Indicator Lamp on Weathertron 3AAT80B1A1 Thermostat

[Don Ocean]

God looks out for fools and drunks.. ;-)

>

[Fartikus]

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.

[Fartikus]

Then this group is in good shape.

[Jeff Wisnia]

Just for Ss&Gs I asked light bulb guru Don Klipstein about the 120 volt bulb with a diode in series with it connected to a 240 volt AC supply. His reply was:

*****************************

240V AC halved by an "ideal diode" (zero voltage drop) will have the same average voltage as unmodified 120V AC. However, the RMS (root mean square) voltage will be different - higher by a factor of square root of 2. That would be 169.7 volts. The RMS voltage is the effective voltage for simple resistive loads such as incandescent lightbulbs and resistive heaters.

Think of it this way: An ideal resistor (resistance not varying with temperature) receives 1/4 as much watts when voltage is halved. Not only is voltage halved, but amps are also halved, and that means watts are quartered.

But a diode merely disconnects the load from the line for half the time, so wattage is merely halved. By Ohm's Law, the effective voltage of AC after a diode is then the original voltage times square root of 1/2.

Incandescent lamps do have the complication of their resistance varying with temperature, but the effective voltage ("RMS voltage") is still the same as if resistance was constant. An incandescent lightbulb receiving

240V AC through a diode is effectively getting 169 volts. After the effect of resistance varying with temperature, power consumption by the lightbulb will be close to 1.65-1.72 times that at 120V rather than double.

Keep in mind that a voltmeter will read a voltage other than about

169 volts unless it is a "True RMS" type. It may read 120 if it has fullwave rectification or possibly either 240 or close to zero due to having mere halfwave rectification. On a DC range, a non-true-RMS voltmeter will read halfwave rectified 240V or fullwave rectified 120V as about 108 volts - the actual average voltage. (Nominal voltage of AC is the RMS voltage, about 11% higher than the average for a sine wave.)

According to what I consider a "usual 1-size-fits-all" rule, life expectancy is reduced by a factor of about 60 - and actual life expectancy results can vary significantly, even greatly.

I would caution that burnout may be more spectacular than at 120V, possibly even unsafe, especially for a diode unless it is big enough to not protect your fuse or circuit breaker by blowing first (possibly explosively) should a "burnout arc" form.

And at 70% overwattage, the bulb may overheat and break - especially in an enclosed fixture or a recessed ceiling fixture.

Lower wattage 120V lightbulbs, such as maybe around or under 25 watts (?), may have their filament vary enough in temperature over each AC cycle when used with a diode and 240V to change results somewhat from results with actual 169-170V - probably for the worse in terms of life expectancy.

- Don Klipstein ( [e-mail])

***********************************

That seems to agree with Mark's explanation....

Jeff

[Fartikus]

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.

[Fartikus]

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.

[Mark]

nce.- Hide quoted text -

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

[kool]

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?