Thermostat Indicator Bulb Type?

As an engieer, doing the measurement will be unavoidable. :)

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Jeff.. Why not measure voltage and use a LED? WW

I could use a white LED, resistor (and prolly a diode across the LED too since the voltage is AC.) But, as its taken 23 years for that first incandescent bulb to burn out because it's probable it was designed to run on the cool side, a replacement bulb will likely outlive me.

Jeff

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Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)
The speed of light is 1.8*10^12 furlongs per fortnight.

On Thu, 13 Nov 2008 20:54:25 -0500, Jeff Wisnia put finger to keyboard and composed:

Does this help at all:

It looks like you may need a 24V bulb. AFAICT, the most common rating appears to be 60mA.

- Franc Zabkar

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Thanks...The schematic of the thermostat in that link will be handy.

Jeff

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Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)
The speed of light is 1.8*10^12 furlongs per fortnight.

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.

Taking another closer look at the schematic in that link you gave me shows the "arrow half" of the symbol for that diode, but it was very easy for me to overlook. (And perhaps you too, since you thought I'd need a 24 volt bulb. )

Jeff

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Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)
The speed of light is 1.98*10^14 fathoms per fortnight.

On Sun, 16 Nov 2008 12:17:08 -0500, Jeff Wisnia put finger to keyboard and composed:

Yes, I missed it. But even if I had seen it, I would have still needed to be told that it was a diode.

- Franc Zabkar

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Please remove one \'i\' from my address when replying by email.

But, I think I may have grabbed a bulb from the kid's model RR junkbox that had a higher voltage rating than the 12-14 volts I thought it had.

Mentioning the successful repair to someone else got me a response that running a 120 volt bulb off of 240 vac in series with a rectifier is NOT equivalent to running it off of 120 vac without the rectifier and the bulb will blow out rather quickly, because its filament is overvoltaged quite a bit of the time.

I've got to think that one out a few more times before I'm ready to say yes or no to it. Perhaps someone reading this post can pouint me to a cite on the subject.

Jeff

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Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)
The speed of light is 1.8*10^12 furlongs per fortnight.

There was a lot of discussion, and arguments on news:rec.antiques.radio+phono a while back. Basically, a diode removes one half the cycle, or applies power for half the normal time. you have to multiply by .707 to get the equivalent voltage.

240 VAC *.707 gives an equivalent of 169.68, minus the .6 volt drop across the diode, which leaves approx. 169 volts. Channel Master built some B&W tvs with a 'dropping diode', and a series filament string totaling 84 volts.

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