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Differential o-scope

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dpcook

Tue Feb 08 2011, 01:57AM

Registered Member #3233 Joined: Thu Sept 23 2010, 06:50PM
Location: Berkeley, California
Posts: 8

I've been working on an induction forge project for the smithy at a place here in the Bay Area called the Crucible. Very new at this whole thing so the learning curve has been quite steep. We basically started with a design that I found on the internet that Jonathan Kraidin had put up.

Now that we are seriously into the testing stage of things, I need to be able to do differential measurements on the MOSFET gate signals, amongst other things I'm sure. I was quite successfully at burning out several ICs and jumper wires before I learned that ground is not always ground...

Patrick

Tue Feb 08 2011, 03:00AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

You'll find it easier to just scrounge on Ebay for one that will fit your needs, theres no use giving you a list of probes types that you can't buy, afford or find.

Weston

Tue Feb 08 2011, 05:34AM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

There is a way to make differential measurements with two normal probes, using the add function while inverting one channel. This uses up two scope channels though.

If you are looking for external differential probes, they run a hefty $400 or more on ebay and you don't have much of a choice about which model. However, Elektor magazine 2010 September issue covers the design and build of a differential probe amplifier. the design works up to about 1000 volts and about 1Mhz if I recall. Not to shabby and very cheep to make. There is even PCB artwork available. I think it would be the cheapest solution.

Good luck with your induction heater. Sounds like a interesting project.

Steve Conner

Tue Feb 08 2011, 01:54PM

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

Here is a good thread on induction heaters from the archives.

Kim Ladha's design posted in that thread is the best "bang for your buck" I've seen, but there's a more complicated one by Uzzors with auto-tuning and so on.

I think Jonathan Kraidin is our "iamsmooth", and we helped him out a lot with his 10kW design.

If you're building a forge, you need IGBT bricks, not measly MOSFETs.

dpcook

Tue Feb 08 2011, 03:39PM

Registered Member #3233 Joined: Thu Sept 23 2010, 06:50PM
Location: Berkeley, California
Posts: 8

Weston wrote ...

There is a way to make differential measurements with two normal probes, using the add function while inverting one channel. This uses up two scope channels though.

How exactly do you hook them up? Do the two ground clips on the probes go to ground or do they get clipped together?

Crunchy Frog

Tue Feb 08 2011, 08:46PM

Registered Member #2422 Joined: Tue Oct 06 2009, 02:41AM
Location:
Posts: 85

dpcook wrote ...

Weston wrote ...

There is a way to make differential measurements with two normal probes, using the add function while inverting one channel. This uses up two scope channels though.

How exactly do you hook them up? Do the two ground clips on the probes go to ground or do they get clipped together?

To ground. Technically it doesn't matter, but you'll get less interference that way.

doctor electrons

Tue Feb 08 2011, 11:31PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Using the add and invert on one channel is a good way to take a differential measurement. You can remove
the ground clips from both probes being as your test point is not referenced to ground. When you do this, think
of it as using your scope as a multimeter, one probe being the red and the other the black! Doing this is safer in
certain circumstances than the conventional way of using probes to take readings. Hope this helps a little bit, if not,
go to the tektronix website! There you will find a pdf guide to taking differential measurements with any probe and scope
combo.

radiotech

Wed Feb 09 2011, 12:09AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

I"m looking for information on differential probes. I have a couple of Tektronix o-scopes that I'm using. One is a relatively new TDS 1001B and the other is an older Tektronix whose number I don't recall at this point in time and I realize that I accidentally erased the user manual that I was able to scrounge.

What is a differential probe? In industrial work with chart
recorders, and TEK Scopes, the term diffenential just referred
to the mode of operation of the 2 channel operation settings of the scope, and with the chart recorders, a high voltage, 2 terminal, isolated amplifier was used.

Here's a P6009 TEK x100, rated 2000 volts peak, and 2 of
them are needed for differential measurements.

Steve Conner

Wed Feb 09 2011, 11:03AM

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

I don't really trust the dual probe method. The two probes and the two scope input amplifiers will always have slightly different HF responses, and the result is that the leading edges of common-mode pulses aren't rejected.

You can maximise your chances by using the same make and model of probe for both, and connecting the two probes to the scope's calibration output, and adjusting the trimming screw on one probe to minimise the pulses you see. You can also twiddle the variable gain on one channel.

But be aware that if you switch to a different volts per division, you may have to redo the above.

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