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# Charge time for 4 capacitors in parallel? Answered

I was wondering how long would it take to charge a capacitor at 5volts if there are 4 parallel. This is the details on the Capacitors. 330v 120uF each. So what is the best way to charge them by usb which gives me 5v or using a 9 volt battery and for how long.

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The time it takes to charge a resistor R in series with a capacitor C depends on R*C.  This product, R*C, or RC, has units of time, and is called a time constant.  It is the amount of time it takes to charge the capacitor to 63% full.   For example multiply R=50 ohm times C = 480 microfarad and get
50 *480e-6 = 24000e-6 = 24e-3 = R*C = 24 millisecond to charge to 63% full.

The actual formula for this is:

VC/V0 = (1 -exp(-t/(R*C)))

That looks complicated, but what it monkeys out to is this.
At  t=0,         VC/V0 = 1-exp(0) = 0%
At  t=RC,      VC/V0 = 1-exp(-1) = 63%
At  t=2*RC,  VC/V0 = 1-exp(-2) = 86%
At  t=3*RC,  VC/V0 = 1-exp(-3) = 95%
At  t=4*RC,   VC/V0 = 1-exp(-4) = 98%
At  t=5*RC,  VC/V0 = 1-exp(-4) = 99%
...etc.

As you can see, the capacitor voltage asymptotically approaches the voltage of charging source.  Practically speaking it takes around 2 or 3 time constants to get about 90% of the way there ,which is usually close enough.

http://en.wikipedia.org/wiki/RC_circuit

I think the reason you're confused is because you don't know what I'm talking about.

But then I realized I might have the same problem with understanding what you're saying, and it occurred to me: These capacitors you've got, they're intended for a camera flash, right? You said they 120 uF with a voltage rating of 330 V. This means you probably want to charge them up to something close to their rated voltage, circa 300 volts, and you want to do this using a 5 volt, or 9 volt, supply. Why would you want to charge capacitors like this to merely 5 or 9 volts?

If this is what you were planning on doing, charging your capacitors to a voltage of around 300 V, it will take more circuitry than just a 50 ohm resistor to do it.  You'll need some kind of charging circuit, like the kind that came with the camera.

This question has been asked before, as recently as a few days ago, here:
Do you mean flashlight or flashgun ?