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Posted by Stormin Mormon on November 17, 2008, 11:12 am
Last time I measured the resistance of a half wave rectifier for a DC bulb,
the half wave rectifier was about 2k ohms resistance. Depending on the
amperage draw of the bulb, that could be the correct dropping resistor
value.
--
Christopher A. Young
Learn more about Jesus
www.lds.org
.
Stormin Mormon wrote:
> Last time I used a VOM on a diode, it was about 2K ohms. Might be the
> right
> value for a dropping resistor in this case. Depending on the miliwatt draw
> of the bulb. Could work. Could blow out. He'll find out.
Its a halfwave rectifier for a DC bulb.
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Posted by Jeff Wisnia on November 17, 2008, 3:24 pm
Mark wrote:
>>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
>>-
>
>
> that is a common wrong mis-conception. A 12V bulb hooked up to 24 V
> AC with a diode does not see 12 V. It sees 24 Volts for 1/2 of the
> time. During the time it sees 24V, it is getting 4x the power because
> P=V^2/R. But since it seeing 4x power only half the time, the net is
> 2x power. So a 12V bulb hooked up to 24 VAC through a diode is
> seeing 2x the correct power, it should be very bright and will burn
> out soon.
>
> If you don't belive me, try it with a 120V bulb and a diode hooked up
> to 240V. It does not work. The bulb will burn out in a short time.
>
> Mark
>
>
Mark, I think you may have got me there!
I probably got lucky when I grabbed bulb from the leftovers from the
kid's model railroading experiences of some 20 years ago, and it
probably was one intended to be run on higher than 12-14 volts. (No
markings on it.)
But, after I soldered it in I looked at it lit (from the back) before I
replaced the thermostat on the wall and compared its color to that of
the "emergency heat" bulb when that one was turned on. The filament of
the bulb I installed was lit to just about the same color as the
"emergency heat" one, so I guess it's a "right fit", huh?
Jeff
--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.8*10^12 furlongs per fortnight.
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Posted by Mark on November 17, 2008, 6:56 pm
> Mark wrote:
> >>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
> >>-
> > that is a common wrong mis-conception. =A0A 12V bulb hooked up to 24 V
> > AC with a diode does not see 12 V. =A0It sees 24 Volts for 1/2 of the
> > time. =A0During the time it sees 24V, it is getting 4x the power becaus=
e
> > P=3DV^2/R. =A0But since it seeing 4x power only half the time, the net =
is
> > 2x power. =A0 So a 12V bulb hooked up to 24 VAC through a diode is
> > seeing 2x the correct power, it should be very bright and will burn
> > out soon.
> > If you don't belive me, try it with a 120V bulb and a diode hooked up
> > to 240V. =A0It does not work. =A0The bulb will burn out in a short time=
.
> > Mark
> Mark, I think you may have got me there!
> I probably got lucky when I grabbed bulb from the leftovers from the
> kid's model railroading experiences of some 20 years ago, and it
> probably was one intended to be run on higher than 12-14 volts. (No
> markings on it.)
> But, after I soldered it in I looked at it lit (from the back) before I
> replaced the thermostat on the wall and compared its color to that of
> the "emergency heat" bulb when that one was turned on. The filament of
> the bulb I installed was lit to just about the same color as the
> "emergency heat" one, so I guess it's a "right fit", huh?
> Jeff
> --
> Jeffry Wisnia
> (W1BSV + Brass Rat '57 EE)
> The speed of light is 1.8*10^12 furlongs per fortnight.- Hide quoted text=
-
> - Show quoted text -
yep sounds like you are OK then...
I remember this from a long time ago we had a temperature chamber that
ran off of 240V and used a 240 V bulb inside for light...the bulb
burned out and we didn't have a 240 V bulb to replace it...one of the
engineers got the bright idea to use a diode and a standard 120 V
bulb, seemed logical,,, so we all got to learn by experience why that
didn't work...
Mark
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Posted by Don Ocean on November 17, 2008, 10:33 pm
Jeff Wisnia wrote:
> Mark wrote:
>
>>> 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
>>> -
>> that is a common wrong mis-conception. A 12V bulb hooked up to 24 V
>> AC with a diode does not see 12 V. It sees 24 Volts for 1/2 of the
>> time. During the time it sees 24V, it is getting 4x the power because
>> P=V^2/R. But since it seeing 4x power only half the time, the net is
>> 2x power. So a 12V bulb hooked up to 24 VAC through a diode is
>> seeing 2x the correct power, it should be very bright and will burn
>> out soon.
>> If you don't belive me, try it with a 120V bulb and a diode hooked up
>> to 240V. It does not work. The bulb will burn out in a short time.
>> Mark
>
> Mark, I think you may have got me there!
>
> I probably got lucky when I grabbed bulb from the leftovers from the
> kid's model railroading experiences of some 20 years ago, and it
> probably was one intended to be run on higher than 12-14 volts. (No
> markings on it.)
>
> But, after I soldered it in I looked at it lit (from the back) before I
> replaced the thermostat on the wall and compared its color to that of
> the "emergency heat" bulb when that one was turned on. The filament of
> the bulb I installed was lit to just about the same color as the
> "emergency heat" one, so I guess it's a "right fit", huh?
>
>
> Jeff
God looks out for fools and drunks.. ;-)
>
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Posted by Fartikus on November 17, 2008, 11:15 pm
> Jeff Wisnia wrote:
>> Mark wrote:
>>>> 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
>>>> -
>>> that is a common wrong mis-conception. A 12V bulb hooked up to 24 V
>>> AC with a diode does not see 12 V. It sees 24 Volts for 1/2 of the
>>> time. During the time it sees 24V, it is getting 4x the power because
>>> P=V^2/R. But since it seeing 4x power only half the time, the net is
>>> 2x power. So a 12V bulb hooked up to 24 VAC through a diode is
>>> seeing 2x the correct power, it should be very bright and will burn
>>> out soon.
>>> If you don't belive me, try it with a 120V bulb and a diode hooked up
>>> to 240V. It does not work. The bulb will burn out in a short time.
>>> Mark
>> Mark, I think you may have got me there!
>> I probably got lucky when I grabbed bulb from the leftovers from the
>> kid's model railroading experiences of some 20 years ago, and it probably
>> was one intended to be run on higher than 12-14 volts. (No markings on
>> it.)
>> But, after I soldered it in I looked at it lit (from the back) before I
>> replaced the thermostat on the wall and compared its color to that of the
>> "emergency heat" bulb when that one was turned on. The filament of the
>> bulb I installed was lit to just about the same color as the "emergency
>> heat" one, so I guess it's a "right fit", huh?
>> Jeff
> God looks out for fools and drunks.. ;-)
Then this group is in good shape.
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> right
> value for a dropping resistor in this case. Depending on the miliwatt draw
> of the bulb. Could work. Could blow out. He'll find out.