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Posted by Jeff Wisnia on November 16, 2008, 12:01 pm
kool wrote:
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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
>>--
>>Jeffry Wisnia
>>(W1BSV + Brass Rat '57 EE)
>>The speed of light is 1.8*10^12 furlongs per fortnight.
>
>
> 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.
>
>
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
--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.98*10^14 fathoms per fortnight.
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Posted by Mark on November 16, 2008, 7:57 pm
show/hide quoted text
> 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
>-
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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Posted by Don Ocean on November 16, 2008, 10:38 pm
Mark wrote:
show/hide quoted text
>> 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
>> -
>
> 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
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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>
>
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Posted by Stormin Mormon on November 17, 2008, 12:35 am
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.
--
Christopher A. Young
Learn more about Jesus
www.lds.org
show/hide quoted text
> 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
>-
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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Posted by Don Ocean on November 17, 2008, 1:37 am
Stormin Mormon wrote:
show/hide quoted text
> 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.
Its a halfwave rectifier for a DC bulb.
show/hide quoted text
>
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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
>>--
>>Jeffry Wisnia
>>(W1BSV + Brass Rat '57 EE)
>>The speed of light is 1.8*10^12 furlongs per fortnight.
>
>
> 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.
>
>