110v bulb to bleed 24v DC cap?

[andybb]

Can I use a 110v bulb to bleed a cap charged with 24v dc current??

 

[picbits]

I don't see why not

 

[andybb]

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]

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]

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.

 

[andybb]

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]

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.

 

[picbits]

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 ?

 

[Nigel Goodwin]

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]

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]

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" !

 

[Nigel Goodwin]

I actually went to find a bulb before posting this - couldn't find anything non "electronic" !

Likewise

Back when I was repairing TV's I always kept a 60W bulb on hand, with a couple of wires soldered to it, I used it as a dummy load on countless applications.

 

[OBW0549]

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:

 

[picbits]

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]

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]

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]

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]

Why not use a 3 or 7W bulb (120V or 240VAC) which will have a lot more resistance than your 100W bulb.

 

[Musicmanager]

Sometimes the old things are cool.

There you go AG, there's hope for you yet !!

 

[gophert]

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.