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Capacitor charge and discharge Rate

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Electronman

New Member
Hello,

Can we charge and discharge the capacitors (specially the electrolyte ones) suddenly? I mean charge them directly via the power source and without any resistor and discharge them by shorting their Pins.
 

colin55

Well-Known Member
Yes. You can do this but it may cause them to "short-circuit" or "go open" due to the high current entering and leaving wwhere the loeads join the foil. They will also heat up and this may cause them to explode or burst. Keep doing it until something happens.
Capacitors are used like this to do "capacitor welding."
 

dougy83

Well-Known Member
You can. It may shorten its life. If you are going to charge and discharge the caps repeatedly, you should have a look at the cap's rated ripple current. As you apply a higher ripple current, the cap will heat up and possibly leak, deform or explode.
 
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colin55

Well-Known Member
What are you talking about?
Ripple current is only a few milliamp.
Charge and discharge current is tens of amps.
 

dougy83

Well-Known Member
@colin55: When I posted I hadn't read your post.

As for ripple current, it can be of high amplitude also. Ripple current on a capacitor results in the capacitor charging and discharging.
 

Electronman

New Member
What about large ones use in TV power supplies? Is there any ready device to discharge them?

The second question is do capacitors have any resistance in series with the capacitor when they are operating in DC supplies like batteries?
 

dougy83

Well-Known Member
What about large ones use in TV power supplies? Is there any ready device to discharge them?
You can use a moderate wattage resistor, maybe 5W, 1k ohm, connected to leads and insulated. Hopefully it won't burn out. Some high voltage capacitors have bleeder resistors across them to reduce the charge in a few minutes after power down.


The second question is do capacitors have any resistance in series with the capacitor when they are operating in DC supplies like batteries?
Do you mean internal resistance? All capacitors have parasitic series resistance or Equivalent Series Resistance (ESR).
 

Sceadwian

Banned
The ESR varies with frequency, and I think it tends to be relatively high with straight DC use. How bad that kind of use is for the capacitor is more a function of how frequently you do it. Like colin was saying ripple current handling is generally very low compared to it's maximum pulse charge/discharge limit, it's more related to temperature than anything else, so with a low enough duty cycle it may not shorten the life of the cap at all. One thing to keep in mind is electrolytics generall have a high ESR, and they're relativly inductive (coil wrapped foil basically) so you will get a very large difference in the peak currents you get into and out of various capacitors. Generally speaking you'll get more bang for your buck out of a higher voltage rated cap with low capacitance compared to a lower voltage rated with with a high capacitance. I've accidentally welded cap leads on several occasions with even low voltages, it's a little difficult to control.
 

Ubergeek63

Well-Known Member
You can use a moderate wattage resistor, maybe 5W, 1k ohm, connected to leads and insulated. Hopefully it won't burn out. Some high voltage capacitors have bleeder resistors across them to reduce the charge in a few minutes after power down.


Do you mean internal resistance? All capacitors have parasitic series resistance or Equivalent Series Resistance (ESR).
Actually they ALL have bleeders by LAW. Pick any safety spec and it will be there... now more recently there are also efficiency specs as well that adds complexity since now the bleeders must be switched if the power line is not.

Dan
 

giftiger_wunsch

New Member
Ubergeek63 said:
Actually they ALL have bleeders by LAW.

By law of which country? Just a quick glance along the location fields on this page you can see at least three different countries. Older devices also may not have bleeders if they were made before that law came into effect.

dougy83 said:
You can use a moderate wattage resistor, maybe 5W, 1k ohm, connected to leads and insulated. Hopefully it won't burn out.

If you suspect it might, or get very hot at least, why not just solder two of these resistors together and connect them across the capacitor in series? That way you double the resistance, as well as halving the proportion of the power each one needs to dissipate. You should essentially reduce the power dissipation of each resistor by a factor of 4.
 
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crutschow

Well-Known Member
Most Helpful Member
What are you talking about?
Ripple current is only a few milliamp.
Charge and discharge current is tens of amps.
The ripple current of a power supply capacitor can be equal to the power supply's output current (the rectifier filter has to supply all the current when the diodes are not conducting current during the low voltage periods of the sine wave).
 

ke5frf

New Member
I think it is always advisable to discharge capacitors with a suitable resistor. You can even build a special circuit for this very purpose, or even a voltmeter if you aren't in a big hurry. Analog type volt meters are lower impedance than digital and work well.
 
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