Q: Regarding Ohm's Law Re: AC - Electrical - DIY Chatroom Home Improvement Forum

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07-16-2007, 12:12 PM   #1
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## Q: regarding ohm's law re: AC

Hi.

I have a hobby project which requires a small amount of heat, by which to operate an LTD Stirling model engine.

One watt of heat energy would suffice. I'll use a metal cased wire wound resistor.

Q: I don't know how to calculate the resistor's value for the source 120V AC current.

Like said, I'd like to make roughly 1W of dissipation in the resistor. What would be an appropriate value, please?

Thanks for any answers,

Reid

Btw:
Stirling models can be seen at various places on the net.
I got mine from Kontax UK; the KS90 hand-held low temperature differential stirling.

image of the model:
http://img394.imageshack.us/img394/4646/bfla9.jpg

the resistor is to be mounted to a sub-plate (aluminum disk),
and the model sits atop the sub-plate.

Last edited by Reid; 07-16-2007 at 12:16 PM.

07-16-2007, 12:58 PM   #2
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1 Watt / 120V = .00833~ Amps

120V / .00833 Amps = 14,405.8 Ohms

 07-18-2007, 12:36 AM #3 retired   Join Date: Jul 2007 Location: Idaho Posts: 108 Rewards Points: 75 I would suggest you get a 1 to 3 watt 12v dc wall wart (power adapter) and use a 144 ohm resister. For a hobby setup like you're doing 12v will be a h*** of a lot safer to play with. Jerry rigging a heater using 120v is courting with disaster, if not for you, then for your engine! Last edited by dmaceld; 07-18-2007 at 12:40 AM. Reason: correct error
 07-19-2007, 03:32 PM #4 Newbie   Join Date: Jul 2007 Posts: 2 Rewards Points: 10 Thank you both! Say, yes, the wall wart would be "safer", agreed, a bit wasteful in losses. Still best, I suppose--the low ohm resistor would be most robust. Only if I can get a metal encased, high value resistor of the heat-sink type, will I go the direct-drop way. Of this sort: http://www.rcd-comp.com/rcd/rcdpdf/600-FA018.pdf ------ Secret Squirrel, does your calculation include/allow for the RMS heating power of AC current? Wait....dummy here now presumes "120VAC" is equivalent in resistance heating to 120VDC. Is that the case? (thanks for bearing with me). With appreciation, Reid Last edited by Reid; 07-19-2007 at 03:42 PM.
07-19-2007, 03:49 PM   #5
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Quote:
 Originally Posted by Reid Secret Squirrel, does your calculation include/allow for the RMS heating power of AC current?
Yes... figured as RMS. Peak to peak is a different scenario.

07-19-2007, 04:13 PM   #6
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Quote:
 Originally Posted by Reid Only if I can get a metal encased, high value resistor of the heat-sink type, will I go the direct-drop way. Of this sort: http://www.rcd-comp.com/rcd/rcdpdf/600-FA018.pdf
Looks like those would be a bit pricey to me. Wire wound ceramic encased resistors that will give 1 watt of heat are mere pennies, well maybe a hundred or two, at Radio Shack.

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