Control AC device with a DC circuit

Discussion in 'The Projects Forum' started by ke5nnt, Jan 31, 2010.

  1. ke5nnt

    Thread Starter Active Member

    Mar 1, 2009
    A friend has asked me to look into making a device for him that will control the on/off cycle of an engine block heater based on outside temperature.

    Essentially the heater is a 1KW heater, in an effort to save energy running it all the time, he wanted something that would switch it on and off at intervals that vary for certain temperature ranges.

    So, my question would be, can I do something like, connect a relay to the A/C heater and control the on/off cycle of the relay with a DC device controlled by a PIC?

    I'm not sure how relays compare to things like BJTs and MOSFETs when it comes to using them as switches. Something simple like switching on/off a LED using a MCU and a BJT is probably different than using a MCU and a relay to switch on/off a 120V AC device...

    If what I read about relays is right, then basically a 12V (or so) DC circuit controlled by the MCU would energize an electromagnet and complete a separate circuit that would basically close the connection between the 120V AC main and the heater? If that's true, then the DC circuit and AC circuit are isolated from each other?
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    Check out Solid State Relays (SSRs) at Mouser, Digikey , surplus dealers and even EBay.
    I can post a thermistor/opamp circuit that will switch an SSR on/off
  3. ke5nnt

    Thread Starter Active Member

    Mar 1, 2009
    Yeah I was looking at SSRs on mouser, had no idea those things were so pricey. $30 for one capable of

    If you want to post your circuit, I'd love to see it. Always nice to see how other people do things.
  4. KMoffett

    AAC Fanatic!

    Dec 19, 2007
  5. Wendy


    Mar 24, 2008
    Look at Tanner's. $5-10 each for AC SSRs, and you can use MOSFETS for DC.
  6. retched

    AAC Fanatic!

    Dec 5, 2009
    Look up air conditioner CRANKCASE heater controls. You can use a OTS thermostat for your control using time or temp or both. They heat the oil in the bottom of a A/C compressor for a set time before firing up the compressor.

    For your relays, I would recommend HVAC relays or contactors. They are build for vibration, and temperature extremes.

    here are some for around $10
  7. ke5nnt

    Thread Starter Active Member

    Mar 1, 2009
    Thanks all, will definitely plow over all that stuff.