Electronics > Beginners

Why does the "dim bulb test" limit current instead of adding load to the supply?

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Vtile:

--- Quote from: Beamin on August 18, 2018, 12:59:19 pm ---This always seemed backwards to me. I always though that if you took a 100 watt load (light bulb) and put it in series with say another 100 watt load electronic device that you would pull 200 watts out of your power supply. But in reality the most it pulls is 100 then when the load is added the current stays at 100 but dims the bulb? To figure out which device; bulb or load gets what % of the hundred watts you would have to measure the ohms of the bulb while its hot and your electronic device? Then E=IR it? Do bulb behave as temp controlled variable resistors? Or is there something special about the bulb where its resistance changes in proportion to filament temp? I always thought 100 watts + 100 watts = 200 watts draw. With no real light bulbs to test I can't do this.


Also instead of a 100W light bulb that's illegal can I substitute it with a CFL with a really high CRI that says it's as bright as a 100 watt on the box?  :-DD Will I get arrested if I plug in a 100 watt filament bulb by the signals it sends to the power company? :)

--- End quote ---
(V:100Vdc)---[100R]---[100R]---(GND)  Voltage over the two resistors are 100V.
(V:100Vdc)---[100R]---[666R]---(GND)  Voltage over the two resistors are 100V.
(V:100Vdc)---[1R]---[100R]---(GND)  Voltage over the two resistors are 100V.
(V:100Vdc)---[100R]---(GND)  Voltage over the one resistor is 100V.

The trick is that the total power will be the inverse sum of the impotences in series connection (in voltage dominant network). However in current dominant network the total impotence would be the inverse sum of powers.

Vtile:
One of our member kindly pointed out that I made a typo with the electrical 'dances', lets go through some of them...

Resistance
Conductance
Impedance
Reactance
Admittance
Susceptance
Impotense

Uh...

Impotense = 1/P

🤣

Beamin:

--- Quote from: Vtile on August 19, 2018, 08:45:33 pm ---
--- Quote from: Beamin on August 18, 2018, 12:59:19 pm ---This always seemed backwards to me. I always though that if you took a 100 watt load (light bulb) and put it in series with say another 100 watt load electronic device that you would pull 200 watts out of your power supply. But in reality the most it pulls is 100 then when the load is added the current stays at 100 but dims the bulb? To figure out which device; bulb or load gets what % of the hundred watts you would have to measure the ohms of the bulb while its hot and your electronic device? Then E=IR it? Do bulb behave as temp controlled variable resistors? Or is there something special about the bulb where its resistance changes in proportion to filament temp? I always thought 100 watts + 100 watts = 200 watts draw. With no real light bulbs to test I can't do this.


Also instead of a 100W light bulb that's illegal can I substitute it with a CFL with a really high CRI that says it's as bright as a 100 watt on the box?  :-DD Will I get arrested if I plug in a 100 watt filament bulb by the signals it sends to the power company? :)

--- End quote ---
(V:100Vdc)---[100R]---[100R]---(GND)  Voltage over the two resistors are 100V.
(V:100Vdc)---[100R]---[666R]---(GND)  Voltage over the two resistors are 100V.
(V:100Vdc)---[1R]---[100R]---(GND)  Voltage over the two resistors are 100V.
(V:100Vdc)---[100R]---(GND)  Voltage over the one resistor is 100V.

The trick is that the total power will be the inverse sum of the impotences in series connection (in voltage dominant network). However in current dominant network the total impotence would be the inverse sum of powers.

--- End quote ---

I think I see what you getting at but I'm losingsomething in translation.

Also my original question was more then just the simple ohms law question but I was thinking that a tungsten filiment had some special property where the change in temp vs resistance was coincidentally proportional to the way it regulated voltage. Like how the diameter of the moon just happens to make a perfect eclipse, or the earth is just the right size to resovle mm wavelengths like they are using in the event horizon telescopes; a flukeof nature. I also like how meters nicely match frequency vs wave length in 10's and 3's due to the 300,000 km/s of light. 300Mhz = 1 meter 100mhz = 3m 30Mhz= 10 meters how easy is that?

sokoloff:
Taking only your subject line question, the answer is "for the same reason that a dropping resistor on an LED limits current rather than adding load to the supply".

ebastler:

--- Quote from: sokoloff on August 21, 2018, 06:48:28 pm ---Taking only your subject line question, the answer is "for the same reason that a dropping resistor on an LED limits current rather than adding load to the supply".

--- End quote ---

Or: "Because it is connected in series with the device unter test, not in parallel."

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