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Capacitor discharge in pulses - Questions

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Sparky_s

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I'm just experimenting with electronics and I'm thinking in the possibility to make a simple test.
I want to lit a light bulb (a 12 V one) from the energy stored in a capacitor. I know that is possible, but the question is:
Is it possible to run the light bulb using small amounts of the energy stored in the capacitor? I mean, instead of using all the energy stored in the capacitor at once, discharge the capacitor little by little, instead of at once.

The circuit basically lits a light bulb using the energy from the cap, in small amounts instead to use all the energy in the capacitor in the same time.
 
the filament in the bulb is going to behave like a resistor (if you were to leave the bulb on and let it heat up, the impedance will decrease). Once connected to the capacitor the bulb will likely turn on brightly (depending on the voltage across the capacitor) and then quickly dim out exponentially. I doubt that chopping the signal will help much - it would result in the dimming being stretched out but the initial brightness would be diminished due to the average power being lower.
 
Thank you very much, so I think it's possible to build a circuit to perform that. I was not sure if it was possible to build that.
I knew it was possible to build a circuit to lit the light bulb using the energy from a capacitor, but I was not sure about using a small fraction of that energy of the capacitor. I mean, using only a small amount of the energy stored in the cap to lit the bulb.
 
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The following isn't meant to degrade or disparage you; merely to educate you some.

The word "lit" is the past-tense of "light" - ie: "Light that light bulb, please." vs. "The light bulb was lit." - understand?

:)
 
Start thinking about an flashing LED ... this one, it will do the job by it self.
But the timing will be fixed.
 
Your idea works better with a LED since the current requirement is constant whereas the current in the lamp is high at first when the filament is cold.
 
The idea has a major flaw. Each subsequent time the switch is closed, the current through the LED is smaller because each pulse leaves less charge in the capacitor. Look at the plots of the remaining capacitor voltage, and the current though the LED each time the switch closes....
 

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Not if you use a constant current supply...
 
The following isn't meant to degrade or disparage you; merely to educate you some.

The word "lit" is the past-tense of "light" - ie: "Light that light bulb, please." vs. "The light bulb was lit." - understand?

:)
Thank you so much for the correction. As you may know, english is my second language.

Your idea works better with a LED since the current requirement is constant whereas the current in the lamp is high at first when the filament is cold.

Yes, but I'm mainly interested to light a bulb instead of a LED. But your comment can help me in a future.

The idea has a major flaw. Each subsequent time the switch is closed, the current through the LED is smaller because each pulse leaves less charge in the capacitor. Look at the plots of the remaining capacitor voltage, and the current though the LED each time the switch closes....

Yes, after thinking a bit about the comment I made, I thought about that.
Maybe the solution for that would be to use only the first portion of energy to light the bulb and then recharge the cap again?
I mean, to charge a cap with 15Volts (for example) and then use only the first pulse to light the bulb. This first pulse is using only a part of the energy stored in the cap.
After using the first (and unique) portion of the energy of the cap, the cap is re-charged again to full it with energy. Then the operation is repeated again and again. Instead of using all the energy in the cap pulsing all the energy stored in it (I mean to pulse the cap up to the cap is empty), use only the first pulse (the more energetic one) and then recharge the cap.
But here there is a second question....
Is useful or there is any problem if I try to recharge the cap without discharging all the energy stored in it?
If I use the first part of the energy of the cap, then there is still energy in the cap while the cap is recharging.
 
Hi, Sparky

there's no problem to " top charge " ( say add charge to full capacitor capacity ) in between, but, as previously told, You'll need a high value capacitor as most of the energy you will use will be to heat the filament of the bulb ... while producing no visible light !!!

If I understand you want to blink a lamp (???) ... a solution is to let a small amount of current flow through the bulb ( say 7 or 8 volts supply, i.e ), just to keep it to the limit filament is visible, - and just let extra current flow to produce light ( full 12 volts supply ).

the overall energy draw will be at it's minimum ... and the lifetime of your lamp will seriously be increased ...

