I'm testing a Switch Mode Power Supply that is not outputting the required voltage. It's only two prong, hot and neutral, and I wanted to get clarification on the best way to approach measuring the hot and cold sides respectively with my oscilloscope. The usage of the circuit is to charge a +12V battery. Just so it's clear, I'm an instrument technician by trade so I'm familiar with the dangers of mains voltage, 120VAC in my case, but I'm a few years out of school so I'm rusty on the usage of my scope.

First question is this: Even with a two pronged mains input, is an isolation transformer still needed?

When probing the hot side of the transformer, if isolated, can I use the common lead on my probe or will I need a differential probe since there's no ground and the reference will be neutral mains or the negative output of the full wave bridge rectifier?

Lastly, in this instance would I be able to use the "Bat-" as my ground reference for the cold side with my common lead on my probe?

Thank you everyone for your time and please let me know if I wasn't clear enough about something.

This is the SMPS that I'm testing at the moment, which is used as a +12V battery charger: -

enter image description here

  • 3
    \$\begingroup\$ This phrase sounds contradictory to me: "When probing the hot side of the transformer, if isolated". I assume by "hot side", you mean the side connected to the mains. If so, how can it be isolated from the mains? Perhaps you mean if you have already used an isolation transformer to isolate the primary? If so, then no diff probe needed. \$\endgroup\$ – DKNguyen Mar 5 at 14:34
  • \$\begingroup\$ The side of the transformer with the mains voltage has always been labeled the hot side on boards that I've seen and in my teachings. Having it isolated would be having it connected to an isolation transformer, like you said. So running it through the isolation transformer would make it safe to use the common lead on the probe because the neutral and earth pins are no longer connected, correct? In this way the common lead is no longer creating a ground loop? \$\endgroup\$ – Basscross64 Mar 5 at 15:09
  • 1
    \$\begingroup\$ Yes. no problem \$\endgroup\$ – DKNguyen Mar 5 at 15:10
  • \$\begingroup\$ Thank you so much. I was thinking that an isolation transformer was only needed if the supply had a grounding pin, which this one doesn't. But from further reading it look like ground and neutral are connected at the panel, requiring an isolation transformer. My mistake. \$\endgroup\$ – Basscross64 Mar 5 at 15:13
  • \$\begingroup\$ Ground pin is for safety and should not normally carry current. It would be connected to something like the case so a loose wire touching the case doesn't make the case live. \$\endgroup\$ – DKNguyen Mar 5 at 15:16

The correct answer is in the comments, but a future reader may find it hard to follow. Just to make it clear:

enter image description here

If you add an isolation transformer: enter image description here

| improve this answer | |
  • \$\begingroup\$ Thank you very much for adding those schematics; I appreciate it. \$\endgroup\$ – Basscross64 Mar 5 at 19:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.