Electronic – How should I probe the primary side of a SMPS


I need to probe the primary side of an SMPS in for repair. This side is not mains isolated, and could be hot/live, at around 330V.

My oscilloscope has only a 250Vpk input rating and can only reliably read up to about 100Vpk, so I would at least need a 1:10 probe. However, I don't feel happy connecting a £6 eBay probe to an expensive scope when magic smoke just loves to leak out. Is there a way to make it safer? I guess I could spend some more money on a high voltage probe, but that would cost me more than a new SMPS.

Also, I heard I should get an isolation transformer for the power supply so that I don't short anything out to earth. Is this a good idea? (The power supply is from a TV and does not use an earth pin.)

Best Answer

You need isolation to measure anything connected to the mains, since parts of your scope will also be. One way is to isolate the power supply with a 1:1 isolation transformer, or you can use an isolation probe (costly!). I've known scopes being fried because they weren't isolated.

To measure the high voltage you can simply make a resistor divider. Don't forget to take the scope's input impedance into account, and make sure your resistors are OK for the high voltage. If necessary use multiple resistors in series.