How to test if mains PSU has isolated output?

Hello,
Is the way to do this to use Ohmmeter from mains neutral to output ground and see if its connected? (then do the same for mains earth wire to output ground and see if that's connected)?

Be very cautious about using an Ohmmeter on power lines. First set the meter up as a voltmeter and measure the voltage between the two points you plan to measure for resistance. If there is a voltage already there, don't try to measure resistance. The voltage present will blow up your Ohmmeter, or worse. Only after you have verified that the voltage is essentially zero between two points can you try to measure resistance. Then you can use the Ohmmeter to tell if the PSU has isolated outputs. Also, make sure nothing is connected to the output of the PSU which could provide an alternate path for the Ohmmeter to detect a connection. A truly isolated output can be forced to have any reasonable offset voltage when compared to mains ground. If you connect a small 1.5 volt battery between the PSU neutral and mains ground, the voltage across the battery will happily stay at 1.5 volts. If the PSU outputs are not isolated, the 1.5 volt battery will be shorted to zero volts. Don't do this for very long, or the battery could get very hot.

Some "isolated" SMPSs, have an output circuit which is voltaically isolated (no DC path), but have a low value capacitor connected from the mains neutral to one of the output terminals. This means that noise on the neutral could be injected into the DC output line.
Frank

have a low value capacitor connected from the mains neutral to one of the output terminals.

Click to expand...

..Thanks, is it possible to get a minor electric shock via that capacitor?
-And is that why these capacitors sometimes have 1 megohm resistors across them?

I would say first check if there is a schematic available and look for obvious connections, then in this order:

1. if there is an Earth connection on the power plug, check for continuity to the output wires. If there is continuity the PSU will be isolated from the line and neutral but obviously not Earth.
2. do the same for both line and neutral to the outputs, check for a resistance of at least 470K Ohms. If it's 470K or higher it probably is isolated.
3. if 1 and 2 say it's isolated, use a small 9V battery and 1K resistor in series, connect one side to line and then neutral and see if current flows from the other side to the output wires. Don't use 1.5V in case a bridge rectifier is in the input circuit, it may not be enough voltage to make it conduct.

It is quite normal to see a high value resistor and parallel capacitor between the neutral input and output sides of a PSU. They are there to leak away any voltage capacitively coupled to the output through the transformer. Without them, an otherwise isolated output could 'float' at any potential present in the input side of the transformer, often this is >300V so could momentarily cause damage if it discharges into a sensitive load. The resistor value is high enough to prevent a dangerous current flowing to the output but low enough to discharge the weak coupling between transformer windings. This is similar to the way an anti-static wrist strap works. The capacitor is also a low enough value (typically 470pF) that AC currents are kept at safe level but it keeps induced noise 'discharged' to a known point. Note that these components have to be rated for high voltage, often 1KV or more and are omitted altogether in power supplies used in medical applications where only extremely low leakage is allowed.

Brian.

When I worked at Panasonic, on the TV psu's, people, for a joke, would snip the 1 meg resistors off, and then you'd get a shock when you touched the circuitry even tho' it was supplied thru an isolation transformer)....we wanted to touch it to see roughly how hot it was getting etc