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heater circuit

Discussion in 'General Electronics' started by pil, Dec 22, 2004.

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  1. pil

    pil Guest

    I need to control a simple heating element which will run off 12Vdc. It does
    not need to get very hot.

    I need to make it variable. At the moment I have a 555 running in astable
    mode pulsing two MJ13005 transistors which are directly connected to the
    element. The transistors seem to get too hot. What other methods can I use?

    My element usualy are a bunch of fine stranded copper wires and the one I am
    currently using has a resistance of 5ohms.

    I need to run this off 12Vdc.
  2. I think the power transistors you are using are high voltage, low gain
    types. So they need a lot of base current to get fair saturation. If
    you replace them with a low resistance N-channel mosfet, that should
    run much cooler. They are also much simpler to connect to the 555.
    The gate can be connected directly to the 555 output.

    Go to Digikey and find the data sheets for IRFZ44V, IRF540, IRF3704
    for examples that should work. One of these can handle your load with
    no heat sink.
  3. use?

    Are you using the transistors in parallel? What frequency is your 555
    running at? Duty Cycle? Schematic?

    All that aside, worst case gain for that transistor is only 8 (Ic=2A)
    which means you will need to supply at least 300mA (each) of drive into
    the base to turn the transistors on all the way. AFAIK, the 555 can't
    supply that much current by itself. What you need is a nice MOSFET,
    preferably one that uses a "logic level" input.
  4. Guest

    Couldn't find much on your transistors.
    2 things, Use a better transistor like a MOSFET that has a low switched
    on resistance.
    2nd, use the pulse width function of the 555 to adjust the temp. you
    will only need one transistor instead of 2 drives.
    A predrive may be needed. You are running about 2.5 amps
    There are many devices out there that would work I would get one about
    5 amps and a med. heat sink.
    Mr. Eric Spain
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