Re: Need heater strip circuit

From: Pete (lansma_at_bigpond.com)
Date: 08/21/04 

Date: Sat, 21 Aug 2004 22:53:24 GMT

John, I have now located this schematic:

http://www.astrosurf.com/benschop/DewRemov.htm

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:
> On Sat, 21 Aug 2004 16:21:35 GMT, "Pete" <lansma@bigpond.com> wrote:
>
>> 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.
>
> ---
> 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.
> ---
>
>> 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.
>
> ---
> 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?

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