Tips for Safely Probing Secondary (DC) Side of SMPS with DMM

[Trurl]

I've "completed" a preliminary schematic of the secondary (low voltage) side of the auxiliary PCB of a HP HSTNS-PD43 server PSU (albeit only the fully accessible side of a double sided PCB - the inner side is almost entirely blocked by the large AC capacitor). I've attached a photo of the auxiliary PCB below for reference. My intention is to probe the section to the right of the opto-couplers (about 2/3 of the PCB).



I'd like to "probe" the secondary side's voltages with a multimeter so I can compare actual voltages with those that show up in the schematic's simulation.

If I only probe the secondary section with the positive probe while the negative is clamped to the main output section's ground, I'm assuming it should be fine, but I wanted to check with those that have more experience, just in case.

What are some safety tips and warnings that should be kept in mind? 

[aliarifat794]

Beware of the capacitors. Capacitors can hold a charge even after the power is turned off. You should discharge capacitors safely before probing the circuit.

[Laval]

I'd like to "probe" the secondary side's DC voltages with a multimeter so I can compare actual voltages with those that show up in the schematic's simulation.

Probing voltages using a multimeter should be relatively safe since it is a high impedance device. You need to take care not to short anything when probing. It is easy on a board like that, which is populated with many small components close to each other, to short something when the probes touch contacts. Use small needle tip probes and some magnifying glass if needed. Of course, keep your hands off the board while probing and your meter close so you don't have to turn around too much to read the value.

[Doctorandus_P]

You can put a plastic container over the mains voltage section, or wrap it in sturdy plastic and fixate this with for example some pieces of tape. This prevents accidental touches of the primary section while poking around on the secondary side.

Do note this is mostly a personal safety thing (it won't kill you anymore), but it's no guarantee for the circuit itself. If for example the feedback circuit Often TL431 with opto coupler) gets disturbed, this may lead to the primary section attempting to deliver full power to the secondary section, which results in over voltages and may blow out IC's and capacitors.

The good old trick of incandescent lightbulb in series with the primary is still a valuable trick (although these things are becoming rare). The lightbulb is a PTC. It allows full voltage to the primary, but if the current in the primary becomes too high the light bulb "trips" and limits the current (and also give visual feedback). Just measure the DC resistance of a cold light bulb, and compare it with the nominal voltage / power of it to see how big this difference is.