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Noob needs help designing a simple on/off timing circuit

  1. Feb 11, 2012 #1
    I am a good DIYer, so I know I have the skills to solder a circuit together, but I can't design anything! Hoping a kind soul here can shed some light.

    I have a space heater that is being used to heat a very small space, and the problem is that on its lowest setting once the area comes up to temp there is still so much residual heat left in the heater that the area coasts up a few more degrees than desired. I want to design a 120v circuit that will cycle the heater on and off, effectively reducing the power output. For example, on and off every 30 seconds may be a good place to start, or on for 20 seconds and then off for 40, continuously. It has to be mechanical, or analog, meaning, it can't use any kind of logic because when the desired temperature is reached the power is removed (its hooked up to a temperature-sensing relay).

  2. jcsd
  3. Feb 12, 2012 #2
    Something like this, I reckon?


    This one is a bit more sophisticated, and uses a temperature sensor (LM35) to control the heater. By the way, BT136 will only support a pretty small heater (4A @ 110V = 400W); you'll probably need a bigger TRIAC.

  4. Feb 12, 2012 #3


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    There is a device called a simmerstat which does exactly what you need.

    You have probably seen them on electric heaters and ovens.
    They have a knob on the front and this varies the duty cycle of the output.

    Have a look here for some pictures:

    You would have to locate one for your voltage and with enough current switching ability.

    I have added these to equipment and they worked OK.
  5. Feb 12, 2012 #4

    jim hardy

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    In process control it is common to place the thermostat very close to the heater so that it arrives at temperature slightly before the process does. In your case the 'process' is your space.
    By judicious tinkering one can make the "coast" deliver just what he wants. That's called "Feed Forward", and is closely related to oft-mentioned "derivative control action".

    So, before building a complex electronic gizmo i'd experiment with creative ductwork, just keep it away from the red hot parts.
  6. Feb 12, 2012 #5
    If you cant find a simmerstat then following on fsb, get a simple triac dimmer meant for lighting at the hardware store. Mount it in a junction box with an outlet and you're good to go.

    Make sure it's rated for at least the wattage of the heater but 1500 watt dimmers are common and cheap.
  7. Feb 13, 2012 #6
    thanks for the great suggestions! i originally wanted to build a circuit and the simmerstat looks great, but the idea of just hooking it up to a dimmer has sort of blown my mind w/ its simplicity. Tomorrow I'm going to try just that, and i'll put my kill-a-watt between the circuit and the outlet and see how effectively it reduces the output.

    update: I found an "infinite switch" on ebay for $6 including shipping. I'm going to give this a try, and report back.
    Last edited: Feb 13, 2012
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