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Posted by kool on November 19, 2008, 2:13 pm
>>>> Jeff Wisnia wrote:
>>>>> kool 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.
>>>>>>> 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
>>>> 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.
>>> 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.
>> 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.
> Kool, the original poster Jeff Wisnia, as quoted above is the one that
> said he traced a single diode in the return leg of the 2 bulbs. The diode
> is built into the thermostat and half wave rectifies the 24VAC used in the
> HVAC control supply.
> Just for fun today I looked up what a grain of wheat bulb is rated at. I
> found at least one that is rated at 75 milliamps at 12 volts.
> I hooked up a transformer that has a 28 volt secondary, measured it to
> actually be 30.5 VAC and hooked up a 1N4007 ( a common switching diode) in
> series with 3 120 ohm resistors (total 360 ohms). That 360 ohms across the
> 30.5 volts AC in series with the diode draws .084 amps, similar to the
> .075 amp rating of the grain of wheat bulb.
> I measured the voltage across the resistors total resistance and read 13.3
> volts DC with a brand new Fluke meter. 13.3 volts will not significantly
> reduce the life of a 12 volt bulb. I did not measure the voltage with a
> scope. All light bulbs are rated at a voltage. This voltage rating is the
> RMS voltage. All AC voltage does have a peak to peak value which works out
> to be 2.8 the RMS voltage. , yet the 120 volt bulb lasts for years.
> Mark said the voltage to the bulb would be 16.5 volts, but that is the 0
> point to 12 volt RMS level x 1.4 peak multiplier math. The half wave
> rectified voltage to the bulb with the bulb loading this down somewhat
> measures out at 12 volts DC. If I get a chance tomorrow, I will hook up a
> scope and see what the peak voltage levels to the load is, just out of
> curiosity.
So I assume that a diode is all that is required to convert 24 VAC to 12VDC
in this simple circuit. I know very little about electronics but this seems
way too easy. Since there are 2 bulbs, would they be 180 ohms each?
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>>>>> kool 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.
>>>>>>> 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
>>>> 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.
>>> 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.