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
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
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.
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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