I am wondering if I can use two normal transformers to form a center-tap transformer..

Here's what it looks like: If I use the right number of turns, would the transformer on the left same as the center-tap transformer on the right? Or how could I make them equivalent?

I am trying to get the left transformer working as the center tap transformer: provide positive and negative voltage at pin 3 and pin 4 with pin 5 connects to ground.


enter image description here

  • \$\begingroup\$ You will need a common core at the very least... \$\endgroup\$ – Eugene Sh. Feb 27 '18 at 22:21
  • \$\begingroup\$ yes, but not exactly the same ... if the two transformers are the same ... it also depends on the application ... note: you could connect the primaries in parallel \$\endgroup\$ – jsotola Feb 27 '18 at 22:26
  • \$\begingroup\$ @EugeneSh. Where should I place a common core? \$\endgroup\$ – Missfresstyle Feb 27 '18 at 22:32
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    \$\begingroup\$ Where the core usually is... inside the coil. \$\endgroup\$ – Eugene Sh. Feb 27 '18 at 22:32
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    \$\begingroup\$ @EugeneSh. afaik that's not really necessary for this case. But I'd parallel the primary windings and not series them as drawn above. \$\endgroup\$ – brhans Feb 27 '18 at 22:42

I think the wording of your question is leading some readers astray.

You can indeed use two identical transformers to give the effect of a single centre-tapped transformer. You would connect them like (assuming 120V/12V transformers):


simulate this circuit – Schematic created using CircuitLab

If the full output is zero rather than 24 volts, reverse the secondary connections on one transformer.

You can also buy transformers with dual primaries and dual secondaries - they can be connected as above, or with the primaries in series for 240V input.

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  • \$\begingroup\$ Thank you Peter. I tried the simulations, and it works! By connecting the two identical transformers in parallel, it works exactly like a center-tap transformer. \$\endgroup\$ – Missfresstyle Mar 1 '18 at 14:26

The problem with two separate transformers with their primary windings in series is considered best under no-load or light-load conditions on the secondaries. What would guarantee that the primary voltage seen on one transformer would be the same as the primary voltage seen on the other?

With light-loading conditions, the dominant current flowing through a transformer’s primary winding is the magnetisation current or, put another way, current due to the primary inductance and its reactance. The magnetisation inductance is not so well defined; one transformer might be 10 henries while another (from the same production line) might be 20 henries. This would lead to big difference in the primary voltages under light load conditions.

For instance, with the inductance difference as above, one transformer primary would receive two thirds of the overall applied voltage. This means that it’s secondary would produce a voltage that is twice the voltage compared to the other transformer’s secondary.

Under full load conditions, the dominant primary-referred impedance is that of the secondary load so, with similar loads, the two secondary voltages would tend to be more equal but, what if the loadings were grossly different? You would have significantly different secondary voltages unfortunately.

Would this happen with a centre tapped secondary? No, because the magnetisation flux in the core is common to both split halves of the secondary and, because of Faraday we know that voltage is proportional to the number of turns hence, either side of the centre tap, the voltages are much more equal even with big loading asymmetries.

Putting the primaries in parallel is a big step to making things more similar.

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  • \$\begingroup\$ Yes Andy! Putting the primaries in parallel is the correct way! Thank for your explanations! \$\endgroup\$ – Missfresstyle Mar 1 '18 at 14:27
  • \$\begingroup\$ @Missfresstyle you have accepted a good answer but you can also upvote answers that please you. \$\endgroup\$ – Andy aka Mar 1 '18 at 14:45

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