems to be considered very good for leakage? What is "non-xfornered"? Sorry for all the questions :) I'd be tempted to see if I can modify the components to reduce this etc, but that'd be not a great idea for a beginner honestly. I do feel like I'm a bit too paranoid about mains.
Here is what I get when I go the link you provided.. I never saw an item restocked when it is marked like this.
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I think KungFuJosh was the only one suggesting floating a scope. I explicitly described how to use it between her non-xformered SWMPS (which has been discontinued
No! I stated floating the scope should NOT be done. I was referring to an idiot on youtube who suggested doing that. My primary point was to not mess around dangerously and/or ignorantly.
And in Josh's defense, I don't think he said that, but I'll leave it to him to clarify. ;)
Indeed. Thank you. ;)
Note the "I THINK". This was not a definite statement. Apologies for the ambiguity.
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Note the "I THINK". This was not a definite statement. Apologies for the ambiguity.
No worries. I just don't want anybody else thinking I suggested that. ;)
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Hmm so what I'm hearing overall is that even if you're circuit is isolated, just don't probe it as is without a differential probe? and that I should ground my circuits unless for some reason, it must not be grounded:? I'm a bit confused. the 60-200ua of current going into the ground of my scope is apparently just fine, the tiny spark is the spooky part. (not sure how that's happening)
I drew a picture to illustrate what the main problem is:
(https://www.eevblog.com/forum/testgear/confusion-around-probing-isolated-circuits-is-this-a-safe-practice/?action=dlattach;attach=2737829;image)
As mentioned, all inputs of a scope are connected to each other (unless you have a scope where the channels are individually isolated).
However, this also means that if you want to measure with several channels, for example, but at different (separate) potential circuits, their galvanic isolation from each other is thereby eliminated.
The consequences can range from malfunctions to destruction of the circuits.
An isolation transformer is used to isolate the potential from the DUT to the mains.
A scope should not be connected via an isolation transformer.
Why use an isolating transformer on the DUT?
If the device is not grounded because it belongs to protection class II.
These devices usually have a 2-pole power plug that can be plugged into any outlet.
However, this also means that the line can then be reversed with the phase.
If you then connect the negative terminal of a probe to the phase, you short-circuit the phase to ground – not good...
(https://www.eevblog.com/forum/testgear/confusion-around-probing-isolated-circuits-is-this-a-safe-practice/?action=dlattach;attach=2737833;image)
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BTW, is there a "go-to" design for a DIY HV differential probe with a reasonable bandwidth and CMRR high enough to be usable for probing generic SMPS'es?
I remember there have been quite a few threads to that effect, is there some design that would stand out in being not overly complex and good enough at the same time? Surely someone tried to build one?
I've seen quiet a few! Although not many of them are properly tested or succeeded well. Some of them don't do some things I've heard is good to do, but it's designed by people who surely know a lot more than I do.
https://resources.altium.com/p/differential-oscilloscope-probe-project (https://resources.altium.com/p/differential-oscilloscope-probe-project)
This one doesn't look... extremely good? https://www.instructables.com/DIY-1GHz-Active-Probe-for-Under-20/ (https://www.instructables.com/DIY-1GHz-Active-Probe-for-Under-20/) it's an active fet tho not differential.
https://xellers.wordpress.com/electronics/1ghz-active-differential-probe/ (https://xellers.wordpress.com/electronics/1ghz-active-differential-probe/) this one didn't really work well up to 1ghz, but it still worked at lower freq? also these days 4 layer PCBs with specific layouts barely cost a few dollars. I can do 50x50mm for 2$ at
https://hackaday.io/project/167197-poor-mans-1ghz-differential-probe (https://hackaday.io/project/167197-poor-mans-1ghz-differential-probe)
JLCPCB!! Rogers is 96$ for a 2 layer 100x50mm tho, 48 for teflon.
I thought about building one of these!
These are (mostly) different beasts: RF active probes, not necessarily differential. They serve a different purpose, not that of safely probing between two arbitrary points in DUT.
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BTW, is there a "go-to" design for a DIY HV differential probe with a reasonable bandwidth and CMRR high enough to be usable for probing generic SMPS'es?
