Electronics > Beginners

What should isolation transformer read like on DMM

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lordvader88:
With the mains line wire connected to the COM probe of a DMM, I checked the output of a 120VAC:120VAC line isolation transformer


what do u think the voltages are when the red probe is connected to the 3 outputs ?


so 120VAC mains line VS each of the 3 secondary wires (its a 3 prong plug)

helius:
Voltage measurements between isolated nodes can be anything at all. This is not the correct way to perform an insulation test.

Brumby:
You are going to have some capacitive coupling - so with a high impedance DMM, you are likely to get some worrying numbers - but they aren't going to be very useful.

ArthurDent:
As others have stated, what you are doing will give meaningless results. If you want to check if the output of an isolation transformer is isolated (it will be) from the input, unplug it and use an ohmmeter. The only continuity between primary and secondary connectors will be on the ground (earth) lead which is carried through for safety reasons.

In your referring to the "three outputs", there is only one output to a 3-prong connector.

lordvader88:
Well they had the chassis mains earth grounded, and they also attached a ground prong on the output plug to the chassis. It's not connected to the neutral-output tho.

So the voltages I measured were
line to sec. neutral ~80V
" to sec. line ~178V
" to sec. gnd ~120V


Should the output gnd be connected to anything ?

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