# Capacitor Charge Time

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What is the forumla to work out how long it takes to charge a cap to max value?

i know there is one because i've seen it before but cannot remember it and cannot find it any books ive got at home nor the net... does anyone know it?

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i dont think u CAN charge a capacitor to the max. i think it involves

1-e^(-t)

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well maybe it was only 2/3 full... i just KNOW i saw a formula to work the charging or partial charging time of a capacitor... anyone know?

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You have to find the time constant, which is R (in ohms) * C (capacitance in farads)

Each time constant, the current falls by 1/e

After about 5 time constants the capacitor is considered charged.

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i wonder why they choose the time constant to the value of time for current to fall by 1/e

was it just for simplification of notation, cos when u make t the subject of the formula u get logs.

why not 1/2 or 1/10

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ok, moving on...

if i have (i do) a 200V 470uf cap behind me and i charge it for say 1/2 the charge time required and put something across the two legs of it does the 'something' get 100V 470uf, or 200V 470/2uf or 100V 470/2uf or what?

remember that is has only been charged for 1/2 the time it needs to be charged.

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I could be wrong, but Im sure its something like 66% per second, and then 66% of the remaining amount and so on to infinity (more or less).

it`s been a long time since I did this stuff

cap types and temp all apply too, this is a general figure.

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well when the pd across a cap reaches 2/3 of supply voltage the cap discharges.

so is that the 66% you were referring to?

also if you have a mega big cap surely it'd fill up less than 66% per second because of its sheer size?

but whatever the time is imagine it was 1/2 full: if i have (i do) a 200V 470uf cap behind me and i charge it for say 1/2 the charge time required and put something across the two legs of it does the 'something' get 100V 470uf, or 200V 470/2uf or 100V 470/2uf or what?

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Great topic....some neat notes:

A capacitor never charges complety.

A capacitor never completly discharges

Voltage don't pass through a capacitor...

Complicated buggers..

jim

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ok, so take a 1 farad 100V cap, charging at 10V, how long would it take to fill most of the way to the top? so i get a nearly 100V nearly 1f discharge?

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i dont think u CAN charge a capacitor to the max. i think it involves

1-e^(-t)

I have worked out the voltage-time relationship of the capacitor-resistor network using 1st principles.

The solution is attached in the word document I have uploaded.

V = v0 - v0*exp(-t/RC) for charging case.

Cheers!

The general solution of the de.doc

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