I have a resistive heater with a resistance of 18 Ohms. The heater is mounted on a job which has a thermocouple mounted on it. I need to use a SSR based controller to control the temperature measured by the T/C. As per my calculation I need to produce 1500 W. I have at my disposal a 230 V, 50 Hz power source, rated at 16A.

I plan to use an isolation transformer to drop the 230V to 165V with the SSR and the heater mounted on the secondary side.

My question: An SSR will generate a number of harmonics due to "chopping" of the sine wave. Question is will these harmonics lead to heat generation in the secondary winding of the transformer?

  • \$\begingroup\$ I agree with @Neil_UK's answer below. I would tend to rate the transformer as though I was using full-wave output into that resistance and then the reduced power due to phase angle control will compensate for any harmonics induced losses. \$\endgroup\$
    – Transistor
    Commented Jul 2, 2016 at 14:57

1 Answer 1


The SSR generates current harmonics, which in a single phase system will only disturb the power factor slightly. It's only in a 3 phase system that such harmonics do other bad things like exciting neutral currents.

While a reduced power factor means there will be more heating in your secondary (and primary) for the same heater power than would be caused by a unity power factor, the effect with an SSR is going to be negligible.

  • \$\begingroup\$ Tanks Neil!, I was wondering what kind of transformer rating I should choose. Going by @transistor's answer, I should choose 1500/95% = 1578 VA ~1600 VA (assuming transformer efficiency to be 95%). I guess to be on the safe side I can jack it up to 2000 VA. Does it look to be sufficient? \$\endgroup\$
    – Abhishek
    Commented Jul 2, 2016 at 15:18
  • \$\begingroup\$ One another question, will the situation be any different if I use a variable autotransformer rated at 16A, with the secondary set at 165 V? \$\endgroup\$
    – Abhishek
    Commented Jul 2, 2016 at 15:22
  • \$\begingroup\$ no different on power and heating, but sounds a bit of a waste of a varaible transformer. BE AWARE an auto transformer is NOT an isolation transformer. If isolation was an important spec point for you, then you won't get it from an autotransformer. As it's heat, why not ditch transformers totally, and acheive the right average power output by on/off control of the SSR? \$\endgroup\$
    – Neil_UK
    Commented Jul 2, 2016 at 15:34
  • \$\begingroup\$ thanks for the comments, yes its true that I can configure the controller to never exceed 165 V RMS while supplying the power from the 16A socket...actually what I have simplified the question, I actually want to run two such heaters on a single 16A, 230V socket... hence I would be using two transformers in parallel (or a single transformer with two secondaries, each at 165 A)... also in another case I may need to use a heater which requires more than 16A at voltages much less than 230V, so using a transformer makes sense - although a separate transformer in each case. \$\endgroup\$
    – Abhishek
    Commented Jul 2, 2016 at 15:46
  • \$\begingroup\$ In an isolation transformer do I need to tie one of the secondary wires to ground? \$\endgroup\$
    – Abhishek
    Commented Jul 2, 2016 at 15:49

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