I have a cheap little PoScope USB oscilloscope. It is limited to +/-20 V RMS. If you try to test a circuit with over 30V DC, it will freeze up, but not smoke. Luckily, I haven't broken it yet!

I need to test a three phase DC motor that uses 120V AC. My scope can't handle that, so I'm wondering how I can make a high voltage probe with some sore of voltage converter circuit to work so I can just use simple math, such as a multiple of 5x to the voltage read off the scope to get the actual line voltage. I've looked, but am not finding anything on this.

Thanks for any suggestions to help, and especially those that don't just suggest getting another scope...

 

Hello,

You could try to get hold of 10:1 probes.
That way you can measure upto ±200 Volts with your scope.

Here is an example of such probe at Digi-Key:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=290-1001-ND

There will be more around.

Bertus

 

Thanks for the tip. If I'm using a probe with 10:1 attenuation, do I multiply the measured voltage on the scope by 10x? My little scope doesn't allow you to specify the probe type.

 

Thanks for the tip. If I'm using a probe with 10:1 attenuation, do I multiply the measured voltage on the scope by 10x? My little scope doesn't allow you to specify the probe type.

Yes, but to be sure that the attenuation is correct, you will need to check that the input impedance of your scope is suited to the probe.

The most common sort of probe is designed for an input resistance of 1MΩ, in parallel with perhaps 10pF to 20pF capacitance - typically the probe has a pre-set adjustment to accommodate a moderate range of capacitance.