Author Topic: Can I blow the crap out of my scope if I probe this?  (Read 2876 times)

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Offline king.oslo

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Can I blow the crap out of my scope if I probe this?
« on: April 24, 2017, 01:11:01 am »
Hello there,

I recall videos where Dave has commented that if you probe stuff that is earth grounded together with your scope, you can get short circuits. However I don't recall what the justification for the statement was.

Basically I want to probe an FPGA demo board, which is powered by a PC USB, and I don't want anything to blow up. Is this safe?

Thanks
 

Offline joeqsmith

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Re: Can I blow the crap out of my scope if I probe this?
« Reply #1 on: April 24, 2017, 01:42:55 am »
Nothing is safe but your scope should survive looking at your FPGA demo board. Have fun and post what you are doing.
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
 

Offline rstofer

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Re: Can I blow the crap out of my scope if I probe this?
« Reply #2 on: April 24, 2017, 01:49:47 am »
I do it all the time.  I assume that my logic ground is equivalent to earth ground so I connect the probe ground to logic ground.  Then I merrily probe away.

There might be some oddball way that logic ground is some voltage level away from earth ground but I haven't run into it.

Where the big problems come in is with mains powered switching mode power supplies.  Or perhaps trying to take a differential measurement across some device where neither side is equivalent to earth ground.  You will notice this right away when you hook up the ground lead and get sparks.

There is probably a case to be made for measuring between where you plan to connect your ground lead and earth ground.  The voltage should be zero or very, very close.

And watch Dave's video again!
 

Online Ian.M

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Re: Can I blow the crap out of my scope if I probe this?
« Reply #3 on: April 24, 2017, 01:54:18 am »
For 99.9% of all scopes out there, the probe ground clip is directly connected via the lead, input socket shell, PCB ground tracks right back to the ground pin of the mains lead, and eventually back to a mains supply ground rod.  The only exception is entirely battery powered scopes and ones that specifically have isolated inputs.  Additionally, on all scopes without individual isolated inputs, the ground clips for all the inputs are connected together.

That means you must *NEVER* connect the ground clip to any point in the circuit under test that isn't already grounded, unless you know that the circuit is electrically floating, and that no other test equipment has introduced a ground. 

You also have to be careful about ground differences between different mains circuits.  Its safest to make sure that all grounded test equipment is running from the same mains circuit, preferably from the same wall outlet plate.

Furthermore if you connect more than one ground clip to a circuit, all of them *MUST* go to points at the same voltage - ideally points that are already directly electrically connected.

A PC (apart from some laptops with two pin mains power cords) has a grounded chassis and 0V rail, which will introduce a ground to your FPGA board.

If in doubt try a test lamp of an appropriate voltage for the board under test between the ground clip and the point you want to connect it to. If it lights up at all *STOP*!
« Last Edit: April 24, 2017, 10:52:54 pm by Ian.M »
 
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Offline rrinker

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Re: Can I blow the crap out of my scope if I probe this?
« Reply #4 on: April 24, 2017, 03:20:38 pm »
 Highlight that last line and put it in a big font size. Unless you are absolutely sure (and even then), TEST. The scope you save may be your own. There is no shame in double checking and finding out that yup, it IS perfectly safe. Much better to take a second and be doubly cautious than have something blow up in your face.

 

Offline king.oslo

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Re: Can I blow the crap out of my scope if I probe this?
« Reply #5 on: April 24, 2017, 10:17:17 pm »
Thanks, is it possible to buy some ac-ac transformer that will isolate my scope from ground? Or can i just cut the ground wire whilst I am probing? Will this eliminere the problem?

Thanks
 

Online tautech

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Re: Can I blow the crap out of my scope if I probe this?
« Reply #6 on: April 24, 2017, 10:47:47 pm »
Thanks, is it possible to buy some ac-ac transformer that will isolate my scope from ground? Or can i just cut the ground wire whilst I am probing? Will this eliminere the problem?

Thanks
It's much safer to isolate the DUT from mains ground. Isolating a scope can elevate all metallic points and surfaces to the same as the DUT and expose a user to potentially fatal shocks.
MAINTAIN EARTH TO YOUR SCOPE.
Avid Rabid Hobbyist
 

Online Ian.M

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Re: Can I blow the crap out of my scope if I probe this?
« Reply #7 on: April 24, 2017, 10:51:08 pm »
No. 'Floating your scope' is generally a very bad idea. 

The front panel controls aren't designed to provide high voltage isolation, so even if it has a plastic case, it becomes hazardous to use.  If it has a metal case, and anything else on or near your bench is grounded, its a death-trap. 

Also its power supply insulation isn't  designed to 'stand off' more than the nominal supply voltage + a safety margin, so you risk blowing up the scope anyway, and probably doing a lot more damage than melting a probe wire that's the typical result of a ground clip mishap on a scope with robustly grounded input sockets.

