Hello, my question is how to tell whether a transformer includes a centre-tap from a typical product spec given by retailers.

For example I'm looking for a centre-tapped transformer to construct a linear dual voltage power supply with +/- 25 V DC outputs, and I have seen various transformers specced as 2 x 35 V output which might be what I need, but how can I tell if it's centre-tapped?

Thanks for any advice!

 

That would be best.

The 2x35V output sounds like two independent secondaries that can be connected in series or parallel. In series, the connection between the two becomes the center tap of a +/-35V secondary. In parallel, you get one 35V secondary with twice the output current. You want the former.

Note that this assumes independent 2x35v secondaries. Without a datasheet, you'll have to buzz it out.

Finally, note that the secondaries are polarized i.e. must be run in phase for this to work. Usually, there is a dot or some other mark indicating the phasing of the secondary coils. If you get them wrong in your center-tapped scheme, the net voltage on the outer wires will be 0 (the two phases cancel out). If this is the case, reverse the connections on ONE of the secondaries.

 

35 volt for your project look to me a kind of high. After the rectifier the caps will charge up to about 35x1.41 volt max . Minus the rectifier diode voltage drop that is. It may be to much for your regulators

 

Thank you both for your advice JohnInTX and t06afre, it is very much appreciated.

As I'm pretty much a total novice I didn't pick up on the series/parallel connection of the two secondaries but reading your post it now makes sense and looking back at the spec of that 2 x 35 V transformer it does indeed state "can be wired in series or parallel" so thank you for pointing me to wiring the two in series and taking the centre tap from the connection.

Also thanks for the advice on polarity and also voltage level which I now realise I should double-check against the other components ratings.