View Full Version : USB oscilliscope isolation

11-29-2006, 12:47 AM
Fellow Stampers..
I recently tried to use the parallax digital USB oscilliscope to measure AC voltage pattern of a household drill. The drill is a universal motor with stator and rotor in series along with some undetermined switch device (probably phase ctrl). I first ran wires out to a bread board and then measured the voltages with a digital volt meter and got number ranging for 10 to 75 volts AC rms I then created a resistor bridge circuit to drop the voltage down below 5 volts AC rms such that the voltage would not be too high for input to the scope I then connected the scope to observe the wave form.... zap, I got a short thru the scope, thru the usbcable, thru the pc power supply... I guess to household ground The scope probes were well welded to the wires.. but I had not intended to make a welding machine!

I would like to be able to measure these voltage without destroying another scope. Any ideas how?

Thanks, John

11-29-2006, 02:17 AM

You've definitely learned a valuable lesson, without loss of life!

This happens with many scopes since not all have isolated power supplies or measuring circuits.

To be clear, your measurement CAN be done safely, but it requires that you know the AC neutral connection of all devices involved is the same.

It was quite common industry, before the days of battery-operated scopes, to always have the scope fed from an isolating transformer. As well, no one would ever touch the metal casing of the scope while connected to the circuit under test.

The safest arrangement for the hobbiest using computer-based instruments, is to use a laptop ( preferably with a completely plastic case) and operate it on battery-only power during measurements of line-operated equipment.

One final note is that the policy for people working for me was to ALWAYS use fused probes on test equipment. In your case, the only damage would have been to change the fuse in the test lead........not the smoke and fireworks you observed!

Don't buy a lottery ticket today - you already used your good-luck for today!


Tom Sisk


Martin Hebel
11-29-2006, 02:22 AM
Also, the scope is only rated for 25V I think. Use a 10Meg resistor in series with the probe to make it a x10 probe.


Martin Hebel
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12-01-2006, 11:23 PM
I had a similar experiance while working on an old tube radio with a standard·oscilloscope. I attributed the problem to having the a/c polarity reversed on the radio (Hot connected to the metal chassis).·You·won't notice a difference in the operation of the drill, but if you ground the hot connection it creates a very dangerous situation.http://forums.parallax.com/images/smilies/smhair.gif

12-01-2006, 11:59 PM
Could you AC couple the scope to the drill by putting a capacitor in series with the scope lead? Wouldn't his effectively isolate the scope from the drill?

12-02-2006, 03:30 AM

That would not help if the a/c polarity was incorrect because the capacitor would be an a/c short to ground.

12-02-2006, 11:29 AM
Thanks to all for your suggestions.. I was thinking that perhaps I could use an an analog optical isolator.. the input voltage and ground level wouldn't matter and its power could be connected to the PDB board ground. I've seen 2 chips that might give direct or proportional representation of the input signal.. ps8601, iso124 5-$15