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Capacitor theory

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curry87

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Hi i have a question about capacitor theory:

Suppose i had a standard led(1.7vf) connected though a 10k resisitor and discharged a capacitor though it how would i work out how long say a 100uf capacitor would be able to substain enough current to visable light the led ? please give an example

Also how would i work out how much capictance i would need to substain a charge of 9v at 3ma for 3min ?


Thanks
 
Hi i have a question about capacitor theory:

Suppose i had a standard led(1.7vf) connected though a 10k resisitor and discharged a capacitor though it how would i work out how long say a 100uf capacitor would be able to substain enough current to visable light the led ? please give an example

Also how would i work out how much capictance i would need to substain a charge of 9v at 3ma for 3min ?


Thanks
hi,:)
There are formula's for charge discharge times for capacitors
look at the link.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capdis.html#c2
Capacitor Discharging
 
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You can't sustain a voltage of 9V, as soon as the power is removed the voltage from the cap will start to drop so you have to list what your limits are, if 9 volts is your lower limit then you have to chose an upper limit then you can calculate the time it would take the capacitor to discharge from the higher state to the lower one, the brightness of the LED will gradually fade, and the capacitance is going to be pretty high. I don't like the math myself which is why I use a simulator. If you start with a 10V charge and discharge to 9V you'll need about 3/4's of a farad worth of capacitance to last 3 minutes, mind you you'll get MANY MANY more minutes of gradually dimming light, depends on exactly what you need. If you're talking about running the LED down to just barely visible you can get away with a much lower capacitance.
 
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