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# How to calculate the discharge rate of a capacitor?

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#### magello

##### New Member
Hey guys, i know how to calculate the charge rate, but how do i calculate the discharge rate of this circuit?

This is a dimmer light, and an you please give me the formula of the discharge rate of the capacitor and how long will the LED stay on for?

Thanks

Magel

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Capacitor Discharging

Q=CV and if you do the math the capacitor voltage changes about 63% over the time RC. In your case it would be the difference between the current voltage and the 1.7V LED voltage.

Dan

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If you know how to calculate the charge rate, then you know how to calculate the discharge rate, as the equations are the same; they depend on the capacitance and the circuit resistance. In your circuit, the capacitor discharges through the combanation of the 515 and 1K resistors. Thus, one time constant ( the time it take for the capacitor to discharge to 63% of it's voltage ) will be 550*10exp-6 * 1515 = 833mS. The voltage at that time will be (1-.63)*V2. It looks to me like the light will fade out in just under 1 second.

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