How to connect scope & DUT to not blow up

Posted by: travelerdbg on 01/06/2012

Forgive me, but I do not have much experience with off-line SMPS measurements.

I have a LNK500 circuit that I'm trying to diagnose and have a question about connecting my scope.

The scope is NOT floating (and I know it's not safe to float it) and line voltage goes to my PCB through a variac. The variac has a 3-prong plug but earth is not connected to my PCB. I grounded my probe to my PCB ground (bridge minus terminal). I wanted to view my switching transformer primary (LNK500 source), but ended up smoking my HOT input trace.

Would using an isolation transformer before my variac solve my issue? Also, can I safely view my switching transformer secondary without an isolation transformer and can I ground my probe to PCB GND to view the secondary?

Thanks,
David

Comments

I use a 500VA isolation transformer before the variac, so I can ground the scope with no problems. Once, I forgot to connect my circuit on the isolated voltage. What happened was a boom, and some PCB tracks melted.

One thing to note is that if you are using more than one channel, all probes must me grounded on the same point of the circuit. The ground of the probes are interconnected inside the scope, so if you put them on different locations of the circuit, these location will be short-circuited.

Thanks a lot for the response. That is what I suspected but wanted confirmation.

A building's Neutral has a low impedance connection to Earth Ground. So if you don't have an isolation transformer, the scope ground will short whatever you connect it to, to Earth and thus Neutral.

You must use an isolation transformer when probing in the primary side. This is also important for your personal safety.

Additionally, even with an isolation transformer, you should not clip your probe/scope ground to a node that has significant dv/dt or switching voltages. Even with an isolation transformer in front of your PSU, because the scope ground clip has significant capacitance to Earth, you will load the node you clip to, with this large capacitance. Clip it to a "quiet" node.

For example, if you want to examine the voltage waveform on an output diode whose anode is connected to the secondary winding, and whose cathode is connected to the output voltage place your ground clip on the cathode and not the anode. Of course you cannot clip a 2nd probe ground to anywhere else in the secondary. The scope will show the anode-to-cathode voltage. If you clip ground to the anode, the capacitive load will distort your reading.