[SOLVED] Basic capacitor question

[Zak28]

Its been too long since I learned this stuff. A capacitor rated to 900v & 600uF is accross a 500v 1amp power supply. The questions I have are
1. Whats the max charge in micro farads will capacitor obtain from supply?
2. How long to receive that charge?

I know the formula for charge in a capacitor is Q=C*Vsupply [1-e^-t/R*C]; were R=cap esr = 3mOhms & C = 600uF but I am uncertain because the cap rating is above power supply volts.

 

[BradtheRad]

With supply 500V @ 1A, implies a resistance of 500 ohms.

Therefore RC time constant is 500 x 600 x 1 millionth, or 0.3 second.

Convention says a capacitor is fully charged after 5 time constants.

 

[Zak28]

Will it actually charge to 600uF? 500v is far below its rated voltage.

 

[mjuneja]

900 V defines the maximum capacity of the capacitor. If you are charging using 500 V, it will get charged uptill 500 V.

That is Q = 600uF x 500 V.

 

[BradtheRad]

The cap rating needs to be greater than maximum voltage which you expect it to charge to. This means the 900V rating is extra safe. You could get by with a 550 V rating.

 

[FvM]

Unit of charge is As, not Farad (or As/V).

Capacitance of film capacitors is completely voltage independent, capacitance of electrolytic capacitors almost.

 

[Kajunbee]

Since I don't trust my math skills I rely on the online calculators. For capacitor charging I go to
"Capacitor charge and discharge - Must Calculate". When I plug the OPs values in I get a charge time of approximately 1.4 seconds at 1 amp. But brads number gives a charge time of 0.3 seconds.
I'm wondering why the online calculator is so much longer. The only thing I can figure is that i am not using the calculator properly. These are the numbers I plugged in.

From voltage- 0
To voltage - 500
Supply voltage- 500
Capacitance - 600u
Resistance - 500
Time - 1.5 seconds

- - - Updated - - -

I think I see my mistake. The 0.3 represents 1 time constant and not complete charge time.

 

[KlausST]

Hi,

Brad´s math calculates tau. Tau = R x C.
It is a non_constant_current charging of a cpaacitor.

After one tau the voltage of the capacitor rises from 0 to about 63% of the supply voltage. In other words the error is about 37% or 0.37
After n x tau the error will be 0.37 ^n
exactly the remaining error is 1/euler which is: 0.36788...
1 tau => error of 0.37^1 = 0.37, charged to 63%
2 tau => error of 0.37^2 = 0.135, charged to 86,5%
3 tau => error of 0.37^3 = 0.0498, charged to 95%
...
This is true for every (constant) R and (constant) C.

The error will never reach 0, but it becomes very close to 0. The capacitor voltage will never reach supply voltage.

*****
Brad calculated correctly tau to be R x C = 500 Ohms x 600uF = 0.3s.

Klaus

 

[BradtheRad]

Yes, since one time constant calculates as 0.3 sec, then 5 time constants make 1.5 sec which is close to your result for overall charge time.