my transformer has a secondary winding of 0 - 12v - 24v

Is it just labelled differently and is it really a center tap where 12v would be the center, 0 would be -12v and 24 would be 12v?



  • \$\begingroup\$ It's not an autotransformer is it (basically just a big inductor with multiple output taps)? You can check by measuring resistance to see if the 0V and 0V are electrically connected to each other \$\endgroup\$ – DKNguyen Feb 28 at 21:41
  • \$\begingroup\$ @Toor thanks will check, in the mean time their website in french clearly makes a distinction between a regular transformer and autotransformer and the label says "transformer" so I would guess this is not an autotransformer. Getting the multimeter.. \$\endgroup\$ – Mike Gleason jr Couturier Feb 28 at 21:46
  • \$\begingroup\$ @Toor Can confirm: 0 and 0 are not connected to each other.. 0 and 12 and 24 are on the secondary, obviously 0 and 220 are on the primary side \$\endgroup\$ – Mike Gleason jr Couturier Feb 28 at 21:49
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    \$\begingroup\$ Yeah, so a normal center-tap transformer just labelled specifically for its intended use then. \$\endgroup\$ – DKNguyen Feb 28 at 21:59
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    \$\begingroup\$ You can. Just note that one half-winding will be loaded a more than the other half and will affect both voltage outputs. \$\endgroup\$ – DKNguyen Feb 28 at 22:06

Not necessarily. If the transformer is rated to produce full output from either the 12V tap or the 24V tap, it could be that the winding from the 12 to 24V terminals is of a finer gage, since the current that could be drawn from the 24V tap would be half that you could draw at 12V. This you can determine by comparing the resistance of the 0-12 and 12-24 windings.

Would that matter for using it as a center-tapped +/- 12V transformer? Not much, the terminal voltage at the high side would be fractionally lower under load, but the overall heating compared to drawing the full power at a single tap would actually be less.

  • \$\begingroup\$ I get 1 ohm resistence between 0 and 12, 1 ohm between 12 and 24 and 2 ohms between 0 and 24... So I guess this really is a center tap? \$\endgroup\$ – Mike Gleason jr Couturier Feb 28 at 21:53
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    \$\begingroup\$ I'd think so. While you can get more out of a given core (since you have to get all the windings through a fixed window size) by fine-tuning the wire sizes, there's the additional cost of winding different wire sizes, so keeping the windings the same is likely cheaper. More effective still for a 12V OR 24V secondary is to have two separate winding that can then be connected series or parallel to maximize winding utilization. \$\endgroup\$ – Phil G Feb 28 at 22:19

I would guess that yes, it is just a regular center tap transformer. Assuming that's the case:

The outputs will be AC, so -12V doesn't really have any meaning. The 12V here will be 12V AC, and the 24 will be 24V AC.

If you make the center tap 0, then "LABEL 0" will be 12V, and "LABEL 24" will also be 12V (but they will be out of phase with each other).

Based on the labeling, I think it's fairly certain that this is a regular transformer, but I would do some continuity tests just to make sure.

  • \$\begingroup\$ Thanks for your answer, As I said above in a comment: 0 and 0 are not connected to each other.. 0 and 12 and 24 are connected on the secondary, 0 and 220 are connected on the primary side. (connected I mean near 0 resistance) \$\endgroup\$ – Mike Gleason jr Couturier Feb 28 at 21:51

0V is arbitrary as it is floating secondary centre-tapped coil.

so you can relabel with 3 choices;

0     : 12Vac : 24Vac   
12Vac : 0V    : 12Vac   +180 deg         
24Vac : 12Vac :  0V

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