I remember there have been quite a few threads to that effect, is there some design that would stand out in being not overly complex and good enough at the same time? Surely someone tried to build one?
https://www.eevblog.com/forum/projects/cheap-hv-differential-oscilloscope-probe/ (https://www.eevblog.com/forum/projects/cheap-hv-differential-oscilloscope-probe/)
https://www.eevblog.com/forum/projects/diy-mains-voltage-differential-probe-for-smps-design/ (https://www.eevblog.com/forum/projects/diy-mains-voltage-differential-probe-for-smps-design/)
https://www.eevblog.com/forum/projects/hv-50mhz-differential-probe-(diy-proyect)/ (https://www.eevblog.com/forum/projects/hv-50mhz-differential-probe-(diy-proyect)/)
https://www.eevblog.com/forum/projects/my-high-voltage-differential-probe-design/ (https://www.eevblog.com/forum/projects/my-high-voltage-differential-probe-design/)
https://www.eevblog.com/forum/beginners/low-cost-diy-differential-probe/ (https://www.eevblog.com/forum/beginners/low-cost-diy-differential-probe/)
Yes, thanks, I can use the search function :).
But you didn't :P
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BTW, is there a "go-to" design for a DIY HV differential probe with a reasonable bandwidth and CMRR high enough to be usable for probing generic SMPS'es?
I remember there have been quite a few threads to that effect, is there some design that would stand out in being not overly complex and good enough at the same time? Surely someone tried to build one?
https://www.eevblog.com/forum/projects/cheap-hv-differential-oscilloscope-probe/ (https://www.eevblog.com/forum/projects/cheap-hv-differential-oscilloscope-probe/)
https://www.eevblog.com/forum/projects/diy-mains-voltage-differential-probe-for-smps-design/ (https://www.eevblog.com/forum/projects/diy-mains-voltage-differential-probe-for-smps-design/)
https://www.eevblog.com/forum/projects/hv-50mhz-differential-probe-(diy-proyect)/ (https://www.eevblog.com/forum/projects/hv-50mhz-differential-probe-(diy-proyect)/)
https://www.eevblog.com/forum/projects/my-high-voltage-differential-probe-design/ (https://www.eevblog.com/forum/projects/my-high-voltage-differential-probe-design/)
https://www.eevblog.com/forum/beginners/low-cost-diy-differential-probe/ (https://www.eevblog.com/forum/beginners/low-cost-diy-differential-probe/)
Yes, thanks, I can use the search function :).
But you didn't :P
Well, search wouldn't pick me the one that's most reasonable of all, if there is one, that's why I asked if maybe someone knew one from their experience.
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This makes me concerned that the currents flowing to the ground terminal of my scope, although very small could potentially damage it. Is probing isolated circuits like this a good/common practice? Is this simply nothing to worry about and normal?
This is entirely normal and one of the functions of the ground is to conduct those currents. µA currents will not harm you or your scope.
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Here is what I get when I go the link you provided.. I never saw an item restocked when it is marked like this.
That's because I use Amazon.ca and you're not in Canada :)
PS: I can see your postal code there if you're bothered by that. can be used to track you potentially.
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Hmm so what I'm hearing overall is that even if you're circuit is isolated, just don't probe it as is without a differential probe? and that I should ground my circuits unless for some reason, it must not be grounded:? I'm a bit confused. the 60-200ua of current going into the ground of my scope is apparently just fine, the tiny spark is the spooky part. (not sure how that's happening)
I drew a picture to illustrate what the main problem is:
As mentioned, all inputs of a scope are connected to each other (unless you have a scope where the channels are individually isolated).
However, this also means that if you want to measure with several channels, for example, but at different (separate) potential circuits, their galvanic isolation from each other is thereby eliminated.
The consequences can range from malfunctions to destruction of the circuits.
An isolation transformer is used to isolate the potential from the DUT to the mains.
A scope should not be connected via an isolation transformer.
Why use an isolating transformer on the DUT?
If the device is not grounded because it belongs to protection class II.
These devices usually have a 2-pole power plug that can be plugged into any outlet.
However, this also means that the line can then be reversed with the phase.
If you then connect the negative terminal of a probe to the phase, you short-circuit the phase to ground – not good...
Oh yes I understand that well. My question was more in the specific scenario of only using one probe (leaving others disconnected to stop myself from forgetting) and not isolating the scope, but rather having iosolation on my power supply side. (meaning the output is also low voltage(<30v) and isolated).
-
BTW, is there a "go-to" design for a DIY HV differential probe with a reasonable bandwidth and CMRR high enough to be usable for probing generic SMPS'es?
