Application: A machine that seals boxes with plastic wrapping. The box is sealed by applying a heating element to the plastic for a few seconds.

I have a 800VA (220VAC to 54VAC) transformer connected directly to a heating element.

My requirement is to make the transformer work with the heating element so that I can adjust the temperature, and also not break the transformer.

Initially I wanted to switch the input voltage by controlling a zero-cross solid state relay (Celduc SO842074) with a PWM signal (Arduino) on the primary side. I then learned that this may cause a DC-component or voltage spikes which could break the transformer.

My current idea is to place the zero-cross solid state relay on the secondary side of the transformer (untested). I'm uncertain as to why switching the primary side would cause a DC component but doing the same on the secondary side would not?

Will this secondary switching be safe for the transformer or will there still be a DC component / voltage spikes?

What exactly happens to a transformer when switching rapidly on the primary side compared to the secondary side?

Bonus question: What would have been a better solution?

Thank you for the help in advance


1 Answer 1


The standard solution when the heater does not react very quickly is to use proportional time control.

enter image description here

Figure 1. With zero-cross switching the result is that the waveform consists of multiple complete half-cycles. Image source: Opto-triacs, solid-state relays (SSR), zero-cross and how they work.

Now you have the advantage of being kind to the transformer, lowest possible EMC because of the zero-cross switching and a simple control strategy. For most applications a duty cycle of a few seconds works well but this depends on the heater's time constant.

There's further information on the topic in (my) linked article.

  • \$\begingroup\$ Thanks for your reply. What about applying this switching to the primary side of the transformer? \$\endgroup\$ Sep 8, 2021 at 10:39
  • \$\begingroup\$ That's common practice. \$\endgroup\$
    – Transistor
    Sep 8, 2021 at 11:13

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.