I am using the following circuit to turn ON/OFF a heater. Works like a charm. However, I would like to limit the maximum power delivered to the heater. The reasons for this are:

  • the heater is a fire hazard when running full on
  • the triac has to be sized for quite large current when powering the load full out

What is the simplest (fewest added components) way to limit the power, that a triac delivers to a resistive load? enter image description here

  • \$\begingroup\$ From what I can work out of the question you probably just want to turn the heater on and off at various duty cycles to control the temperature? In that case maybe a 555 timer is a simple way. \$\endgroup\$
    – PeterJ
    Jan 23 '13 at 12:25
  • \$\begingroup\$ The usual way to do this is to detect zero crossings of the AC voltage, and delay triggering the triac for some selected delay after that. This doesn't really regulate the power though, it just decreases the RMS voltage delivered to the load, which only indirectly changes power, and is unregulated. Is that what you are hoping to do? \$\endgroup\$
    – Phil Frost
    Jan 23 '13 at 12:29
  • \$\begingroup\$ @ Phil Frost, exactly. Please propose/edit the title to reflect the idea clearly! \$\endgroup\$
    – Vorac
    Jan 23 '13 at 12:35
  • \$\begingroup\$ @PeterJ, my aim is to limit the maximum heat output of the heater, as that is too powerful and creates a fire hazard. Therefore, pulses of full wall power, longer than 500ms are not allowed. \$\endgroup\$
    – Vorac
    Jan 23 '13 at 12:49
  • \$\begingroup\$ I think you need more details on just how you want to limit the power. Limiting to 500ms pulses is not the same as limiting to say, 50W. If you allow only 500ms pulses, how much idle time must there be between pulses? 1ns? 10 days? \$\endgroup\$
    – Phil Frost
    Jan 23 '13 at 14:51

If the existing heater is a fire hazard, you must select a different heater! It is far too easy for some part of your "regulator" circuit to fail in such a way that full power is applied to the heater.

One simple solution would be to wire two heaters in series — each heater would operate at 1/4 power, producing lower temperatures, and the total power would be half that of the original heater alone.

  • 1
    \$\begingroup\$ Another would be a rectifier in series (or change the triac to an SCR). That gives half of each cycle and thus half the power. \$\endgroup\$
    – gbarry
    Jan 23 '13 at 23:30

Would this work?

  • use a ready-made SSR with zero crossing instead of rolling your own
  • use standard PWM to turn the header on/off for a different amount of time to gain different total heat power.

TRIAC Angle of Firing vs Power Delivered to Load, how calculate?

  • \$\begingroup\$ This answer is very late (> 2 years), a bit garbled - zero-cross in one line with TRIAC angle on another - and doesn't address the fire hazard in the event of a triac failure. \$\endgroup\$
    – Transistor
    Apr 1 '16 at 16:11
  • \$\begingroup\$ Hm, true it is late. I dunno why stackexchange pops up old questions. I believe we can not solve the fire hazard in pure electronics, so that is out of the question. Can you elaborate what is garbled? I suggest not using triac firing angle control, but rather use an SSR with zero crossing and implement a slow PWM. More details are in the linked answer, which is actually pretty much the same question. \$\endgroup\$
    – Gee Bee
    Apr 2 '16 at 0:19

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