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I want to 'dim' some 3kW heating elements to provide variable output i.e. between 0 & 100%.

I intend to use zero-crossing solid state relays.

let's say, for example, I want to run the heater at 50%, a couple of options could be:

  1. repeatedly turn the heater on for 3 minutes, then off for 3 minutes; or
  2. repeatedly turn the heater on for 20 milliseconds, then off for 20 milliseconds (20milliseconds = 360deg on a 50Hz frequency).

The question is, which option will promote a longer lifespan for the heating element? option 2 has more frequent switching, but much less temperature swing, so I'm not sure which will be better for the element.

For the purposes of the question, let's say that I don't care about the longevity of the control equipment (relay, contactor etc.) and i don't care about the temperature change on the building elements around the heater

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  • \$\begingroup\$ What will you heat? If you have a large thermal mass temperature will change slowly and it doesnt matter if you switch every 3 min or 50 per second. \$\endgroup\$ Commented Aug 4 at 13:28

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Since heater elements are really just resistors, they don't care about switching, as long as that doesn't come with a lot of thermal stress – and as you, I'd come to the same conclusion that the short-period switching means less thermal stress. (Note that this is different to many types of incandescent lamp, which are also "just" resistors, but ones that want to be operated at a high temperature.)

One thing to note though: heating-wise, 50% duty cycle at 50 Hz periodicity might be more power than 50% duty cycle at 0.00556 Hz – as you correctly say, in the 3 minutes, the heating element gets hot, and that increases its resistance, meaning that the average power during 3 minutes on-time is lower than it is during 20 ms on-time.

Note that for exactly 50% duty cycle, a half-wave rectifier would do – a simple diode, or an SCR with its gate tied to the anode, or any other way of achieving a slightly more ideal diode.

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    \$\begingroup\$ Electricity suppliers generally don't like large loads being supplied by a half-wave rectifier. It effectively adds a large DC current on the AC supply, which can saturate the local transformer. \$\endgroup\$ Commented Aug 4 at 12:54
  • \$\begingroup\$ Yeah, that makes sense, @SimonB. I don't know whether that's relevant in this case (this might well be a residential installation, and the polarity of the half-wave rectification should on average cancel if a lot of people do that); you're right, a nicer circuit would effectively be a triac-based bipolar dimmer, and just leave out every other period (or do say 25 full periods of 50 per second, or such). \$\endgroup\$ Commented Aug 4 at 13:01
  • \$\begingroup\$ Thanks for the feedback. The truth is, I'm trying to do a range of percentages, but used 50% as a simple way to explain it. Also trying to balance out the DC-bias. \$\endgroup\$ Commented Aug 6 at 4:41
  • \$\begingroup\$ But yes, it's the thermal stress that I'm stressing about... When you say it's different to incandescent lamps, how so? I think one of my heater elements is 'open coil', so essentially just a big version of an incandescent lamp, right? I have another application where we'll be using Mineral Insulated Metal Sheathed elements. \$\endgroup\$ Commented Aug 6 at 4:41
  • \$\begingroup\$ When we were testing our solid state relay with a mineral insulated metal sheathed element, if we switched power on for 1 ac cycle, then off for 9 (on repeat), we could actually hear it! It made the same sound that you hear when elements are just turning on, but normally that sound gradually fades as the element heats. I suppose that makes sense because in our test, we were running it at 10%, and it didn't make that sound when we ran it at 90%. But if i can hear it make noise, it does make me worry... \$\endgroup\$ Commented Aug 6 at 4:41

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