Author Topic: Afraid to use my oscilloscope?  (Read 5383 times)

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Offline Ian.M

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Re: Afraid to use my oscilloscope?
« Reply #25 on: November 24, 2021, 09:09:23 am »
In my understanding the DUT (device-under-test) shall be floating and thus connected to mains using an isolation transformer, and the measuring instruments shall be connected to the protective ground. NOT another way around.
The problem is *keeping* the DUT floating. If its really floating, accidental contact with it with one hand cant electrocute you - there is no path for current to flow through your chest and heart.  You can still get a nasty shock of course with the risk of electrical burns and even local nerve damage, but you will be alive to regret it.  However you loose that small amount of safety from a floating DUT as soon as you connect anything grounded as although still isolated, it is no longer floating as soon as you make that ground connection,, which could be via your ordinary scope probes, most of your test equipment, or even your bench PC and it isn't always obvious *what* will introduce a ground to the DUT.  e.g. roof mounted aerial systems are often grounded and almost invariably so if they feed multiple outlets.

And do not solder live high voltage circuits even when the DUT is floating (or, you THINK it is floating).
Do not solder live circuits full stop.
Even if its low voltage, you can get in big trouble.  Most soldering irons and soldering stations have the tip grounded to prevent leakage currents damaging highly ESD sensitive components, and its usual for the tip to be hard-grounded with no series resistor to limit discharge currents.   If you attempt to solder anything connected to one terminal of a LiPO battery pack (or other high current battery) with the other terminal grounded, e.g. via your scope's ground clip, you will regret it as the battery shorts out through the ground loop formed by your solder station and scope.
If  batteries are involved *ALWAYS* totally disconnect and isolate the board or pack while soldering.
« Last Edit: November 24, 2021, 09:13:25 am by Ian.M »
 

Offline Steffalompen

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Re: Afraid to use my oscilloscope?
« Reply #26 on: November 26, 2021, 06:12:55 pm »
I wish all the videos on the subject would stop talking about "the probe" and call it the ground clip, to lessen confusion.

As for me, I'm sat in the second floor of my wooden house, and there isn't a ground wire to be found on this floor.
So then I suppose at worst the floating charge potentials could zap me or my gear? (Unless I manage to ground clip the scope chassis to L or N and contact the other)
 

Online tautech

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Re: Afraid to use my oscilloscope?
« Reply #27 on: November 26, 2021, 09:30:07 pm »
I wish all the videos on the subject would stop talking about "the probe" and call it the ground clip, to lessen confusion.

As for me, I'm sat in the second floor of my wooden house, and there isn't a ground wire to be found on this floor.
So then I suppose at worst the floating charge potentials could zap me or my gear? (Unless I manage to ground clip the scope chassis to L or N and contact the other)
Welcome to the forum.

Actually it's more correctly called the probe reference lead which for any signal you probe is referenced against.
Most scopes of course are mains ground referenced and their BNC shells are at that potential for safety reasons which brings us to your concern that your scope and other instruments may not be grounded which indeed due to the X/Y caps in the mains input filters could couple a small charge into the instruments chassis.
This could certainly potentially damage sensitive circuits and also not having a mains earth close to the bench gives you no way to manage ESD so a dedicated lab earth feed could be well worth considering.
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Offline Steffalompen

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Re: Afraid to use my oscilloscope?
« Reply #28 on: November 26, 2021, 10:58:54 pm »
Thank you  :)
So like an ESD strap I could have the entire feed high impedance and still referenced to mains earth? I like that. Feels safe.
 

Offline cdev

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Re: Afraid to use my oscilloscope?
« Reply #29 on: November 27, 2021, 02:36:13 am »
Get a voltmeter with a sensor for high voltage built in. Its often called NCV (non contact voltage) probe. It will sense voltages high enough to give you a shock or damage your scope with a light or a beep. No contact, just proximity is all the probing you need to do. My Uni-T clamp meter includes this handy feature. It beeps to warn me of any place where a dangerous voltage is. Its very handy. Often it warns you of problems you would likely never have found any other way, except being shocked or similar. .
« Last Edit: November 27, 2021, 02:38:03 am by cdev »
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Offline m k

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Re: Afraid to use my oscilloscope?
« Reply #30 on: November 27, 2021, 08:35:45 pm »
Back in the day we had CRT monitors that occasionally had problems.
Many times those problems didn't behave and on top of that the majority of their numerous heatsinks were live.
I personally do not miss them very much.
Advance-Aneng-Appa-AVO-Data Tech-Fluke-General Radio-H. W. Sullivan-Heathkit-HP-Kaise-Kyoritsu-Leeds & Northrup-Mastech-REO-Simpson-Sinclair-Tektronix-Triplett-YFE
(plus lesser brands from the work shop of the world)
 

Offline Ian.M

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Re: Afraid to use my oscilloscope?
« Reply #31 on: November 27, 2021, 09:59:00 pm »
Yep,  up to about 35KV EHT on a really big color set and typically over 12KV even on small B&W sets at the final anode just waiting to bite you if you had to remove the cap connecting the LOPT EHT output lead to the side of the CRT bowl.  A big tube had a lot of capacitance and could deliver a shock that could throw you clear across the room.   Also static would build up on the face of the tube when you switched it on or off, as the final anode charged or discharged, so if you switched a set off, disconnected it, and grounded that static by picking it up with the tube face against your chest (which you had to do for balance as that's where most of the weight was), that would charge up the guts of the set to several KV, just waiting to bite you if your grip touched a connector at the back or the mains plug.

There were also various voltages on the tube pins associated with the electron gun and focus electrodes of up to several KV.
« Last Edit: November 27, 2021, 11:35:34 pm by Ian.M »
 

Offline james_s

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Re: Afraid to use my oscilloscope?
« Reply #32 on: November 27, 2021, 10:30:38 pm »
The EHT sounds scary but the B+ of 180V or so is more dangerous because it can deliver so much more current. I've been zapped by both on a few occasions and the EHT feels like a powerful static zap, the B+ feels like a properly dangerous shock. A regular switchmode power supply is arguably as dangerous to work on as a CRT display.
 

Offline Gyro

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Re: Afraid to use my oscilloscope?
« Reply #33 on: November 27, 2021, 10:53:05 pm »
...
Also static would build up on the face of the tube when you switched it on or off, as the final anode charged or discharged, so if you switched a set off, disconnected it, and grounded that static by picking it up with the tube face against your chest (which you had to do for balance as that's where most of the weight was), that would charge up the guts of the set to several KV, just waiting to bite you if your grip touched a connector at the back or the mains plug.

I remember doing that exact thing with a MicroVAX II workstation colour monitor that I had just powered down. Picked it up and my finger found one of the BNC connectors round the back. The punch in the chest was so strong that I don't think I noticed anything in my finger. Luckily I kept my footing... and the monitor, but those things weighed a ton!
Best Regards, Chris

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Offline Ian.M

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Re: Afraid to use my oscilloscope?
« Reply #34 on: November 27, 2021, 11:46:44 pm »
It was also enough voltage to cause your isolation transformer to flash over if you got bit by the EHT while the set was actually floating, which could cause insulation breakdown, ruining the isolation transformer.   I expect that back in the bad old days, a few scopes floated by cheater plugs died that way as well.
 

Offline SilverSolder

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Re: Afraid to use my oscilloscope?
« Reply #35 on: November 29, 2021, 03:12:21 pm »
The EHT sounds scary but the B+ of 180V or so is more dangerous because it can deliver so much more current. I've been zapped by both on a few occasions and the EHT feels like a powerful static zap, the B+ feels like a properly dangerous shock. A regular switchmode power supply is arguably as dangerous to work on as a CRT display.

When I was around 7 years old, I had the brilliant idea of researching the question, "Wonder if one can feel electricity with the fingers?".  So I promptly stuck my finger into an unpopulated light bulb socket (in a 240V mains country) to find out.   I found out... 

Ever since then, I have had a healthy respect for working with high voltages!  :)
 
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Offline de.leon

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Re: Afraid to use my oscilloscope?
« Reply #36 on: December 11, 2021, 02:33:46 pm »
I was wondering why Dave did not include the multimeter sanity check thing in his video.  I'm new to all this but it seems like a "best practices" thing to at least confirm your logic and assumptions as to where to ground a scope with a meter first.  My question is how much potential difference between grounding point and scope ground is too much current/voltage or does it need to be 0?  Would any voltage throw out any readings or waveforms or would it just be bled off by the mains earth?
 

Offline m k

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Re: Afraid to use my oscilloscope?
« Reply #37 on: December 12, 2021, 09:55:05 am »
First you must understand what you are doing, otherwise you can miss a point somewhere.
It doesn't mean you can't still miss it after decades of understanding.

One example is isolation transformer.
Maybe you have not needed it and its function has faded.
Then you get an ancient machine with a used to be a 0-class plug.
With luck you burn only a fuse.

I've measured 10A from me to ground and some real amps from RS-232 GND.

I'd say, as a rule of thumb, if you need a meter don't do it, get a guaranteed differential probe instead.
(no, I didn't)

On the other hand, connecting different cases together was a norm once.
But so was waking a battery with a crowbar and connecting bendix gear using the other method and nobody should do those eighter.

On the 3rd hand, I've learned to trust the chassis ground, but a note, do not do the same with an unknown extension cord.
And don't trust that ancient machine has a chassis even if something looks like one.

Maybe one more thing.
Nowadays buildings have 3-phase wall stuff, it means that different wall jack groups can be in different phase.
So to avoid that part of the equation use only one group of jacks or single extension cord.
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(plus lesser brands from the work shop of the world)
 


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