Need heater strip circuit

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.

Pete

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

Thank you.

-- Clear and dark skies!

Pete

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John Fields wrote:

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).

Pete

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John Fields wrote:

--- 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.

-- John Fields

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Thanks!-)

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?

--
Terry Pinnell
Hobbyist, West Sussex, UK

edgarcito had written this in response to

: hola. No entiendo nada de lo que pasa y nada de ingles, pero necesito el circuito de un calentador de induccion. Gracias

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