The figure below shows how a bleeder resistor (R = 256 k Ohm) is used to dischar

Question

The figure below shows how a bleeder resistor (R = 256 k Ohm) is used to discharge a capacitor (C = 73.7 mu F) after an electronic device is shut off, allowing a person to work on the electronics with less risk of shock. What is the time constant? How long will it take to reduce the voltage on the capacitor to 0.100% of its full value once discharge begins? If the capacitor is charged to a voltage V_0 through a 123 Ohm resistance, calculate the time it takes to rise to 0.865 V_0 (this is about two time constants).

Explanation / Answer

C)

Time constant = T = RC = 123 x 73.7 = 9.1 x 10-3 sec

Using the formula

V = Vo (1 - e-t/T)

0.865 Vo = Vo (1 - e-t/T)

0.865 = 1 - e-t/T

e-t/T = 0.135

-t/T = -2.0025

t = 2.0025 x 9.1 x 10-3

t = 0.0182 sec

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