Thermostat Indicator Bulb Type

Nov 14, 2008 6 Replies
J

One of the GE Weathertron Model 3AAT80B1A1 thermostats in our home is doing its thing and when the temperature setting is increased a few degrees above ambient the auxillary heaters come on OK to supplement the heat pump's output.



The blue "Aux Heat" indicator recently stopped lighting in that mode. I could live with it that way, but being an engineer I'm sort of anal about having everything working the way it was designed to.



Does anyone happen to know the type number or the voltage/current ratings of the "grain of wheat" incandescent indicator bulbs used in those thermostats. (It's soldered in.)



It would be annoying to spend good money to replace an otherwise working thermostat just for the want of a bulb.



I suppose I could measure the voltage and current draw of the working bulb in our other identical thermostat, but I thought I'd ask here first.



Thanks guys,


Jeffry Wisnia (W1BSV + Brass Rat \'57 EE) The speed of light is 1.8*10^12 furlongs per fortnight.
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S

solder in a small led /680 ohm resistor.

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T

The vast majority of US thermostats use 24VAC control voltage. Yours may be the exception. A voltmeter across the connection to the bulb would tell you real quick.

24 or 28VAC grain of wheat lamps are often 5-10mA or less.

If you want to do the LED replacment others have suggested, keep in mind that most LED's do not tolerate much in the way of reverse voltage.

Tim.

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J

Thanks, and the few times I've put an LED indicator on a low voltage ac circuit I alway stuck a diode across it to limit the reverse voltage.

Jeff

Jeffry Wisnia (W1BSV + Brass Rat \'57 EE) The speed of light is 1.8*10^12 furlongs per fortnight.
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J

I soldered in a new bulb yesterday. Tracing the circuit showed that here's a single diode in the common return lead to both the bulbs in the thermostat so the bulbs effectively see only half of the nominal 24 volt AC supply.

I used a 12-14 volt "grain of wheat" bulb left over from my sons' model railroading period of 20+ years ago and it worked fine.

Jeff

Jeffry Wisnia (W1BSV + Brass Rat \'57 EE) The speed of light is 1.98*10^14 fathoms per fortnight.
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M

No, it sees half power, not half voltage, so you have the equivalent of (24 VAC * .707) -.6 V, or 16.368 volts.

http://improve-usenet.org/index.html aioe.org, Goggle Groups, and Web TV users must request to be white listed, or I will not see your messages. If you have broadband, your ISP may have a NNTP news server included in your account: http://www.usenettools.net/ISP.htm There are two kinds of people on this earth: The crazy, and the insane. The first sign of insanity is denying that you\'re crazy.
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J

You are right on that. The bulb I grabbed (Unmarked it was.) was probably an 18 or 24 volt one. Anyway, it's filament lit to the same "less than bright white" color as the other bulb (The emergency heat indicator.) so I'm figuring all's well.

Jeff

Jeffry Wisnia (W1BSV + Brass Rat \'57 EE) The speed of light is 1.8*10^12 furlongs per fortnight.
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