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I need to make an adjustable power controller for a 3kW heater that will run off single-phase 240V AC power.

How should it be done?

Maybe a triac circuit similar to that used for dimming lights. Could this work? (Household heaters don't seem to ever be 'dimmable', is this just a cost saver, or is there another reason?)

An alternative idea was switch the heater on and off with a minimum interval of once per second in a PWM fashion, using a zero-cross solid state relay. But I don't think the neighbours would be happy with their lights flickering at 2Hz.

Any thoughts or other ideas would be appreciated

Jodes
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No need for phase angle modulation like on a dimmer for lighting, no need for PWM nor for switching on/off every second. It will introduce interference and the thermal capacity of the heater will be so large that you can easily switch on and off for full mains cycles and even every so many seconds (if your neighbors don't mind).

Switching at zero crossings is good practice, and a Solid State Relay can help you with that and is safe when properly connected.

jippie
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  • I didn't think light dimmers do any phase angle modulation? AFAIK they chop up the AC signal, for example for 50% power, switching on at 90 and 270 degrees, then switching off at 180 and 360? I didn't think that's phase angle modulation? – Jodes Apr 18 '13 at 11:55
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Two thoughts:

The traditional way is to use a solid-state relay. It won't be particularly cheap, but it will be properly heatsinked and will have a zero-crossing circuit built in. Once you get that, you can easily do PWM on it. In either case, you will get the same effect as if you were turning it off and on with a switch.

There are also some newer devices that can be used to PWM AC at a higher rate.

Eric Gunnerson
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Suitable switching rate depends on what you are heating. Some calculations of its thermal inertia are worthwhile : start with its mass and heat capacity, and see how long it would take 3kw to heat it by one degree. You might find that PWM at a frequency of 1 cycle per minute or per 10 minutes is appropriate.

If it's a tank of water, you may need to override the controller when drawing off hot water. Crude way : If a flow switch runs for a minute, run at full power for 3 minutes or something. Better way : read up about PID controllers)

Switching devices? I cannot add anything to the excellent suggestions to avoid phase angle modulation at these power levels, and use zero crossing SSRs.