Re: [ATM] dew heater circuit questions-MORE Q's

[George Anderson <georgea@cam.org>]

3 Watts at twelve volts is equal to 0.25 amperes (3/12).
0.25 amps at 12 volts requires a resistance of 48 ohms (12/0.25).
The physical size of the fuse will depend on which fuse style you
choose. The old 3AG type ran around 1/4" in diameter and about 1 1/2"
long. If size is critical then a solder type pico-fuse could be used
which is about the same size as a 1/5 watt resistor. The drawback is
having to gain access to the circuit board with a soldering iron when
the fuse blows (Murphy's law, cold night with pristine viewing
conditions from a dark site...). A better choice would be a chassis
mount fuse holder with the fuse wiring in line with the output plug.

George Anderson
Montreal Canada

Clear skies and good health

Dave Smith wrote:
> 
> Hey Y'all,
> If I run 1 to 5 watts from 12 volts then I'm looking
> at 30 to 140 ohms. Correct?
> 
> If V=I*R, then my current is between 2.5 and 12 Amps.
> Correct?  That sure seems high! Now I realize that the
> circuit will not be utilizing the full 12 volts since
> the  design is only to keep the system ~2 degrees
> above ambient. However, (and maybe Ken or Don can
> address this) at what current (fuse capacity) do you
> set the current maximum for the fuse in the circuit
> layout??  And what size is the fuse (physical
> dimensions) in the pcb layout?
> Any help would be appreciated!
> Thanks,
> Dave
> 
> 
> 
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