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oscilloscopes: measuring signals safely

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ragnar

Tue Feb 28 2006, 06:07AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
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Posts: 1425

'afternoon all,
after the recent arrival (intact, *phew*) of my oscilloscope (after my first one promptly failed three weeks after I got it following a suspiciously hot day), I've just got a quick question regarding a trick EVR recommended.

IIRC, EVR had some good advice to the effect "connect the ground probes together, add/invert etc, and measure with both probes".

My oscilloscope says "600v pk" above each input. I'm guessing that I can't use this trick to probe halfbridges or anything without an isolation transformer, as I'm still relying on the internal isolation in the scope... AND things can ring to higher voltages etc.

I can, however, use this to look at something which might be floating e.g. 80V above ground? or am I totally confused?

WaveRider

Tue Feb 28 2006, 09:42AM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

For high frequency, high voltage/current stuff (> 100 KHz), I always use a non-contacting "sniffer" probe with my scope. It is essentially 3-4 turns of AWG 14 insulated wire soldered to a female BNC connector...which is then connected to the scope input using a 2m RG58U coaxial cable (usually terminated at the scope end with a 50 ohm load..to prevent coax resonances from disturbing the waveform). The coil diameter is about 12-13mm. This also has the benefit of not significantly loading the circuit under test (gate drive circuits can be sensitive to this). Of course, you will not know the exact magnitude of the current/voltage in the circuit, but it can often be deduced from other measurements (like by using a peak voltage measurement circuit). Better to err on the side of caution to avoid expensive (and possibly fatal) mistakes.

ragnar

Tue Feb 28 2006, 11:53AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

So in short, regardless of whether I connect the groundclips together and whatnot, for nonscalar measurements (e.g. I know there's XYZ voltage swing on my fullbridge, but I'd like to see what the shape of the wave is and if it has ringing") I'll make a sniffer =)

Any last tips/words of advice on groundclip-to-groundclip then measuring from prope-to-probe after add-and-invert? hehe :P

HV Enthusiast

Tue Feb 28 2006, 12:23PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

blackplasma wrote ...

'afternoon all,
after the recent arrival (intact, *phew*) of my oscilloscope (after my first one promptly failed three weeks after I got it following a suspiciously hot day), I've just got a quick question regarding a trick EVR recommended.

IIRC, EVR had some good advice to the effect "connect the ground probes together, add/invert etc, and measure with both probes".

My oscilloscope says "600v pk" above each input. I'm guessing that I can't use this trick to probe halfbridges or anything without an isolation transformer, as I'm still relying on the internal isolation in the scope... AND things can ring to higher voltages etc.

I can, however, use this to look at something which might be floating e.g. 80V above ground? or am I totally confused?

BlackPlasma,

This really isn't a trick. This is a standard industry measurement procedure and the reason your oscilloscope as the ADD, INVERT functions in the first place.

However, even though you are measuring differentially, you STILL must make sure that the voltage inputs at each probe do no exceed the rated voltage for the probes. If you are using 1x probes and 50V is the maximum voltage your oscilloscope can measure at the input, then obviously you can't perform this measurement for 600V signals.

I use either 10X or 100X probes when doing these measurements depending on the voltage level i'm trying to measure.

If your oscilloscope says 600V peak above each input (and to me that sounds high for some reason), then with 10X probes you should be fine. Even 1X probes should work for most bridge work provided you don't push it up too high.

ragnar

Tue Feb 28 2006, 01:23PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Thanks for clearing it up for me EVR,
I'm not quite a complete scope noob, but this is the first time I've had a scope with add/invert functions, which caused me to notice your post =)

I'll buy some 10:1 probes and then perhaps I can scope with confidence. "Trick" was thoroughly the wrong word. I'll read some of those tektronix primers and manuals on the net before asking any more stupid questions.

cheers,
Matt

HV Enthusiast

Tue Feb 28 2006, 02:22PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Well, there is no such thing as a stupid question. I think tektronix might have some app notes on differential measurements as well. Check them out.

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