I remember there have been quite a few threads to that effect, is there some design that would stand out in being not overly complex and good enough at the same time? Surely someone tried to build one?
I've seen quiet a few! Although not many of them are properly tested or succeeded well. Some of them don't do some things I've heard is good to do, but it's designed by people who surely know a lot more than I do.
https://resources.altium.com/p/differential-oscilloscope-probe-project (https://resources.altium.com/p/differential-oscilloscope-probe-project)
This one doesn't look... extremely good? https://www.instructables.com/DIY-1GHz-Active-Probe-for-Under-20/ (https://www.instructables.com/DIY-1GHz-Active-Probe-for-Under-20/) it's an active fet tho not differential.
https://xellers.wordpress.com/electronics/1ghz-active-differential-probe/ (https://xellers.wordpress.com/electronics/1ghz-active-differential-probe/) this one didn't really work well up to 1ghz, but it still worked at lower freq? also these days 4 layer PCBs with specific layouts barely cost a few dollars. I can do 50x50mm for 2$ at
https://hackaday.io/project/167197-poor-mans-1ghz-differential-probe (https://hackaday.io/project/167197-poor-mans-1ghz-differential-probe)
JLCPCB!! Rogers is 96$ for a 2 layer 100x50mm tho, 48 for teflon.
I thought about building one of these!
These are (mostly) different beasts: RF active probes, not necessarily differential. They serve a different purpose, not that of safely probing between two arbitrary points in DUT.
Oh yes I included those too because I found them cool :) I wanted to make one because I don't have a spare 5,000$!
Someone here actually made one, but it's not opensource, they sell it for 250 euroes I think? for now i'll give up on 6mhz+ noise on my power supply and measure ripple at 1:1 and maybe 10:1 (tbh, 5mv noise is still going to be much more than any switching power supply I could design!)
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This makes me concerned that the currents flowing to the ground terminal of my scope, although very small could potentially damage it. Is probing isolated circuits like this a good/common practice? Is this simply nothing to worry about and normal?
This is entirely normal and one of the functions of the ground is to conduct those currents. µA currents will not harm you or your scope.
Awesome! what about the sparks? What can I do to eliminate them?
TVS diode was one idea I had, but i'm confused about how they even happen. I'm aware of sparks from suddenly disconnecting high current loads due to inductance, but 200uA is barely anything to generate high enough voltage to ionize the air and jump to the scope! I also know during connections sparks happen with high loads as the connector surface isn't perfect and you can make a very high resistance connection and weld the tip. It might have something to do with electromagnetic fields too. not 100% sure. (if the distance is very small, the electrons might just start jumping?)
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In case the OP hasn't seen it:
https://www.youtube.com/watch?v=xaELqAo4kkQ (https://www.youtube.com/watch?v=xaELqAo4kkQ)
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In case the OP hasn't seen it:
https://www.youtube.com/watch?v=xaELqAo4kkQ (https://www.youtube.com/watch?v=xaELqAo4kkQ)
Oh I've actually seen that video a few times :) It was the first video I watched when I bought my oscilloscope :) It's a very good and useful video.
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Here is what I get when I go the link you provided.. I never saw an item restocked when it is marked like this.
That's because I use Amazon.ca and you're not in Canada :)
PS: I can see your postal code there if you're bothered by that. can be used to track you potentially.
What privacy?
I managed several databases in the 80s and and 90s when SS numbers were still sacrosanct. Doge gave them all away, Flock cameras, Pegasus, Cambridge Analytical, etc. OTOH, I knew enough to keep my digital footprint pretty small. When I do deep searches on my and my wife's name, very little comes up. And I have been on since 1983 (DarpaNet).
Back in the 1990s, when our daughter was in middle school, I showed her the router logs. I made it clear I would never monitor them, but I wanted her to be aware that others, particularly employers, will.
In the 1950s my mother applied about my missing father to the federal office that located missing people by SSN (via Hollerith sorts!). The reply was yes, he is alive, but we cannot tell you anything else because such info is protected by law. I have the letters.
This lies at the heart of why many boomers prefer direct interactions.
So it goes. (KV)
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The arc happens because as you disconnect the scope ground lead, the point it was connected to is no longer shorted to ground, and the AC current flowing through* the Y capacitor allows the potential to rise to a level that an arc occurs.
