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Very Newbie Oscilloscope Questions.

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Gabriel35

Wed Aug 29 2012, 01:10PM

Registered Member #2310 Joined: Wed Aug 19 2009, 08:04PM
Location: Santa Catarina - Brazil
Posts: 169

Hi folks!

So, finally I got a Scope here!, For measuring that misterious waveforms that always can be a problem on some circuits.

So I have some basic and easy questions to do.

I heard that in some circuits, like Induction heater or Tesla coil oscillators, there are some measurings that are impossible to be done without using a differential probe or something like that, because if you do that with a passive probe, depending on the points that you are measuring you can lead to a short circuit. Is that true?

Do you have some examples of situations that it can happen?

What are the possible measurings to do on a Induction heater oscillator for an example? Gate drive signals and H-Bridge output waveform?

Sorry for the newbie questions but I need to know what I'm doing, to don't throw my money away, damaging my scope for some reason...
And 4hv was always a place for learning a lot, since I started dealing with electronics!

Thank you so much!

Pinky's Brain

Wed Aug 29 2012, 02:35PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

A true short circuit is not a problem unless you do something stupid like not using an isolated power supply for your circuit under test and then using your scope ground clip at a point which is not ground.

The problem is that some signals are not ground referenced at all, like the gate signal of upper device in a half bridge ... now you might think "oh I'll just use an isolation transformer for either the circuit or the oscilloscope and then I can attach the ground clip wherever I want" but it's not generally a good idea (especially not the latter, since you don't generally expect the case of your scope to be live).

The ground/power planes of the circuit effectively have a large capacitance to true ground, both from the isolation transformer and simply because they are comparatively large chunks of metal ... same is true for the scope case. So if you attach the ground clip to the middle of a half bridge you're effectively adding that capacitance to the bridge output.

Here is a recent thread about the same topic :

Gabriel35

Wed Aug 29 2012, 03:02PM

Registered Member #2310 Joined: Wed Aug 19 2009, 08:04PM
Location: Santa Catarina - Brazil
Posts: 169

Thank you!, Very informative Link! Seems to be the same doubt that I had.

HV Enthusiast

Wed Aug 29 2012, 06:37PM

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

You're not going to be able to measure the output of your half-bridge or full-bridge with a single probe with your oscilloscope. You will need to use two probes in a differential measurement configuration.

Basically connect one probe to OUTPUT+, the other to OUTPUT- (ground leads left hanging or connected together - but NOT to circuit).

Then set Channel 1 as NORM, Channel 2 as INVERTED and then ADD the two channels. This will give you a differential measurement.

Make sure the probes can handle the voltage you are measuring though.

This same technique will need to be used for gate drive in the bridge as well if its under power.

Sigurthr

Fri Aug 31 2012, 01:12PM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Thanks for posting that, EVR! I was wondering the procedure for "poor man's differential probe setup". Too bad the OS-7040 (which I LOVE) doesn't have the channel inversion or mixing/adding features.

I've been wanting to scope my SSTC's half bridge but I'll have to wait until I can pick up a differential probe. Heck, I still need to scope the gates with the TC's DC buss unpowered though, but neither the TCs nor the scope is really "portable" and they're about 20ft apart, haha.

Gabriel35

Fri Aug 31 2012, 03:05PM

Registered Member #2310 Joined: Wed Aug 19 2009, 08:04PM
Location: Santa Catarina - Brazil
Posts: 169

Very good answers folks! Thanks, that helped me a lot!

What are other important waveform measurings that can be made on a H-Bridge Induction Heater?
Are there measurings on this kind of circuit that can be done with normal 10x passive probes?

What happens if I leave my scope with the ground unplugged, and try to measure, for an example, the output of a h-bridge, simply conecting the probe to OUTPUT+, and the GND clip to the OUTPUT-?
Would it result in and explosion or it will just give me the wrong waveform?

I just need to make some measurings to the circuit searching for parasitic oscilations, or ringing, not for exactly measuring gate drive or bridge output voltages and current, so what do you think?

Steve Conner

Fri Aug 31 2012, 04:07PM

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

If the scope is mains powered, this is not recommended. Even if you disconnect the ground wire, you still have to leave the live and neutral connected to power the scope. These are often connected to the scope's ground by EMI filter capacitors, and on the mains side, the neutral at least is grounded. So there is a path for high frequency current to flow. Best case you will get garbage waveforms, with lots of ringing that isn't actually present on the signal. Worst case, the EMI filter caps in the scope could explode, or the power supply could burn out.

An old-fashioned scope may not have an EMI filter, but the transformer can have considerable inter-winding capacitance, causing the same problem.

Long story short, floating scopes is not recommended unless you really know what you are doing.

Gabriel35

Fri Aug 31 2012, 08:35PM

Registered Member #2310 Joined: Wed Aug 19 2009, 08:04PM
Location: Santa Catarina - Brazil
Posts: 169

Oh, I understood Mr. Steve, Thanks!

So, what are other important waveform measurings that can be made on a H-Bridge Induction Heater? Other than Gate-Source, and H-Bridge Output waves?

Are there measurings on this kind of circuit that can be done with normal 10x passive probes?

If don't, so I'll be very bored, coz I got my money away on a Scope and there's only a few things that I can measure with it. =//

If I want to measure my Power Supply, seraching for ringing on the Waveform (1 big transformer - 50v, rectified and with some filter caps)

Whats the procedure for doing that? are there any risks of shorting everything too?

Concluding, the best bet is to get a scope with 4 channels and using them in pairs for measuring diferential, or getting a differential probe that cost 3x the price of the scope? Damn...

Gabriel35

Tue Sept 04 2012, 02:06AM

Registered Member #2310 Joined: Wed Aug 19 2009, 08:04PM
Location: Santa Catarina - Brazil
Posts: 169

Someone?

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