I have some old school lab power supplies that have a ferro-resonant isolation transformer (FRIT). The other day I was running it and it just quit. When I checked the output it was 25VAC. Historically, with 116VAC input I'd see about 128VAC output from this 1:1 transformer. Not certain the transformer failed I started checking everything else; switches, wires and a circuit breaker. All appear to be in good order. This afternoon I pulled the FRIT and directly wired it to 116VAC. The final output is 25VAC. Since I believe transformers don't often fail I was wondering could it be the capacitor that might have failed? I'm pulling the cap now to see what happens.

Here's a schematic of the cap. Not yet sure about the cap value - haven't pulled it yet.

 

Cap is 2µF @ 660VAC. My Extech meter has a capacitance setting. I've removed one wire and am waiting for the cap to charge up from the meter. With one wire removed the cap is reading 2.6nF. Have no confidence in my meter when it comes to caps, but 2 nano's is a far cry from 2 micro's.

After being connected for a while, the meter now reads 83nF. Getting there, but don't know how long it will take. OK, now the reading is bouncing all over the place.

I guess the bottom line is: what can I expect the FRIT to do with a bad cap?

 

With the cap out of circuit, I get 77VAC on the secondary. Other than the diagram I have (and gave you) I don't understand how it's supposed to work. Is this, I'll call it the resonant coil, suppose to be in opposition to the secondary coil?

[edit] cap back in and I get 25VAC again. [end edit]

 

With the wrong value of capacitance the resonant frequency doesn't match the line frequency and it stops working.
The LC tank circuit resonates at line frequency, the resonance would like the voltage to keep on increasing for ever, but the voltage reaches a point where the magnetics saturates and that limits the voltage.
From experience, they run for about 10 years, then the capacitors start to fail. Replacing the capacitors brings them back to life. Very occasionally a winding burns out. We used them for emergency lighting - 108V DC square-wave in: 230V sinewave out, up to about 30kVA.

 

Thanks Ian0 these ARE old units. From a local high school who wanted to clean house. I got my hands on them from a teacher/friend who asked if I'd be interested in some equipment. Also got a couple O-Scopes.

I'm going to go grab one of the other units and pull the cap and swap it. Though I have other units, they're all in rough shape from students messing with them. The wiring in them is atrocious. This particular unit - I've rewired it CORRECTLY and it has worked great for a few years. All of a sudden I need a variable AC source and yesterday morning it had failed.

I'll swap the caps and see what happens. Thanks again.

 

Ours have several 50uF polypropylene "motor-run" type capacitors in parallel, up to about 400uF @ 450V AC on the larger ones. We look for duff ones with a current clamp meter to find one that is taking no current. Replace it, and generally it comes back to life.
We stopped using them because LED lighting with regulated outputs draws more current as the voltage reduces (to keep the power constant), that's negative resistance. Negative resistance plus tank circuit equals oscillator = flickering LEDs.

 

Well, thanks Ian0 changing the cap has brought it back to life. It now has an output of 127.7VAC. That with 113.1VAC input. Nice how these things regulate the output. When the input drops the output remains fairly constant.

PROBLEM SOLVED!

[edit] the top of the cap appears to be slightly domed. [end edit]

 

The offender: