Oscilloscope question

Hi, Please don't take offense but it concerns me that someone about to measure in the area with a couple hundred volts would ask theses questions. The X10 means the voltage is 10 times higher than the display shows. Over-voltage will cause the display to go off scale. Within the limits of the vertical amplifier the scope will not be damaged. Anywhere you clip the probe alligator clip will now be earth ground. If you are isolating your device then clipping the probe will un-isolate it. I do not recommend you bypass the scopes ground. If anything I said confuses you then don't do it until you understand more. Best, Tom

Greetings Tom, I'm not offended. Basically what I want to know is what is safe for the 'scope. Using the TEK 10x probe lights up the 10x numbers on the selector dial on the 'scope so the numbers do indicate the proper voltage. I know that using an isolation xmfr to power the power supply means that the voltages present in the supply won't have a potential to ground like the power from the mains will. I also understand that connecting the 'scope ground makes the isolation xmfr useless because then the voltages present in the power will have a potential to ground through the 'scope ground. The reason I asked if the 'scope ground should be connected to the power supply is because I don't know if the 'scope will show the proper voltages and wave forms if not grounded to the power supply being tested. And if it does need to be connected to the power supply ground and this is done through yet another isolation xmfr will this change the display enough to make it meaningless? Thanks, Eric

What does the scope manual say? It is on the internet, if you don't have one.

A 10x probe will divide the voltage. If I connect 400 volts, then the scope input sees 40 volts. Measuring up to 1000 Volts should be no problem then.

If you erroneously switch to 1x, you buy a new input amplifier. No problem. Better use fixed 10x probes.

Measurements on HV power supplies are done with two probes, dual channel and switch to A minus B thus displaying the difference between the two channels.

Scope ground should always be connected to protective ground. I did in rare circumstances disconnect the scope ground wire to get rid of line noise, but I do not recomment this to you.

w.

** Means it can display a 400V peak -peak signal. ** Some of the signal will be off the display and you will need to use vertical shift to see it. ** No.

** No.

But any scope has a maximum ( no damage) continuous input voltage which should be in the specifications.

Generally it is about +/-400V or 280VAC and applies to all vertical ranges.

Using a 10:1 probe extends this to the safe limit of the probe - which is in the specs for the probe.

** The WHOLE idea of using an isolation tranny is SO you can ground any part of the circuit you like with a scope probe earth clip.

... Phil

If you're going to be looking at the mains side of the PSU, it would be safer to use a high voltage probe. I have one of these

which works fine at low frequency. There's little reason to hang a scope on the HV side for troubleshooting, and you'll live a lot longer if you don't.

You can get scopes with isolated probe pods, so you can hang the probe ground pretty well anywhere, but you do still need to power down the supply and verify that the primary caps are discharged before doing anything around there.

Also most 10x probes are not rated for very high voltage. Virtually none, and certainly none of the switchable ones, is rated for high voltage / high energy circuits. That matters. Once an arc gets started, stopping it is _not_ easy in a high energy circuit.

I'd use a good quality, Cat III rated multimeter such as a Fluke 87V, and keep my fingers out of the HV section.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

Hello,

You have just meet two of the most famous people on sci.electronics.basics

Phill Hobbs is a PhD with many years experience who often works at the very cutting edge of electronics. His answers are well thought out and helpful.

Phil Allison has autism, which has some surprising strengths when talking about electronics and design. In the areas PA knows he is extremely competent and he will give you the exact correct answer, but he may not take into account your personal limitations. For example PA's advice is absolutely correct and a competent technician would have no problem with it. However you don't seem to have that experience and PH's advice will also get you the answer you want and give you a better chance of being alive to appreciate it.

PA is also extremely volatile when anyone posts anything can be seen as depreciating the value of his advice - as you have just seen.

Welcome to sci.electronics.basics!

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