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Practical Electronics for Inventors, Third Edition by Paul Scherz

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Problem 3: If a 100-μF capacitor in a high-voltage power supply is shunted by a 100-K resistor, what is the minimum time before the capacitor is considered fully discharged when power is turned off?
Answer: 50 s. Hint: After five time constants, a capacitor is considered discharged.
Problem 4: An IC uses an external RC charging network to control its timing. To match its internal circuitry, the IC requires 0.667 of the supply voltage (5 V), or 3.335 V. If the capacitor’s value is 10 μF, what value resistor is required in the RC network to obtain a 5.0-s timing period?
Answer: 500 kΩ. Hint: Use
V(t) = V(1 − e−(t/RC)) = 0.667 V
Problem 5: What is the reactance of a 470-pF capacitor at 7.5 MHz and 15.0 MHz?
Answer: XC(7.5 MHz) = 45.2 Ω, XC(15 MHz) ...

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