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?

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

 

1-e^(-t)

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?

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.

 

 

 

Source: http://www.kpsec.freeuk.com/capacit.htm

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

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.

I could be wrong, but I`m sure it`s 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.

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?

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

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?

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