On 03/06/2011 02:10, Dwayne Reid wrote:
> Anyway, it occurred to me that I could simply scale the same circuit
> up to run from rectified, un-filtered AC and simply use the pass
> device as the heating element.  Like I said, I've got lots of
> experience with this.
>
> Back-of-the-envelope calculations say this should work well.  Now all
> I need to do is find a suitable pass device.  N-channel MOSFET,
> because of price and voltage rating.
>
> The device will not be running hot - the unit turns on somewhat near
> 0C and is guaranteed to be off at 10C or so.  Its only purpose is to
> raise the internal temperature of the air inside the enclosure
> significantly above the -45C that the outside of the enclosure is sitting=
 at.
>
> Hope this makes sense!

Yep, makes sense.
I can see how your smaller TIP122 circuits would work well, and should=20
scale up okay providing you can find a suitable power transistor. I=20
realised the suggestion of a 2N3055 was silly as they don't have the=20
required voltage rating. I was thinking more of the TO-3 package and the=20
high power rating.
I'm thinking it might be difficult to meet your specs with just one=20
transistor (and keep the price reasonable) Maybe it would be an option=20
to put a couple in parallel.



--=20
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