Capacitor Action Topics covered in this presentation: Capacitors on DC Capacitors on AC Capacitor Charging Capacitor Discharging

Charging a Capacitor (DC) Before looking at how capacitors charge with an AC supply, we need to first look at the charging action of a DC supply. When a voltage is applied across a capacitor, the positive voltage attracts electrons from the positive plate, leaving it positively charged. The negative voltage repels electrons to the negative plate, making it negatively charged.

Charging a Capacitor (DC) Electrical current is the rate of movement of charge. Therefore, when a capacitor charges a current (I) will flow for a period of time (t) as the electrical charge (Q) moves around the circuit.

Charging a Capacitor (DC) If this supply voltage is then removed, the charge is held in place by the attraction between the opposite charges on the plates.

Capacitor on a DC Supply If a capacitor is connected across a DC power supply, a current will initially flow as the capacitor charges. As the capacitor charges and the voltage across it increases, the current in the circuit will decrease. When the capacitor is fully charged, the voltage across the capacitor will be the supply voltage so no current will flow.

Capacitor on a DC Supply In practice a resistor is used in the charging circuit to limit the current. If the capacitor is not charged through a resistor, the current can be very large, but only for a very short time. Once fully charged the capacitor has the effect of blocking current flow in a DC circuit as the gap between the two plates is effectively an open circuit.

Capacitor on an AC Supply For a capacitor on an AC supply, you need to examine how the capacitor charges and discharges for one complete AC cycle. During the first quarter when a positive going voltage is applied across the capacitor, the capacitor will charge positively. As the voltage across the capacitor increases, the current decreases. When the capacitor is fully charged, the current will be zero.

Capacitor on an AC Supply In the second quarter the capacitor discharges as the voltage across it decreases. As the capacitor discharges a current increasing in value will flow in the opposite direction. When the capacitor is fully discharged, the current will be at a maximum.

Capacitor on an AC Supply In the third quarter a negative going voltage is applied across the capacitor, and the capacitor will charge negatively. As the voltage across the capacitor increases, the current decreases. When the capacitor is fully charged, the current will be zero.

Capacitor on an AC Supply In the fourth quarter the capacitor discharges as the voltage across it decreases. As the capacitor discharges a current increasing in value will flow in the opposite direction. When the capacitor is fully discharged, the current will be at a maximum.

Capacitor on an AC Supply For a capacitor connected across an AC supply, a current flows all the time, almost as if the capacitor was allowing current to flow through it. However, this is not the case. Current flow occurs as a result of the capacitor charging and discharging during each cycle of the AC supply. A capacitor in series with an AC supply will allow current to flow through other components in the circuit.

Capacitor Charging through a Resistor If a resistor is connected in series with a capacitor it will limit the supply current and slow down the rate at which the capacitor charges. Initially the full voltage appears across the resistor. The capacitor will start charging causing a change in voltage across the capacitor. If the initial rate-of-change of voltage could be maintained the capacitor voltage would reach the applied voltage in a time of CR seconds. This is known as the Time Constant (t): t = CR seconds

Capacitor Charging through a Resistor As the capacitor charges, the voltage across the capacitor will increase, so decreasing the voltage across the resistor. The current in the resistor therefore decreases (rate of charge slows down). The voltage rises by 63.2% of the difference between the capacitor voltage and the applied voltage during each time constant.

Capacitor Charging through a Resistor Theoretically the capacitor never reaches the full supply voltage. However, after a time of 5CR seconds, the capacitor will be 99.5% charged and it can be assumed that the capacitor is fully charged. Once charged, the power supply can be removed and the capacitor will continue to hold its charge.

Capacitor Discharging through a Resistor The capacitor can then be discharged through the resistor. The resistor will limit the current as the capacitor discharges and will slow down the rate of discharge. Initially the full capacitor voltage will appears across the resistor and maximum current will flow.. If the initial rate-of-change of voltage could be maintained the capacitor would discharge in a time of CR seconds.

Capacitor Discharging through a Resistor As the capacitor discharges, voltage across the capacitor decreases, so decreasing the voltage across the resistor. The current therefore decreases and rate of discharge slows down. In each time constant the capacitor voltage falls by 36.8%.

Capacitor Discharging through a Resistor Theoretically the capacitor will never be fully discharged. However, after a time of 5CR seconds, the capacitor will be 99.5% discharged and it can be assumed that the capacitor is fully discharged.

Capacitor Half-Life The time taken for a capacitor to charge or discharge to half of the supply voltage is called the half-life of the capacitor. It can be shown that the time taken to reach half of the supply voltage is 69% of the time constant. V = 0.5VS after 0.69CR seconds