# Indicator Lamp on Weathertron 3AAT80B1A1 Thermostat

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• posted on November 18, 2008, 4:13 am

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.
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• posted on November 18, 2008, 5:22 am
Fartikus wrote:

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That seems to agree with Mark's explanation....
Jeff
--
Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)
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• posted on November 18, 2008, 11:13 am

Well, in that case, I lose my argument. life goes on, happy as before for me. :-) I thought I was right, but I have been wrong before at least once.
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• posted on November 18, 2008, 2:45 pm

Hi Fartikus
try the other half of the experiment too...
compare the brightness of the bulb hooked to an actual 12VAC source vs when it's hooked to the 24VAC + diode source. A lightbulb's brightness will follow the true RMS value.
Unless your meter is a "true RMS meter" it will be lieing about the rectified waveform reading. Most meters are not "true RMS" meters and they read the correct RMS value only for a good sine wave which is what we read most of the time.
Mark
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• posted on November 18, 2008, 11:34 am

Well, in that case, I lose my argument. life goes on, happy as before for me. :-) I thought I was right, but I have been wrong before at least once.
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• posted on November 19, 2008, 7:13 pm

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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• posted on November 15, 2008, 5:35 pm
On Thu, 13 Nov 2008 13:53:57 -0500, Jeff Wisnia

Blinking lights on the thermostat you describe means it's gonna blow anytime.
I am the real ftwhd and I approve this message.
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• posted on November 16, 2008, 4:22 am
On Sat, 15 Nov 2008 09:35:59 -0800, snipped-for-privacy@home.com wrote: