Connecting the dual secondaries of a mains transformer to increase the current avauilable?

Thread Starter


Joined Feb 26, 2017
Hi All,

I have just ordered a very small mains (230V) transformer that provides 2 secondary outputs on 4 connectors. When looking at the outputs on a scope they identical and in phase.

Is it OK to connect the secondaries together in parallel fashion to increase the current capability?

Many thanks.


Joined Mar 14, 2008
As long as their voltages are nearly identical.
Any voltage difference will cause a circulating current in the transformer equal to the voltage difference divided by the sum of the two winding resistances.
The windings should be indentical.

To phase the transformer in abcence of any markings like dots, connect the secondares in series until the output is 2*V. The other answer will be nearly zero. No damage will happen with this connection.

Once in series. you now know the phasing.
0      V 0           V
(What a pain to do that ASCIIART)
Parallel: connect the 0 points together and the V points together.

Getting the phase of the primary relative to the secondary would be harder.

Thread Starter


Joined Feb 26, 2017
That's good news, many thanks.

....Which leads me to another question:
If each secondary has, for instance, a 500mA rating what would be the calculation for the parallel of the two?

BTW: the secondary OP's are 18.02V & 18.02V
Last edited:


Joined Jul 18, 2013
Many secondaries of the same voltage are wound Bi-filar, which generally means they are very close in value.
1amp in parallel.