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Complete Electronics Self-Teaching Guide with Projects by Harry Kybett, Earl Boyson

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Capacitors in a DC Circuit

34 Capacitors are used extensively in electronics. They are used in both alternating current (AC) and DC circuits. Their main use in DC electronics is to become charged, hold the charge, and, at a specific time, release the charge.

The capacitor shown in Figure 1.15 charges when the switch is closed.

Question

To what final voltage will the capacitor charge? _____

Answers

It will charge up to 10 volts. It will charge up to the voltage that would appear across an open circuit located at the same place where the capacitor is located.

35 How long does it take to reach this voltage? This is an important question with many practical applications. To find the answer you must know the time constant (τ) (Greek letter tau) of the circuit.

Questions
A. What is the formula for the time constant of this type of circuit? __________
B. What is the time constant for the circuit shown in Figure 1.15? __________
C. How long does it take the capacitor to reach 10 volts? __________
D. To what voltage level does it charge in one time constant? __________
Answers
A. τ = R × C.
B. τ = 10 kΩ × 10 μF = 10,000 Ω × 0.00001 F = 0.1 seconds. (Convert resistance values to ohms and capacitance values to farads for this calculation.)
C. Approximately 5 time constants, or about 0.5 seconds.
D. 63 percent of the final voltage, or about 6.3 volts. ...

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