Thermostat Indicator Bulb Type

J

Jeff Wisnia

Guest
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.
 
Jeff Wisnia wrote:
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,
solder in a small led /680 ohm resistor.
Click to expand...
 
On Nov 13, 8:55 pm, Jeff Wisnia <jwis...@conversent.net> wrote:
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.
Click to expand...
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.
 
Tim Shoppa wrote:

On Nov 13, 8:55 pm, Jeff Wisnia <jwis...@conversent.net> wrote:

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.


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.
Click to expand...
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.
 
Tim Shoppa wrote:

On Nov 13, 8:55 pm, Jeff Wisnia <jwis...@conversent.net> wrote:

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.


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.
Click to expand...

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.
 
Jeff Wisnia wrote:
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.
Click to expand...

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


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.
Click to expand...

--
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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.
 
Michael A. Terrell wrote:

Jeff Wisnia wrote:

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.



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



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.
Click to expand...
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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