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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?

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    \$\begingroup\$ Not without breaking apart the windings and un-centertap one of them. \$\endgroup\$
    – winny
    Feb 25 '18 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 '18 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 '18 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\$ Feb 25 '18 at 21:59
  • \$\begingroup\$ digikey.ca/products/en/transformers/power-transformers/… \$\endgroup\$ Feb 25 '18 at 22:01
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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.

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  • \$\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 '20 at 18:53
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It's easy. Give both a bridge rectifier - they give you a DC output. Connect them in series.

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  • \$\begingroup\$ Draw a schematic and you’ll see this will create a short circuit. \$\endgroup\$
    – winny
    Feb 26 '18 at 11:39

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