Furthermore the large capacitance to true ground + noise injected by the scope PSU that normally gets shunted to ground will mess up any readings you take with the probe ground clip on a non-grounded point.
 

Offline palpurul

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Re: Can I blow the crap out of my scope if I probe this?
« Reply #8 on: April 25, 2017, 08:40:04 am »
If your computer is connected to the mains via power brick you want to connect the ground of the probe to the ground of the circuit that you want to probe, nowhere else. If you connect the probe's ground to the ground of the circuit you'll be fine most of the time.
 

Offline Kilrah

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Re: Can I blow the crap out of my scope if I probe this?
« Reply #9 on: April 25, 2017, 08:56:08 am »
Using a laptop on battery power to drive your USB thingy is often an easy solution.
 

Offline homebrew

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Re: Can I blow the crap out of my scope if I probe this?
« Reply #10 on: April 25, 2017, 04:56:49 pm »
No. 'Floating your scope' is generally a very bad idea. 

Really? At least the were manufactured like that ...
Mine is a Philips PM3216 - 35MHz.
And it is completely floating. Never had any real issues ... but I tend to use my brain and do ABSOLUTELY NO high voltage measurements ...
 

Online Ian.M

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Re: Can I blow the crap out of my scope if I probe this?
« Reply #11 on: April 25, 2017, 05:39:47 pm »
The Philips PM3216 is a rather odd beast that falls in the (estimated) 0.1% I excluded from the 99.9%.  I believe that's one of the Philips series of scopes that can also be run from a 24V battery supply.

My caution was against floating a scope by external means.  A scope that's designed to be inherently floating is a different matter - Yours has reinforced insulation on its line frequency mains transformer, and doesn't rely on a filter that dumps SMPSU noise to ground.   However it still has the potentially live chassis and high capacitance to ground issues, so it would be unwise to connect its probe ground to a node more than +/-50V from ground or to a high impedance or fast switching node.
« Last Edit: April 25, 2017, 05:44:01 pm by Ian.M »
 
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Offline bitwelder

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Re: Can I blow the crap out of my scope if I probe this?
« Reply #12 on: April 25, 2017, 06:03:21 pm »
Using a laptop on battery power to drive your USB thingy is often an easy solution.
Perhaps it should be possible to use a simple USB powerbank instead.
 

Offline Nerull

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Re: Can I blow the crap out of my scope if I probe this?
« Reply #13 on: April 26, 2017, 05:06:49 pm »
No. 'Floating your scope' is generally a very bad idea. 

Really? At least the were manufactured like that ...
Mine is a Philips PM3216 - 35MHz.
And it is completely floating. Never had any real issues ... but I tend to use my brain and do ABSOLUTELY NO high voltage measurements ...

What do you mean it's a bad idea to drive a car off a cliff? There are these things called airplanes that are designed to fly through the air! They must be exactly the same!
 

Offline homebrew

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Re: Can I blow the crap out of my scope if I probe this?
« Reply #14 on: April 26, 2017, 08:50:25 pm »
No. 'Floating your scope' is generally a very bad idea. 

Really? At least the were manufactured like that ...
Mine is a Philips PM3216 - 35MHz.
And it is completely floating. Never had any real issues ... but I tend to use my brain and do ABSOLUTELY NO high voltage measurements ...

What do you mean it's a bad idea to drive a car off a cliff? There are these things called airplanes that are designed to fly through the air! They must be exactly the same!

Hmmm, I don't think that it is that clear. Yes, the PM3216 is built to be isolated from ground in the first place. But where is the difference to a 'normal'scope being run from an isolation transformer? In theory one could run any scope from a car battery using an inverter ... Why should one expect any different problems here?

And no, no one was talking about cutting the ground wire and connecting the scope to the grid. THAT would be as stupid as driving a car off a cliff... However, using the right gear it would be rather like putting a car into a plane ...

And regardless of the technique applied it still remains dangerous to measure anything high voltage with a floating scope ...
 

Online Ian.M

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Re: Can I blow the crap out of my scope if I probe this?
« Reply #15 on: April 26, 2017, 09:29:56 pm »
Hmmm, I don't think that it is that clear. Yes, the PM3216 is built to be isolated from ground in the first place. But where is the difference to a 'normal'scope being run from an isolation transformer? In theory one could run any scope from a car battery using an inverter ... Why should one expect any different problems here?
Philips have designed your scope AS A SYSTEM to function correctly and to specification and to be safe floating, within a reasonable voltage of ground (I don't have its specs handy) as that's its normal operating state.   On a scope designed to be grounded, it could be something as simple as barely recessed grubscrews on control knobs that render it unfit to float, or any one of MANY other issues that a competent engineer would have to check for and decide if the issue could be mitigated and how much it would compromise safety and/or performance.
And no, no one was talking about cutting the ground wire and connecting the scope to the grid. THAT would be as stupid as driving a car off a cliff... However, using the right gear it would be rather like putting a car into a plane ...
That's what the O.P apparently wanted to do:
Thanks, is it possible to buy some ac-ac transformer that will isolate my scope from ground? Or can i just cut the ground wire whilst I am probing? Will this eliminere the problem?

