Electronics > Beginners
Why does the "dim bulb test" limit current instead of adding load to the supply?
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IanB:
Also, nobody seems to have directly mentioned that two 100 W, 120 V bulbs in series becomes equivalent to a 200 W, 240 V bulb. So then you can just obviously ask the question, what happens if you attach a 200 W, 240 V bulb to a 120 V supply?

(But, really, the OP's question is so far out of left field that I have no clue where it came from. I think Beamin just asked it as a joke to wind everyone up. I seem to recall she has done the same kind of thing before.)
amyk:
I agree with the others here than this feels more like a bad trolling attempt.

But in an attempt to enlighten(!), consider this: if you think a 100W bulb will always consume 100W of power, regardless of the voltage across it, then what happens at 0V? Do they light when disconnected from a socket and have their terminals shorted to each other? :o


--- Quote from: IanB on August 18, 2018, 01:50:07 pm ---It is a curious thing that a filament bulb can act as an approximate constant current regulator. It was once used as such in lead acid battery chargers, limiting charging current when the battery was empty and dropping out when the battery was in the CV stage.

--- End quote ---
https://en.wikipedia.org/wiki/Iron%E2%80%93hydrogen_resistor
james_s:

--- Quote from: IanB on August 18, 2018, 01:50:07 pm ---It is a curious thing that a filament bulb can act as an approximate constant current regulator. It was once used as such in lead acid battery chargers, limiting charging current when the battery was empty and dropping out when the battery was in the CV stage.

--- End quote ---

They are used as ballasts in self-ballasted mercury vapor lamps too. Not nearly as common anymore as they used to be but they are still available. They contain a standard mercury arc tube in series with a tungsten filament in the same envelope. When power is first applied the arc tube voltage is low and the filament lights brightly but as they warm up the filament gradually dims as the arc tube gets brighter. At one point they were an economical replacement for incandescent lamps that did not require the installation of expensive ballasts. Efficiency was not much better than incandescent but they did last 10 times as long. 
Beamin:

--- Quote from: IanB on August 18, 2018, 04:04:57 pm ---Also, nobody seems to have directly mentioned that two 100 W, 120 V bulbs in series becomes equivalent to a 200 W, 240 V bulb. So then you can just obviously ask the question, what happens if you attach a 200 W, 240 V bulb to a 120 V supply?

(But, really, the OP's question is so far out of left field that I have no clue where it came from. I think Beamin just asked it as a joke to wind everyone up. I seem to recall she has done the same kind of thing before.)

--- End quote ---


I always just kind of built circuits from examples and never really got into the basics that much. It's a problem of the Arduino generation where to blink an LED requires writing lines of code. But sometimes I get these seizures and it will wipe out some bits of my memory. Like I know more about RF and making antennas then some of the more simple concepts. I tend to focus on really specific things like radio stuff. Most of my circuit building is just guestimating the parts from a schematic and physically trying different values. The soldering and putting in a nice box is the fun part so I haven't spent much time recently relearning ohms law again for the who knows how many times.  Unfortunately with my condition I never know what I am going to forget. I was also thinking the light bulb was drastically changing in resistance from orange to white that just happened to be a really good way to current limit. One of those "It just happens a light bulb works  by a fluke (27) of nature". I try not to troll this forum even though 90% will find it funny 10% will click report. Where would find a data sheet for something like tungsten's temp coefficient? I tried light bulb manufactures but their customers really only  care about on and off.

So what is physically different in a 12 and 120Vlight bulb? They are all tungsten do they use longer lengths or thicker filaments?



--- Quote from: Hero999 on August 18, 2018, 01:47:31 pm ---
--- Quote from: Audioguru on August 18, 2018, 01:40:55 pm ---
--- Quote from: Rerouter on August 18, 2018, 01:17:07 pm ---a 100W bulb when it is cold draws more than 100W, as the coppers resistance is lower while its cold
--- End quote ---
The filament is not copper that would melt, it is tungsten. The resistance of tungsten is also lower when cold.

--- End quote ---
Yes and the resistance of all metals has a positive temperature coefficient.

--- End quote ---

Are there some material with negative temp coefficient that results in runaway? Isn't that's what transistors and PN junctions do?


I know I have asked this before but what are good examples of beginner versions of the spice program? There was one where you would see red and green lines that show current flowing through them. Are there any that you can download rather then web base? I have lost the URL but since you can't save your circuits it gets annoying. I learn best by seeing visual like in those youtube videos by that Russian guy using whats sounds like a computer voice. Potential is shown as raised lines and current is dots that move around.

https://youtu.be/G3H5lKoWPpY


Spice is too technical where it gives you the box with numbers in it, very useful if you are doing complicated stuff but I need something in between that and the web based tool. One where you could simulate poking a scope probe around or DMM.


So "series string TV sets" use the filaments that all total 120 volts each one dropping down. So the first tube sees the full 120 volts then the next 90 and so on or is it an average; say 10 12V tubes all in series. But if it has a transformer then each heater is run in parallel.


While we are on heating up wires: If I want to build a 120V isolation transformer using a torroid I would just take 10 turns of 12 ga wire on one side and 10 turns 12ga on the other? When this is plugged in is there enough magnetic field induced that the wire on the line side wouldn't turn into a heater when the secondary/load side is disconnected? Does the creation of the magnetic field make it efficient enough to carry away most of the current energy into magmatism to stop it from burning up in a hurry?


Thanks for the patience in all that answered seriously rererelearning things is frustrating as hell but I often have luck rereading my own threads here.
james_s:
A thicker filament requires higher current, a longer filament requires higher voltage. If you take 10 12V bulbs that draw 1A and wire them in series, you are stringing those 10 filaments together and you end up with a 120V bulb that draws 1A, ie a 120W bulb. The filament diameter is the same as any bulb that draws 1A, the length will depend on the rated voltage.

If you wanted to make a bulb of a given wattage, you would decide on the voltage you want it to operate from, then use Ohms law to calculate the current required for the desired wattage at the selected voltage, then you would select a filament diameter corresponding to that current, then select the length for the voltage you want.

The reason many specialized lamps such as those for slide projectors and other applications requiring precise optical control use odd voltages is that they need to use a short filament for optical reasons and this dictates using a low voltage. There are other factors too, a long thin filament is mechanically fragile and suffers a greater amount of thermal losses, especially convective losses for gas filled lamps. This is why 240V incandescent lamps are substantially less efficient than 120V lamps. You can eliminate the gas fill and use a vacuum but then your tungsten evaporation rate is much higher resulting in shorter life. Everything is a compromise.
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