Hello all, I was reading this post on calculating the capacitor charge time: Code (Text): 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 dont know the R value, is it something i chose arbitrarily or should it be in a datasheet. Thanks.
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.
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.
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 I^{2}R 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.
here's a pretty good overview of large capacitors http://www.vishay.com/docs/28356/alucapsintroduction.pdf