Calculating Capacitor Charge Time: 1000 uF, R Value

[OZwurld]

Hello all,

I was reading this post on calculating the capacitor charge time:

https://www.physicsforums.com/showthread.php?t=145203

.

My problem is i have a capacitor (1000uF) and i want to determine its charge time but don't know the R value, is it something i chose arbitrarily or should it be in a datasheet.

Thanks.

 

[MrSparkle]

It should come from the circuit. If your resistance is at or near 0, then your current is very large and your capacitor would fill up instantaneously.

 

[OZwurld]

is there a typical value that i should use, or does that all depend on how much current i want?

 

[sophiecentaur]

is there a typical value that i should use, or does that all depend on how much current i want?

In the limit, it absolutely depends upon the resistance (and the Inductance) of the rest of the circuit. In real life, a Capacitor is never charged "instantaneously". The leads and internal structure of the C will also have resistive and inductive components; when using Capacitors at RF, the self Inductance can be very relevant; any Capacitor can resonate and behave as a short circuit, open circuit or even an Inductor, above self-resonance.

 

[Baluncore]

The critical parameters relating to minimum charge time will be specified in the manufacturers data sheet.
1000uF will almost certainly be electrolytic, so the parameters will be;
1. ESR, the effective series resistance. 2. Ripple current, I_ripple. 3. Temperature rating.

If you try to charge an electrolytic capacitor too quickly, the high current flow may fuse the internal foil. That can also happen if you short circuit a capacitor. If you repeat the charge-discharge cycle often, the I²R power dissipated in the capacitor ESR may overheat the electrolyte. Keep the current below the ripple current specified.

To charge a capacitor, C, from a fixed DC voltage, V, through a resistor, R, will require the minimum resistor value to be R = V / I_ripple.

 

[jim hardy]

here's a pretty good overview of large capacitors

http://www.vishay.com/docs/28356/alucapsintroduction.pdf