This isn't the '70s.  Reasonable bandwidth differential probes are available and no longer need a moon mission budget to purchase them.  If you want to go back to the bad old days of floating test equipment not designed for such use, while you are throwing out what has been learned about test equipment safety in the last forty years, why not throw out such advances in automobile safety as seat belts, air bags and anti-lock brakes?  After all, they also cost more and can occasionally inconvenience the user . . . .  :horse:
 
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Offline vk6zgo

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Re: Can I blow the crap out of my scope if I probe this?
« Reply #16 on: April 27, 2017, 05:34:34 am »
Hello there,

I recall videos where Dave has commented that if you probe stuff that is earth grounded together with your scope, you can get short circuits. However I don't recall what the justification for the statement was.

Basically I want to probe an FPGA demo board, which is powered by a PC USB, and I don't want anything to blow up. Is this safe?

Thanks

This thread has suffered massive "mission creep", so I will initially address the actual question.

If your USB GND connection is returned to the chassis of the PC, & hence Mains Earth, ( as it probably will be on a desktop PC), if you get your probe connections wrong, & put the "earth (ground) clip" on the +5 volt supply, you will short circuit that supply, see a spark, & probably kill your USB port, but it is most unlikely to damage your Oscilloscope.

With a laptop, the 5v supply will normally be "floating" with respect to Earth, so there will be no damage to either the 'scope or the USB port.

That said, some peripherals connected to the laptop may place an Earth connection on the USB port GND pin..

Another possibility is that a desktop PC may have its USB GND connected to chassis, but have no Earth conductor in the power cord.
In this case, it is not unusual for the chassis to be at around half the Mains voltage w.r.t Earth, so connecting the probe correctly with the ground clip to GND may cause a tiny spark.
This will not damage your 'scope, as the current available is tiny.

As is usual with questions re probing circuitry, the thread went off at a bit of a tangent, with "much wailing & gnashing of teeth" about the dangers of probing Mains, etc.

Most Mains Electricity supplies are similar to the Australian MEN system, which, for historical & good Engineering reasons, returns one side (called the Neutral) of the Mains to Earth ( the "Earth" itself, with earth stakes), both at the Power Station, & at various points along the way, including at the entry point to your premises.

The single phase wiring usually used runs a "Protective Earth" (PE)conductor from that point to all the power sockets (GPOs in Oz speak).

If you measure across the Active & Neutral pins of the GPO with a (suitably rated) DMM, you will see
(in Oz),nominally 230v RMS.
Between Active & PE, it will be "near as dammit" the same.

Measuring between Neutral & PE you may see a volt or less.(but not always!)*

The big bugbear with people measuring Mains with an Oscilloscope, is that, if  you get it "wrong", & put the "groundclip on the Active, you will see a blinding flash, & hopefully the Residual Current Device will trip, protecting the circuit.

If you don't have a RCD, it is up to the "old school" fuse in the "meter box" to blow, protecting both you & hopefully your 'scope.

Classic Oscilloscopes had their groundclip connected back to the chassis via the BNC connector, & hence to the earth conductor in the 'scope's power lead.

Unfortunately, some modern 'scopes run the Earth connection as a track on the main PCB.
This may burn out, so that you no longer have the protection which several generations of hardworking EEs built into the power distribution system.

Without that intact track, next time you make this same mistake, there will be no "blinding flash" & everything will work OK, BUT!! the BNC connectors, & everything else normally returned to Earth in the instrument will be at Mains Active.

If you now touch any of that metal, you will get a dangerous shock.
If you have a RCD fitted, it should hopefully not be fatal.
If not, send us your address beforehand so we know where to send the flowers! ;D

The same applies if you deliberately remove the power lead Earth.

*But, Wait!---There's more!.

It would seem that you are OK,if you put the groundclip on the Neutral, BUT!!  sometimes if there is a lot of current being drawn by a device on the same circuit, the Neutral may be elevated enough due to volt drop in the line, that it can still cause a spark, killing your PCB track.

This is why, people such as myself who worked on SMPS used an isolation transformer on the DUT,
( not the 'scope).

OK, you're pretty much convinced about the Mains thing---how about  power supplies inside equipment?
Yep!, some of them can supply enough current to damage the PCB earth track in just the same way.
 
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