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Old Sep 21, 2001, 03:52 PM
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Colorado
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Capacitors :confused:

Just a few questions

The largest amount of energy a capacitor can hold is at the specifide voltage right?

Can the capacitors handle higher voltages?

Can they really be fully charged at lower voltages?

And if so what if the voltage was lower to begin with and when charged the voltage went up?

Thanks.
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Old Sep 21, 2001, 04:45 PM
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London UK
Joined Sep 2001
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Capacitors will charge until they reach the voltage placed across the terminals.

The maximum voltage that a capacitor can handle is usually printed on the case.

The amount of charge (equates to current) that a capacitor can hold is fixed although at higher voltages there is more energy (watts) stored by the device.

I hope this helps

Neil
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Old Sep 21, 2001, 09:12 PM
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Rochester,MN
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A capacitor is like a bucket that holds water, only the capacitor holds electrons not water. The capacitor is made of parallel plates with an insulator between the plates. If the voltage gets to high the insulator breaks down and the capacitor can blow-up like a firecracker. This is where the voltage rating comes from. It's the voltage where the capacitor blows up. To charge the capacitor (fill the bucket) it takes a certain amount of current (like water flow) for a certain amount of time. You don't want to charge a capacitor up to its voltage rating. There would be to much risk of it blowing up. It isn't like a battery that you want to charge as full as possible.

You can get the same charge in a small capacitor at a high voltage or a large capacitor at a low voltage.

Q = C x V where Q is charge, C is capacitance, and V is the voltage. To get more charge, Q, you can increase the voltage, V, or Capacitance, C.

Does this help?
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Old Sep 21, 2001, 09:31 PM
rebmeM roineS
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Rockingham, Perth - Western Australia
Joined Apr 2001
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Can the capacitors handle higher voltages?

It is not a good idea to exceed a capacitors voltage rating....or it will go boom. Especialy if your using electrolytic 'can' type caps or tantalum 'bead' type. I've seen a demonstration at college of a few of these explode violently, shooting the caps can over 20 feet across a room.
Ive even heard of bombs being made with large electrolytics.

Use a cap rated about 25%-50% higher than the intended operating voltage.
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Old Sep 22, 2001, 03:23 AM
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Using a capacitor as an energy source is fairly messy. Batteries are much easier. For some capacitor types the quoted voltage is not the working voltage but the breakdown voltage. Working voltage should be much lower. Many capacitors also do not like large currents, especially ripple currents (e.g. like the PWM that we normally drive motors with).

What is it that you are trying to do and what type of capacitors are you using ?

Steve
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Old Sep 22, 2001, 03:38 PM
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Well it's actually for a model train engine (though it might have use in planes,) and I want to filter out the small gaps in electricity of the track. So I tried a polarized 16v 3300uf and it only seemed to have an effect when the voltage neared 16 volts, not when needed! Oh well, maybe it won't work, I was more curious about capacitors themselves anyway. Thanks.
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Old Sep 22, 2001, 05:34 PM
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Surrey, BC Canada
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You should be useing 8 mico farrad caps at about 40 volts, this will smooth out the ripple on your track voltage. The way it works is as follows, voltage rises and charges cap to the peak voltage, voltage drops and cap discharges to maintain peak voltage until next ripple. You can place caps in parralel to improve voltage output (IE two caps will provide a smoother output voltage than one).
Robert
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Old Sep 24, 2001, 05:23 PM
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Rhode Island
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You don't want to use a filter cap on a model train locomotive, you won't be able to get it to run as slow. The better train power packs make the train run at creeping speeds by pulsing full power at a low duty cycle similar to the way our ESC's work at low throttle. A capacitor will defeat the pulsing by filtering it.
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Old Sep 25, 2001, 07:22 PM
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Colorado
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BobK, what you're saying does makes sense. I did notice a power loss when I attached the cap using a digital speed controller and not as much with a cheap speed controller. I still might try it but most of what I was trying to do has been defeated, oh well. Thanks guys.
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Old Sep 26, 2001, 03:09 PM
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I'll agree with Bob on this, a electroltic cap will make certain model train motors start hard if the cap is paralell to the throttle output BEFORE it gets to the reversing switch.. For heavens's sake be careful where you solder that cap...! If its direct to the track and you apply reverse voltage, you'll blow the cap, or damage the throttle or both..

My experience with train motors has been that most of the cheaper engines, i.e. Bachmann, Yugoslav AHM, Life-like,..etc. have three pole motors that cog real badly, and they need some sort of low frequencey kick like a half wave ac ripple superimpossed on the dc out to get the train to do a scale "creep". Better Sagami can motors and the better brass engine open frames work better with a more filtered DC. A higher frequencey PWM throttle will work well with these motors. As a side point don't do hours of slow creeping with any motor you treasure, with the cheaper speed throttles as the half wave DC will heat the motor too much.

Looee
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