*technically the current doesn't flow through the capacitor, but that's a different question.
Wait, but unconnected, the voltage on the negative/positive terminal barely goes above 4.8Vp. That's barely enough to create an arc. Why would that happen during a disconnect? I thought it's caused by "parasitic inductance" in most devices, but 96ua is so low. Is there a way to avoid this?
But the main reasons seems to be connecting two different ground clips in places you shouldn't? or the sparks.
I usually avoid using more than one probe at a time when doing this. once another probe is connected to my scope, I make sure everything's earthed and only connected to ground.
The reason you only measure 4.8V between GND and either -ve or +ve is because the meter has a low enough impedance (probably 10MΩ or so) to pull the voltage down to that. The open circuit voltage is going to be higher, and can, depending on what other circuit components interfere, reach pretty much whatever your line voltage is (unlikely to get this high though).
The best way to avoid this, if you are only going to be using the -ve rail as your scope grounding point, is to connect the -ve and GND with a shorting link on the psu; that's what they are there for. If you need to connect the scope ground somewhere that isn't the PSU -ve, you should not short the PSU -ve and GND, and only connect and disconnect the probe ground while the DUT is switched off.
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Here is what I get when I go the link you provided.. I never saw an item restocked when it is marked like this.
That's because I use Amazon.ca and you're not in Canada :)
PS: I can see your postal code there if you're bothered by that. can be used to track you potentially.
What privacy?
I managed several databases in the 80s and and 90s when SS numbers were still sacrosanct. Doge gave them all away, Flock cameras, Pegasus, Cambridge Analytical, etc. OTOH, I knew enough to keep my digital footprint pretty small. When I do deep searches on my and my wife's name, very little comes up. And I have been on since 1983 (DarpaNet).
Back in the 1990s, when our daughter was in middle school, I showed her the router logs. I made it clear I would never monitor them, but I wanted her to be aware that others, particularly employers, will.
In the 1950s my mother applied about my missing father to the federal office that located missing people by SSN (via Hollerith sorts!). The reply was yes, he is alive, but we cannot tell you anything else because such info is protected by law. I have the letters.
This lies at the heart of why many boomers prefer direct interactions.
So it goes. (KV)
lol that's fair. I also feel mostly the same way, that privacy is a lost cause at this point, but I also prefer to be cautious. This stuff makes it easier for non savvy people to do stuff, but honestly? yeah.....
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Wait, but unconnected, the voltage on the negative/positive terminal barely goes above 4.8Vp. That's barely enough to create an arc. Why would that happen during a disconnect? I thought it's caused by "parasitic inductance" in most devices, but 96ua is so low. Is there a way to avoid this?
But the main reasons seems to be connecting two different ground clips in places you shouldn't? or the sparks.
I usually avoid using more than one probe at a time when doing this. once another probe is connected to my scope, I make sure everything's earthed and only connected to ground.
The reason you only measure 4.8V between GND and either -ve or +ve is because the meter has a low enough impedance (probably 10MΩ or so) to pull the voltage down to that. The open circuit voltage is going to be higher, and can, depending on what other circuit components interfere, reach pretty much whatever your line voltage is (unlikely to get this high though).
The best way to avoid this, if you are only going to be using the -ve rail as your scope grounding point, is to connect the -ve and GND with a shorting link on the psu; that's what they are there for. If you need to connect the scope ground somewhere that isn't the PSU -ve, you should not short the PSU -ve and GND, and only connect and disconnect the probe ground while the DUT is switched off.
Ooh that makes sense. It's kind of the observer effect :P but I'm still a bit shocked that it does that. I felt like you'd need a good current too for arcs to make it jump to a few kV.
My feelings towards this is so far to just honestly give up and ground the thing, and leave differential stuff to my imagination/calculations :P It's a shame cause I wanted to probe shunt resistors that aren't necessarily on the low side. I did connect a 20k (or maybe even 1MOhm) to avoid the arcs before hoping it'd be so small that it'd not mess with anything, but also get rid of the arcs, but that seems a bit... not great.
Or let the arcs slowly kill my scope so I have a justifiable excuse for a Rigol mho900/SDS3000x HD :-DD (Joking ofc)
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Or let the arcs slowly kill my scope so I have a justifiable excuse for a Rigol mho900/SDS3000x HD :-DD (Joking ofc)
Ewwwww, don't joke about buying a Rigol. ;)