HOW TO change 2 x 24VAC trans make into 24-0-24

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Joined 2011
Use each secondary to rectify, filter, and regulate 24V.
Then connect the two supplies in series, similar to the drawing (which is for 15V instead).

The regulator and other details depend on your application.
 

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Moderator
Joined 2011
All of the parts depend on the application requirements.
You have to work backwards from there to find suitable components (including transformers).

Here is one example of many possible bipolar linear power supply circuits, just to show a typical approach.
http://audio.gotroot.ca/minipow/

Here is a bipolar kit for a center tap secondary, but it can easily be adapted to two secondaries.
https://glass-ware.stores.turbify.net/psbiposukit.html
Here is a bipolar kit for two secondaries.
https://glass-ware.stores.turbify.net/dulv.html
 
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Just connect the two secondaries together in the middle - no need for two bridges unless it’s required for other reasons. If you get the phase backwards you just won’t get any voltage. If that happens just flip one side of the secondary.

If you accidentally swap one side of a dual primary that’s when you could run into trouble. And why you fire it up on a variac or dim bulb the first time.

Ive seen EI transformers bifilar wound too, to make the CT. Its a valid way to make any CT transformer, provided the voltage between the two secondaries is below about 300 volts. Above that it can get iffy. If you want equal DC resistance and leakage inductance, it’s the easiest way to do that.