I bought 2 center tapped transformers because I need to make 30V 2A center tapped transformer equivalent and I couldn't find anything with a 120V input.

So I got a 18V 2A center tapped transformer and a 12V 2A center tapped transformer.

How can I connect the secondary together so I get a 30V 2A center tapped transformer equivalent?

  • 1
    \$\begingroup\$ Not without breaking apart the windings and un-centertap one of them. \$\endgroup\$
    – winny
    Feb 25, 2018 at 21:47
  • \$\begingroup\$ Would two 15v transformers have given you 30v with a centre tap - why pick two odd ones?? \$\endgroup\$
    – Solar Mike
    Feb 25, 2018 at 21:50
  • \$\begingroup\$ Remember to derate VA rating 30% if you plan on a bridge cap load so you may want 90VA or consider a COTS SMPS, when you buy your next power source. \$\endgroup\$ Feb 25, 2018 at 21:58
  • \$\begingroup\$ You COULD do it if either of those transformers had a split secondary (2 isolated 0-12V windings) instead of centre tapped. \$\endgroup\$
    – user16324
    Feb 25, 2018 at 21:59
  • \$\begingroup\$ digikey.ca/products/en/transformers/power-transformers/… \$\endgroup\$ Feb 25, 2018 at 22:01

2 Answers 2


Basically you can't. If you connect the secondaries of your transformers in series, you will get 30V across them but the common connection will not be a center tap since one side will be at 18V and the other at 12V. If you got 2 of the 18V transformers, you would get a center tapped 36V transformer by connecting the secondaries in series. Two of the 12v transformers connected in series will give you a 24V center tapped transformer. You really need 2 15V transformers and they don't have to be center tapped. When you connect them in series, the common point becomes the center tap of the 30V transformer.

  • \$\begingroup\$ Related to the question, in my case i have 2 SMPS with 24V 5AMP , what wire connections are needed in order to get 24-0-24 5AMP at output ? Do they have to be in parallel ? if so then what will be the center tap point ? \$\endgroup\$
    – Vikrant
    Mar 18, 2020 at 18:53

It's easy. Give both a bridge rectifier - they give you a DC output. Connect them in series.

  • \$\begingroup\$ Draw a schematic and you’ll see this will create a short circuit. \$\endgroup\$
    – winny
    Feb 26, 2018 at 11:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.