Indicator Lamp on Weathertron 3AAT80B1A1 Thermostat

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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.
I suspect it's just a "burned out bulb". The thermostat is old enough so I'd doubt that the indicators use LEDs.
Before I approach it, can someone tell me if replacement lamps are available and easily screwed or snapped in place or will I have to figure out what tiny bulb to buy and use my past years of electronic repair experience to solder it in.
It would be annoying to toss out an otherwise working thermostat just for the want of a bulb.
Thanks guys,
Jeff
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Jeffry Wisnia
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It's a tiny incandescent bulb about the size a Motrin gel-cap , with two little wires protruding from the bottom G.E.# 35E. It just sits in the hole over a blue plastic lens.(the bulb is clear.) Mounts only by the wires soldered onto the wires in the stat.
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kool wrote:

Jeff
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Jeffry Wisnia
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For want of a bulb, the thermostat was lost. For want of a thermostat, the heat pump was lost. For want of a heat pump, the house was lost. For want of a house....
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Christopher A. Young
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On Thu, 13 Nov 2008 21:01:35 -0500, "Stormin Mormon"

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kool wrote:

I soldered in a new bulb yesterday. Tracing the circuit showed that here's a single diode in the common return lead of both bulbs so the bulbs effectively see only half of the nominal 24 volt AC supply.
I used a 12-14 volt "grain of wheat" bulb and it works fine now.
Jeff
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that is a common wrong mis-conception. A 12V bulb hooked up to 24 V AC with a diode does not see 12 V. It sees 24 Volts for 1/2 of the time. During the time it sees 24V, it is getting 4x the power because P=V^2/R. But since it seeing 4x power only half the time, the net is 2x power. So a 12V bulb hooked up to 24 VAC through a diode is seeing 2x the correct power, it should be very bright and will burn out soon.
If you don't belive me, try it with a 120V bulb and a diode hooked up to 240V. It does not work. The bulb will burn out in a short time.
Mark
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Mark wrote:

Nice answer.. consider the fact that bulbs have a DC voltage rating and an AC voltage rating. There is a difference in filament structure and power capability. Now what is the rating of that 12 or 24 volt bulb vs the 120vac bulb?

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Last time I used a VOM on a diode, it was about 2K ohms. Might be the right value for a dropping resistor in this case. Depending on the miliwatt draw of the bulb. Could work. Could blow out. He'll find out.
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Stormin Mormon wrote:

Its a halfwave rectifier for a DC bulb.

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Last time I measured the resistance of a half wave rectifier for a DC bulb, the half wave rectifier was about 2k ohms resistance. Depending on the amperage draw of the bulb, that could be the correct dropping resistor value.
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Mark wrote:

Mark, I think you may have got me there!
I probably got lucky when I grabbed bulb from the leftovers from the kid's model railroading experiences of some 20 years ago, and it probably was one intended to be run on higher than 12-14 volts. (No markings on it.)
But, after I soldered it in I looked at it lit (from the back) before I replaced the thermostat on the wall and compared its color to that of the "emergency heat" bulb when that one was turned on. The filament of the bulb I installed was lit to just about the same color as the "emergency heat" one, so I guess it's a "right fit", huh?
Jeff
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yep sounds like you are OK then...
I remember this from a long time ago we had a temperature chamber that ran off of 240V and used a 240 V bulb inside for light...the bulb burned out and we didn't have a 240 V bulb to replace it...one of the engineers got the bright idea to use a diode and a standard 120 V bulb, seemed logical,,, so we all got to learn by experience why that didn't work...
Mark
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Jeff Wisnia wrote:

God looks out for fools and drunks.. ;-)

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Then this group is in good shape.
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Jeff Wisnia wrote:

Now that would be RMS voltage and that diode would indicate the bulb is DC rather then AC. basically that bulb has a half cycle to cool and a half cycle the heat and produce light. If all of that is not needed, redesign and install a lifetime LED.

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Here is an explanation how it works.
The bulb would get approx 12 VDC pulses and the 0.7v junction voltage of a (silicon) diode would reduce the actual by 0.7V
http://www.wisc-online.com/objects/index_tj.asp?objID=SSE402
By the way, a grain of wheat bulb is incandescant, so it does not matter whether you feed it AC or DC. If an LED wre used (with a current limiting resistor to reduce the voltage to the LED to its rating, feeding it the pulsating approx 12VDC would be fine as long as it was wired for the correct polarity.
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that is wrong..... it is NOT 12 Volt pulses...
24VAC RMS means is +34 Volts and - 34 Volts peak. Through a 1/2 wave rectifier, the bulb will see 1/2 sinewaves of 34-0.7 = about 33 Volts peak.
Due to the time duration of the sine wave shape and the missing 1/2 cycle the RMS or heating value will be equivalent to about 16.5 volts RMS. Connecting a 12 volt bulb to 24 VAC throgh a diode is like connecting it to 16.5 Volts RMS. It will run bright and burn out in a short time.
Mark
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Original bulb is GE 85E not 35E.
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On this application there is no DC involved. The power to the two bulbs comes directly off of the "B" terminal on the stat through the diode to one wire from each bulb. the other wire from the blue bulb (auxillary heat) goes to the "U" terminal and the second wire from the red bulb (emergency heat) goes to the "F" terminal. The F terminal is not fan as it also has a G terminal as well as Y,X2,W, O,R and T. on the BAY subbase. 1981 vintage no electronics other than 1 diode.
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