The charging circuit for the flash system of a camera uses a 110 F capacitor tha

Question

The charging circuit for the flash system of a camera uses a 110 F capacitor that is charged from a 260 V power supply.

Part A

What is the most resistance that can be in series with the capacitor if the capacitor is to charge to at least 88 % of its final voltage in no more than 8.0 s ?

The charging circuit for the flash system of a camera uses a 110 F capacitor that is charged from a 260 V power supply.

Part A

What is the most resistance that can be in series with the capacitor if the capacitor is to charge to at least 88 % of its final voltage in no more than 8.0 s ?

Explanation / Answer

voltage on a cap, charging
v = v[1–e^(–t/)]

v is the voltage after time t
R is resistance in ohms,
C is capacitance in farads
t is time in seconds
RC = = time constant

(0.88)•V = V•[1 e^(-t )]

0.88 = [1 e^(-8.0 )]

= (8.0) (2,12) = 4.726

= RC = R • (110 µF) = 4.726

Rax = 40000 = 40 K

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