Electronics > Beginners

Using Ch1 + (inv)Ch2 on hot side vs unhooking mains ground

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lordvader88:
I want to probe the primary side of computer SMPS's, my DSO138 is too slow for some stuff.

I don't have a working isolation transformer yet, and I just found out that u can use JUST the tips, of Ch1 + (inv)Ch2 as a diff. probe ? I have a Tek2340A

Is that considered fine ? Does it change the wave form much compared to the proper way (I mean 1 Ch w/ isolation transformer). Is there anything/anywhere I should NOT probe using that method ?




And if it is just for a quick check, what about just unhooking the Tek's main's GND from the plug/wall. I have cords without a GND prong. I don't want to run it like that all the time, just for a few minutes. And just be aware that the chasis could be hot if something fails at that time.

james_s:
No, do not do this, I don't know what the max common mode voltage is for that scope but it's really not safe. If you don't have an isolation transformer then just keep out of the mains side of the power supply. I have only very rarely had any need to look at waveforms there anyway.

lordvader88:
I will be getting a proper iso.trans. soon. If I had diff. probes, is there anything on the hot side they can't do ? I'm guessing they are a lot cheaper than an iso.trans. for +150 where I am

james_s:
You should be able to get an isolation transformer for little or nothing. You can make one yourself out of any two suitably large transformers connected back to back.

GregDunn:
If the scope has a math function, subtract the two channels with that.  You won't be exceeding any common mode specs that way.

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