If I have a transformer with X number of taps on the primary side, and I have a fixed input voltage that I'd like to change the output from.

In its lowest state the two outer taps are active and the coil is whole, this is a lowest voltage output in secondary. By switching to the next tap from one end the coil is effectively shortened and the output voltage is higher. This goes on for however many taps there are.

Obviously this can be done with an array of relays, or alternatively a different set of drive transistors on each tap.

What is the best (cheapest/simplest) way I can achieve this? All ideas welcome.

Specs: ~100-200V ~20A

  • 4
    \$\begingroup\$ As with all questions of this type, you need to define exactly what "best" means in your application. You need to prioritize constraints such as cost, space, complexity, time -- both in terms of the engineering (non-recurring) and the product (recurring). By far, the simplest manually-operated solution is a 1-pole N-way rotary switch. Be sure that it's break-before-make. \$\endgroup\$
    – Dave Tweed
    Feb 16, 2017 at 13:17
  • \$\begingroup\$ if possible add a matching image of the transformer and what you need to achieve \$\endgroup\$
    – Raj
    Feb 16, 2017 at 13:29
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    \$\begingroup\$ homemade-circuits.com/2011/12/… \$\endgroup\$ Feb 16, 2017 at 13:44
  • \$\begingroup\$ Simplest way is to use off-load tap selection. Determine the optimum for conditions and fix it. You can also use break-before-make switches. Alternatives used by automatic tap changers to in large transformers are transition switches connected with resistors or inductors to prevent open circuits. Care must be used to prevent overheating of the derated transition components during power system faults. \$\endgroup\$
    – skvery
    Feb 16, 2017 at 15:25
  • \$\begingroup\$ @MarkoBuršič Hee hee, that is not going to cheap or simple but it could be very convenient. Check out some of the handy tap selector switches at the link below. - schurterinc.com/en/Components/Catalog?(PG04)/1 \$\endgroup\$
    – KalleMP
    Feb 16, 2017 at 19:36

1 Answer 1


You can use a solid state relay with zero-cross circuit and a multiplexer to enable each SSR of one single tap. If you turn off the drive signal of the conducting SSR in the middle of AC cycle, the SSR will continue to conduct until the end of AC cycle, meanwhile when turn off the signal of 1st SSR you turn on the 2nd SSR. But the 2nd SSR will not turn on until zero-cross - at begining of next AC cycle. Such way there is almost impossible that two SSR will conduct at the same time.

  • \$\begingroup\$ Isn't it preferable to switch AC power to an inductive load at the maximum voltage? To Zero Cross or Not To Zero Cross - John Dunn, Consultant, Ambertec, P.E., P.C. \$\endgroup\$ Feb 16, 2017 at 19:02
  • \$\begingroup\$ ... also "A zero-crossover solid-state relay may be the worst possible method of switching on a transformer ..": Beware of Zero-Crossover Switching of Transformers \$\endgroup\$ Feb 16, 2017 at 19:08
  • \$\begingroup\$ Those who upvoted may not be aware of the design flaws of a simple switch and ZCS SSR is definitely worse. \$\endgroup\$ Feb 16, 2017 at 19:32
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    \$\begingroup\$ @MarkoBuršič The OP has "a transformer with X number of taps on the primary side" which they want to switch between. \$\endgroup\$ Feb 17, 2017 at 10:01
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    \$\begingroup\$ @AndrewMorton I didn't notice that. The primary is supposed to be connected on a tap with respect to the line voltage. Tap switching on primary is not a proper way for selecting the secondary voltage. \$\endgroup\$ Feb 17, 2017 at 10:30

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