# 12V/24V simultaneously from 12-0-12

Discussion in 'The Projects Forum' started by abuhafss, Aug 26, 2013.

1. ### abuhafss Thread Starter Senior Member

Aug 17, 2010
200
4
I am considering to use a center-tapped 12-0-12 (A-C-B) transformer for getting 12V and 24V simultaneously.

I tried to use one bridge across A and C to get 12V, and another across A and B to get 24V. But when the two negatives are connected, both outputs become 24V.

Any help shall be appreciated.

2. ### DickCappels Moderator

Aug 21, 2008
5,070
1,551
I assume that you want positive voltages, but this method will work to get ±12 and ±24 VDC.

You can ground one end of the winding, then use the center tap to drive a rectifier for your 12 volt output, then rectify the end of the winding to get 24 volts.

This only gives you half-wave rectification so you will need a larger capacitor for a given amount of ripple at a given load, and you won't be able to get as much current out of the transformer as with full wave rectification.

http://postimg.org/image/l28psk007/

You probably don't want all the outputs, so just leave off the diode and capacitor for each output you won't want.

Aug 17, 2010
200
4

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4. ### DickCappels Moderator

Aug 21, 2008
5,070
1,551
I am not familiar with this arrangement, but it appears to be a fully wave voltage doubler. It will still subject the transformer to a high peak/average current and it might be better than the half wave rectifiers. In other words, I don't know. How about simulating the two configurations with SPICE and comparing them? That's one thing that SPICE is pretty good at doing.

5. ### #12 Expert

Nov 30, 2010
18,093
9,683
Regarde...

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Last edited: Aug 28, 2013
6. ### abuhafss Thread Starter Senior Member

Aug 17, 2010
200
4
With LTSpice, your configuration gives 12V and 24V whereas, my configuration gives 12V and 33V. With Falstad's java simulator both gives 12V and 24.

#12's last circuit almost similar to mine. seems to be better.

Last edited: Aug 28, 2013

Aug 17, 2010
200
4
Thank you.

8. ### #12 Expert

Nov 30, 2010
18,093
9,683
It's a matter of slowly stepping through the additions to be sure you got the grounds right.

I got some sort of brain failure looking at your circuit so I just backed up to basics.

9. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
198
Hi abuhafuss,

You do realize that if you half-wave rectify 12 vac for example and add a filter capacitor you will get roughly the peak AC voltage out minus the diode drop, (and ripple), correct ?

Last edited: Aug 28, 2013
10. ### abuhafss Thread Starter Senior Member

Aug 17, 2010
200
4
Yes, I am getting 11.45V and 22.90V.