5614 Posted December 19, 2004 Share Posted December 19, 2004 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? Link to comment Share on other sites More sharing options...
bloodhound Posted December 19, 2004 Share Posted December 19, 2004 i dont think u CAN charge a capacitor to the max. i think it involves 1-e^(-t) Link to comment Share on other sites More sharing options...
5614 Posted December 20, 2004 Author Share Posted December 20, 2004 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? Link to comment Share on other sites More sharing options...
blike Posted December 20, 2004 Share Posted December 20, 2004 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 Link to comment Share on other sites More sharing options...
bloodhound Posted December 20, 2004 Share Posted December 20, 2004 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 Link to comment Share on other sites More sharing options...
5614 Posted December 24, 2004 Author Share Posted December 24, 2004 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. Link to comment Share on other sites More sharing options...
YT2095 Posted December 24, 2004 Share Posted December 24, 2004 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. Link to comment Share on other sites More sharing options...
5614 Posted December 25, 2004 Author Share Posted December 25, 2004 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? Link to comment Share on other sites More sharing options...
Guest tinman Posted January 14, 2005 Share Posted January 14, 2005 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 Link to comment Share on other sites More sharing options...
5614 Posted January 17, 2005 Author Share Posted January 17, 2005 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? Link to comment Share on other sites More sharing options...
cyeokpeng Posted January 21, 2005 Share Posted January 21, 2005 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 Link to comment Share on other sites More sharing options...
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