Alain
 
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Hi,


The tungsten filament light bulb has a unique property that many other electronic lighting components like this dont possess. That is, they have an unusual built-in storage mechanism that allows them to glow for a period of time even after the current stops flowing through the filament. This is due to the heat capacity (however small) of the filament. The filament stores heat and takes time to cool off so the resistance stays higher if the pulses of current are close enough together.
This means that for a pulsing current the average resistance will be higher than that for a bulb that is not conducting at all, even during the time between pulses. The only requirement in order to make a difference is that the pulses be close enough together to not allow the filament to cool off too much, and that can be accomplished quite easily with a high enough frequency.
Note however that with a duty cycle under 100 percent the bulb filament will still not have as high a resistance as when it is driven fully 'on', but it will be higher than just when it is off totally. This means each new pulse does not have to bang a cold filament each time and so does not have to deliver the full start up current. The actual difference will of course depend on the actual bulb used and the frequency, something you might want to experiment with for your actual bulb.

The resistance is of course very nonlinear too though with resistance decreasing with voltage, so no matter how you keep the filament a bit hotter than when 'off' the resistance could still be much lower than when fully 'on' which means more current anyway during the pulse...it just wont be as high as we would expect for a totally cold bulb.

If you drive a bulb with the energy stored in the capacitor that is ok, but you have to realize that eventually the cap runs down and you have to charge it again.

Some pulsing applications like this are made mostly just to dim the bulb, so better help you might tell us more about what you want to use this for.

Interesting, some of the larger bulbs you can actually see the filament glow for a short time period after you turn it off. It lasts maybe one second or less, but for some of the very large 12v bulbs you can see them glow for maybe 2 seconds after turn off.
 
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It takes time for an incandescent light bulb to heat enough to produce light. It takes a lot of energy because most of the power to a light bulb makes heat, not light. Before the light bulb is lit then the capacitor charge will be gone.
 
@Sparky
The best example of an bulb and capacitor , are the flash units that we use in photography.
Study those and you will discover your Atlantis ..
 
@Sparky
The best example of an bulb and capacitor , are the flash units that we use in photography.
Study those and you will discover your Atlantis ..
In Canada we haven't used incandescent camera flash bulbs for about 30 or 40 years. But maybe in Greece you still use gunpowder or heat an incandescent bulb to make the flash of light. We use very bright LEDs.
 
In Canada we haven't used incandescent camera flash bulbs for about 30 or 40 years. But maybe in Greece you still use gunpowder or heat an incandescent bulb to make the flash of light. We use very bright LEDs.

Well from your words , it looks that in Canada , you take pictures with cell phones,
and not using real cameras of 1000-2000$ worth ..

Is it true that up there, your only best friend are only the Bears ..
 
The very bright professional camera flash bulbs are xenon, not incandescent. They use incandescent lights that are continuously lighted, not flashed. Modern personal digital cameras use bright LEDs.
I remember when personal cameras used film and the flash bulbs were incandescent and actually burned out to make the flash so they were used only 1 time.

Bears in Canada are in the woods and in the zoo, not in the cities. In the cities we have many squirrels, some rabbits, some racoons and a few skunks.
 
Modern personal digital cameras use bright LEDs.

I do not know from where you buy cameras , but sorry, their are not that high tech.
LEDs are toys for small children s .

Real TTL flash units , that blow anything at 20 meters like My digital Olympus FL40 , works with an specific manner.

This was my suggestion from the start.

The TTL system can adjust the energy that the bulb will use , at 40 steps ,
so at the low ones , it uses only some of the capacitors energy.

Well Canadian , you have to go back to school.
 
Kirakos,
You are talking about professional cameras and a very high power separate flash unit. They are extremely expensive.
I am talking about modern inexpensive personal cameras that have an LED flash built-in. Mine is made by Hewlett-Packard, is 6 years old and sold for $330. Sony makes similar cameras. They cost much less today.
The original poster in this thread is talking about lighting an incandescent light bulb from a capacitor.
 
Ok guys,
Thanks to each one for the replies. Everyone has helped me a little bit.

To start with, I want to create a very simple circuit. Basically I want to light a light bulb using only the energy from a capacitor.
I will start with a simple 12Volt light bulb.
 
A capacitor can supply a low current for a short amount of time unless it has a high capacitance. Its voltage drops as it is discharged.
But an incandescent light bulb uses a fairly high current for a fairly long time (long enough for it to heat up).
 
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