VA of unknown transformer

I can't find where its in the S.E.R.

You find the voltage drop as you load the outputs individually. The total load of all outputs will cut down on the added individual outputs. Measure the output from no load to drop voltage 10-20%, then measure or compute current.

greg

Given that the transformer is likely to be over 85% efficient, the input power consumption rating printed on the back of the unit will give you a good idea of the VA rating of it

Arfa

Well, there is none !

I expected you'd all take from the fuse rating. Interesting that you didn't... (I made a mistake, BTW : the fuses are on the 16.8 V windings.)

The measure-current-at-10-to-20%-lower-voltage-than-unloaded method seems a little vague to me, not knowing what the V vs. I curve looks like (I know very little about transformers).

Think I might as well increase the load until the transformer becomes "too hot"...

If you do that, it might be the only time that you do, as it may well have a thermal fuse embedded in the primary winding, which will likely fail. Open circuit. Forever ...

What exactly is the issue with knowing the rating of this transformer ? Apparently, it is working, as you are able to measure voltages. Worst case, you could just measure the primary current with it running the tuner. If it's a 'typical' little red transformer that is normally to be found in such items, then just multiply the power consumption calculated from measuring the input current, by about 1.5 to get a ballpark figure for the transformer rating. When I say "ballpark", I actually mean in the same county, but maybe not the same town ...

Arfa

Knowing the total rating gives little help determining individual windings.

Oh. My "too hot" threshold is probably at least 20°C below the manufacturer's, though.

Partly desire to learn what rules of thumb might exist, partly that I'm going to reuse the transformer to power a few dual supply circuits and want to know how much I can count on.

It does have red adhesive tape around the windings.

Thanks.

In theory, that may be largely true, but in practice, it will get you in the area. The winding for the VFD heater can be largely ignored in the calculation, as it will only be good for the few mA that the heater needs. That leaves the two identical 'main' windings, and you wouldn't be too far wrong, given what I said about ballparks, towns and counties, if you just took the figure calculated from the primary current, and split it between the two windings, half each.

Arfa

Increasing load and watching voltage droop is much more effective and safe, but burning it out is another method.

I didn't even read where the transformer came from, but it doesn't matter.