# How do I limit the maximum power, delivered to a heater, through a triac?

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?

• 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. Jan 23 '13 at 12:25
• 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? Jan 23 '13 at 12:29
• @ Phil Frost, exactly. Please propose/edit the title to reflect the idea clearly! Jan 23 '13 at 12:35
• @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. Jan 23 '13 at 12:49
• 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? 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.

• 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. Jan 23 '13 at 23:30

Would this work?