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110v bulb to bleed 24v DC cap?

Thread starter #3
picbits Thank you. I was considering a few options to hook to a momentary switch to bleed it. An LED with a resistor, a 24v fan or just using a small Xmas light bulb which doesn't require me to spend any $ or have to order something online since Radio Shack is non-existent. I have a bleed resistor but adding the bulb would allow me to know/see if they are bled.
 

audioguru

Well-Known Member
Most Helpful Member
#4
You never mentioned the capacitance. If it has a low capacitance then the light duration will be too small for it to warm up.
The Christmas tree lights I used 10 years ago (all LEDs now) were 2.5V I think with 48 of them in series to make them light from 120V.
What will happen to a 2.5V light bulb with 24V on it?
 

Pommie

Well-Known Member
Most Helpful Member
#5
What is the cap supplying? Will the existing components not drain it anyway? Why not just use a resistor, a 110V bulb is rather bulky for such a thing.

Mike.
 
Thread starter #6
It is bleeding a 100,000uf 50V cap in a mig welder that I am converting from AC to DCEN. I can use a small bulb. It will be inside the welder cabinet. Actually, I guess the easiest thing to do is just use the 100W 15ohm bleeder I have and put a meter across it once and see how long it takes to bleed it down and just make sure I hold the switch down that long whenever I'm done. I'm using the switch because I don't want the caps to discharge each time I let go of the trigger but want to bleed it when I'm done. The light just provides visual confirmation. I'm not an EE so this is related to this post. https://www.electro-tech-online.com/threads/momentary-switch-and-led.121208/page-2#post-1346083

I am open for suggestions though.
 

MrAl

Well-Known Member
Most Helpful Member
#7
Hi,

I've done such things with light bulbs in the past. They are resistors when they dont light up.
The resistance of a typical 100 watt bulb is around 10 ohms i think.

I've even used one for a heater to make a heat box oven for testing things at elevated temperatures.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
#9
I would expect a 110v 100w bulb to take around 0.9A (at 110v) and as V=IR or R=V/I then maybe 120R ?
Bulbs aren't resistors - and the resistance at a much lower voltage will be MUCH lower than that at it's rated voltage.

This is why old bulbs commonly blew when turned on, as they take a large surge before they warm up and their resistance increases.

If you stick a meter across a cold bulb you can easily measure the cold resistance.

Or for this application, simply try different wattage bulbs? and see which works best.
 

alec_t

Well-Known Member
Most Helpful Member
#10
I guess the easiest thing to do is just use the 100W 15ohm bleeder I have and put a meter across it once and see how long it takes to bleed it down
It takes 5 secs to discharge to about 1V, or 2 secs down to about 6V.
 

picbits

Well-Known Member
#11
Bulbs aren't resistors - and the resistance at a much lower voltage will be MUCH lower than that at it's rated voltage.

This is why old bulbs commonly blew when turned on, as they take a large surge before they warm up and their resistance increases.

If you stick a meter across a cold bulb you can easily measure the cold resistance.

Or for this application, simply try different wattage bulbs? and see which works best.
I actually went to find a bulb before posting this - couldn't find anything non "electronic" !
 

OBW0549

Active Member
#13
I would expect a 110v 100w bulb to take around 0.9A (at 110v) and as V=IR or R=V/I then maybe 120R ?
The resistivity of tungsten varies with temperature, roughly 0.4% per degree K, so the resistance of an incandescent lamp at its operating temperature of roughly 2800K is much higher than its cold resistance.

I made some measurements of I vs. V on a few incandescent light bulbs and plotted their resistance versus voltage. For a 100 watt, 120 volt bulb, this was the result:

100Watt.png
 

picbits

Well-Known Member
#14
I've just measured a 100W 240V bulb resistance and yes, it is pretty low at 40R (took me ages to find an "old school" bulb) so with a 100W 110V bulb a 10R would be perfectly feasible (I stand corrected).
 

picbits

Well-Known Member
#15
I wonder if I was thinking about the halogen car bulbs I was playing with - I might have to measure those next time I'm out in the workshop.
 

audioguru

Well-Known Member
Most Helpful Member
#16
I actually went to find a bulb before posting this - couldn't find anything non "electronic" !
Yes, antique things are rare. Some people still play vinyl records with amplifiers that use vacuum tubes. Remember them?
I still have a portable MP3 player that uses a heavy hard drive in it.
 

JimB

Super Moderator
Most Helpful Member
#17
Yes, antique things are rare. Some people still play vinyl records with amplifiers that use vacuum tubes. Remember them?
Some people drive old cars, with carburetters, kettering ignition and no catalytic converter, for fun
Some people ride horses for fun.
Some people walk and run for fun.
Some people play with old radios with vacuum tubes and transmit SSB and CW (morse), for fun.

Sometimes the old things are cool.

JimB
 

gophert

Well-Known Member
Most Helpful Member
#20
Yes, antique things are rare. Some people still play vinyl records with amplifiers that use vacuum tubes. Remember them?
I still have a portable MP3 player that uses a heavy hard drive in it.
I must say that the young, aspiring engineers that I know all look on EE-related forums for answers/explanations to their problems (most don't even have accounts). The funny thing is, they all seem to know the name "audioguru" and his Uncle Scrooge avatar.
 

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