Is it safe to use the lower voltage inputs on a mains transformer?

[gnif]

As the subject states, I am purposing an old transformer for a general linear power supply, car battery charger, high current, etc... Because I plan to charge automotive batteries with it I need at least 14.8V after rectification and regulation, and yes, I want to regulate it properly, it will be used for other general stuff too.

The transformer came from an old junk UPS and as such has some very beefy secondary windings (well, they were the primaries), and multiple taps on the primary for various voltage standards. The secondaries are 7.6-0-7.6 CT, and the available primaries are 200, 235, 270V. In our area the standard is 230V, but I measure 240V here . It also has a 17.5V secondary for the control logic, which is what makes this transformer attractive still.

My question is... would it be safe to power the transformer from the 200V input to give a more usable output?

Unfortunately I have no more specifications on the transformer as it is some OEM specific product. I do know though it is happy to push 8-10A across the entire secondary (15.2V) when on the 235V tap.

With my prototype design so far I am able to achieve regulation at 13.5V @ 2A, I am using a MAX6350 (yes, overkill but I have a bunch), a NE5532 opamp, and a TIP112 darlington to drive 3x 2n3050 transistors. I have the series pass transistors and capacitors on the negative side which solved a few issues I have with my other supply for bench use (output filter capacitors taking too long to discharge), and overall better regulation.

[MosherIV]

The problem with unknown transformers is that you do not know what the current capability is for each secondary winding.

[Siwastaja]

My question is... would it be safe to power the transformer from the 200V input to give a more usable output?

Powering the 200V winding from 240V, i.e., 20% overvoltage,  is probably fine, especially if you derate the power capability a bit. This is only true if the transformer is designed conservatively and is of high quality, so that the iron core doesn't start saturating too much even with this new higher flux density.

[Cliff Matthews]

This was an output transformer and the application is now to use it in reverse? The primary (core winding) would have been the LV high power square-wave input. Use at your own risk and maybe not for constant duty until you heat it up by load testing the beast.. Also the internal insulation and shield may not be up to mains standards, so a fuse and TCO would add safety.

[oldway]

My question is... would it be safe to power the transformer from the 200V input to give a more usable output?

No, it is not, magnetizing current can be too high because saturation and overheat the primary of the transformer.

But it was a transformer from a UPS and you must know with what kind of wave (sine or square) this transformer was designed to work.

You can test the transformer with a variac to measure the magnetising current at different voltages.
When magnetizing current begin to increase a lot, that's the max. voltage.
Nominal primary voltage is 10% lower.

[Kleinstein]

A transformer from an UPS might not have sufficient insulation, especially if the secondary is not connected to PE.

Using the 200 V winding with 230 V can cause saturation. So I would prefer the 230 or even the 270 V winding.

[gnif]

Thanks people, good to know, I will opt to purchasing a transformer instead