Playing around with power transformer output voltage

[Glenn L]

I have a transformer from a junked UPS. There are two primarys and a heavy duty secondary. If I feed 240vac into the main primary, I get 15vac on the secondary, and 24vac on the other primary ( yeah I knows its really another secondary, but its wound next to the primary so I'll call it the 2nd primary, and the secondary is doubleinsulated from these ). I need a few extra volts on the secondary. If I wire the 2nd primary in series with the main primary, BUT reverse phased, I get 18volts from the secondary, perfect. But is this save?
I noticed the primary current was 0.3A just using the main primary, but with the reversed winding in series, current jumps up to 0.6 amps.
There apears to be no heat build up.

Thanks
Glenn

[Guest]

You have two opposing magnetic currents, and may burn out a winding or two. If you just wire them in series the output will also drop as you are lowering the turns ratio.
If you fullwave rectify and filter the 15 vac you will have 15 X 1.414 = 21.21 volts no load, Then use an 18 volt regulator, you will have a fairly constant 18 Volt output.