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Oscilloscope probing Do's and Do Not's ???

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LAN_403

bwsparxz

Mon Jun 25 2012, 12:43AM

Registered Member #1928 Joined: Mon Jan 12 2009, 01:51PM
Location: Reidsville, NC
Posts: 10

I am learning switch mode power supplies for building an induction heater and HV power supplies. I was trying to scope the 24 vdc supply from a Kaiser laser power supply that runs from 300+ vdc. I managed to kill the power supply and thought I killed my TDS340 scope (luckily it just needed a reset). I was probing the half-bridge GDT on the primary and secondary. Guess I did something wrong because I saw sparks on either side. Not wanting to kill my scope , any other circuits , OR MYSELF! So How is scoping switch mode supplies done in various parts of the circuit , for example , when and where to use the ground of the probe . I believe that using the ground improperly is what caused this by creating a short. Please forgive my lack of knowledge here but I have not found any clear information on the subject . Any advice appreciated.

Antonio

Mon Jun 25 2012, 01:32AM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

Switched-mode power supplies usually have the primary part directly connected to the power line through a rectifier. Osilloscopes have the ground line of the probes connected to the ground pin of the power outlet. If you try to connect the ground of the probe to anywhere on the primary side you may cause a short-circuit, destroying components on the power supply and possiby the ground line of the probe too.
A dangerous procedure is to disconnect the ground line of the oscilloscope power supply. Not recommended. A safe procedure is to use two probes, with ground at the secondary side of the power supply, that is (must be, verify) grounded or floating, and subtract the signals, what most oscilloscopes can do. Expect a lot of noise.

Killa-X

Mon Jun 25 2012, 02:46AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
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Plus one to Antonio. If you don't use an isolation transformer and probe a bridge, the ground clip will explode it...which, I've done. Luckily a trace blew, and the scope was fine.

I also, second the use of 2 scope probes... Just keep in mind the ground clip on a scope is well, ground. And can cause issues and shorts.

hboy007

Mon Jun 25 2012, 07:58AM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
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When testing line voltage equipment, an isolation transformer that powers the device is recommended. You then could touch any part of the circuit but installing a probe will again define a ground reference in respect to which other parts of the circuit become live.

ps. sorry, just saw you already mentioned the isolation transformer. However, it's really must-have and you'll have less noise 'surprises', especially when working on a switch-mode circuit.

Steve Conner

Mon Jun 25 2012, 09:15AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Sure, but bear in mind that the isolation transformer doesn't isolate at high frequencies, because of the stray capacitance between windings.

So, for example, this means that you can never clip your scope ground clip to the source of the high-side FET in a halfbridge, as there is a couple hundred volts of RF on it relative to any of the AC line terminals.

Mads Barnkob

Mon Jun 25 2012, 09:52AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Having a semi expensive DSO is not worth much without some good probes. Dig as deep in your pockets as you did for the DSO and get a high voltage differential probe.

Killa-X

Mon Jun 25 2012, 05:29PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

Steve Conner wrote ...

Sure, but bear in mind that the isolation transformer doesn't isolate at high frequencies, because of the stray capacitance between windings.

So, for example, this means that you can never clip your scope ground clip to the source of the high-side FET in a halfbridge, as there is a couple hundred volts of RF on it relative to any of the AC line terminals.

Didn't know that. When I scope my drsstc bridge, the high side, I used the scope ground, and a 3KW isolation transformer. Was doing induction tests to 800A primary...70khz. But I didn't have any problems using isolation transformer and used single probe. Who knows. Just how I always ran things :)

Steve Conner

Mon Jun 25 2012, 08:46PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well, even if nothing blew up, you probably got scope waveforms that looked different to how they really are, due to common-mode pickup.

GeordieBoy

Mon Jun 25 2012, 09:33PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

You might be able to get around this problem by putting a whopping great big common-mode choke on the scope probe cable. i.e. Get a big high-permeability Mn/Zn ferrite core and wind your scope probe cable through it several times.

This common-mode inductance might be enough to discourage those sharp spikes of current at the switching transitions from traveling down the screen of the scope probe cable and messing up measurements.

I've always used proper isolation probes for looking at high-side devices waveforms. This is the proper way to do it. However, I frequently use common-mode chokes on oscilloscope probes when looking at low-side power electronics waveforms or scoping digital circuits when I suspect HF currents flowing on the screen of the probe.

A good test to see if common-mode noise is a problem, is to ground the tip of the scope probe with its ground lead clip and turn up the Y-gain of the scope. You should see nothing except input amplifier noise. Then touch the already grounded scope probe to the 0v rail (ground) of your circuit under test during operation. If you suddenly see little wiggles or HF hash in the waveform you know you have a common-mode current flowing between the circuit under test's ground and the scope ground. You should see nothing because the end of the probe is still firmly grounded by the ground lead clip. A common-mode choke round the scope probe lead helps increase the inductance seen by these currents and discourages them from flowing down the scope probe screen where they add to the signal you're trying to measure!

-Richie,

Steve Conner

Mon Jun 25 2012, 09:38PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I agree, I use these tricks too!

The ferrite core on the scope probe lead often helps.

Another thing to watch out for, is when someone removes the mains ground from the scope, then attaches the scope ground clip to something with lots of HF voltage on it. I'm sure I remember seeing a PhD student do this and blow the capacitors in the scope's mains input filter.

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