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Need heater strip circuit

Discussion in 'Electronic Basics' started by Pete, Aug 21, 2004.

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  1. Pete

    Pete Guest

    Hi, I posted recently a problem with a controller circuit kit that I
    constructed using a 7555 timer IC and a BUZ71A power mosfet transistor.
    Kim from this group thinks I blew the transistor, and she is probably
    correct because it did get very hot despite using a heat sink. In truth
    I need a much more robust circuit and I am open to any ideas. I want to
    control the amount of heat generated by a heater strip made from 47 x
    330 Ohm resistors in parallel. This is a commonly used wrap-around dew
    removal method for telescope lenses. As I understand it the power needs
    to pulse to avoid overheating. I would plan to operate this from a 12
    volt 1 amp battery pack plugged into mains power but may end up using a
    12 volt car battery. I am inexperienced in electronics, but can
    construct projects from circuit diagrams.

    Thanks for your help.

  2. John Fields

    John Fields Guest

    The duty cycle (duty cycle is the ratio of the time the pulse is ON to
    the total time between pulses) of the pulse needs to vary so you can
    change the temperature of the heater.
    Commercially available telescope heaters I've seen use that scheme,
    but with a flexible heater which wraps around the barrel and a 555
    pulse-width modulator driving a MOSFET.

    Your 47 X 330 ohm resistor scheme will result in a total resistance of
    about seven ohms, which will allow about 1.7 amps to flow from a 12V
    source, which would cause the resistors to dissipate 20 watts if they
    were on all the time, less if the 555 lowers the duty cycle. All of
    this is eminently doable. Would you like a schematic?
  3. Pete

    Pete Guest

    Yes please, do you have one or know of one somewhere?

    Thank you.
  4. Pete

    Pete Guest

    John, I have now located this schematic:

    BUT it isn't clear how many resistors are used for the heater strip or
    what configuration they are in. I guess this is important because this
    circuit uses similar components to the one I made with the transistor
    that (probably) blew. I already have the strip constructed by a friend
    with the 47 x 330 Ohm resistors spaced perfectly to fit around my 8 inch
    telescope. BTW: The transistor I used was in fact BD679/681 Darlington
    but can be exchanged for the BUZ71A with slightly different pinouts
    (well that's what the diagram I have says).

  5. From the note on that page, it seems they are 'standard Kendrick
    Heaters', which hopefully means more to you than to me, and are
    available in a range, decribed at

    I'd use your own readymade strip, presumably with great care over the
    construction to eliminate any risk of scratching.

    That circuit looks OK, although I expect John will improve on it. BTW,
    I'm not clear why C2 aand R4 are needed?
  6. John Fields

    John Fields Guest

    Also replied via email...

    With the ~ 7 ohm array you have, and a 12V supply, the Darlington will
    be dissipating about 5 watts when it's fully turned on, so you'll need
    a rather large heat sink to keep it from getting toasted, which is
    probably what happened in your original circuit.

    That schematic looks fine, (with a couple of exceptions) and the
    heater your friend put together for you should work well with it as
    long as you use a BUZ71.

    If you use a BUZ71, the changes I'd make to the schematic would be to
    remove R3 and R6, connect the gate of the BUZ71 directly to the output
    of the 555, and connect a 15V Zener (1N4744A) directly across the gate
    and the source of the BUZ71, with the cathode connected to the gate.

    Also, if you use a bipolar 555, you should connect something like a
    10µF aluminum electrolytic and a 0.1µF ceramic cap directly across its
    supply pins.

    If you use the OPTO22 ODC5 you should remove R5 and replace R3 with a
    750 ohm +/-5% 1/4 watt resistor, and if you use the Darlington, you
    should remove R6 and replace R3 with a 240 ohm +/-5% 1/2 watt
    resistor. Again, however, if you use the Darlington, when you've got
    the heater cranked to max the Darlington will be dissipating about 5
    watts, which will require quite a large heat sink to keep it from
    dying, so I recommend, if at all possible, that you forget the
    Darlington and use the BUZ71, which will only dissipate about 200
    milliwatts, worst case, and won't require a heat sink at all.
  7. John Fields

    John Fields Guest

    Me neither. It looks like maybe the author was thinking about getting
    rid of RFI, or maybe softish-off for some reason, but that current
    spike through C2 every time the MOSFET turns on sure looks like a
    source of RFI, as well as a waste of battery and extra work for the

    I'm pretty sure a Kendrick heater is an arrangement like a serpentine
    wire (electric blanket) embedded in a fabric matrix, so it should look
    pretty resistive, deepening the mystery... ;^)
  8. edgarcito

    edgarcito Guest

    edgarcito had written this in response to
    hola. No entiendo nada de lo que pasa y nada de ingles, pero necesito el
    de un calentador de induccion. Gracias

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