For procedures, see your electrical laboratory manual.

Title: Response of AC Circuits to Square Wave


Aim

To observe experimentally transient voltage effects RC and RL circuits.

Theory

When a voltage source is applied to an RC circuit, the capacitor, C charges up through the resistance, R.

All Electrical or Electronic circuits or systems suffer from some form of “time-delay” between its input and output, when a signal or voltage, either continuous, ( DC ) or alternating ( AC ) is first applied to it.

This delay is generally known as the time delay or Time Constant of the circuit and it is the time response of the circuit when a step voltage or signal is first applied. The resultant time constant of any electronic circuit or system will mainly depend upon the reactive components either capacitive or inductive connected to it and is a measurement of the response time with units of, Tau – Ï„

When an increasing DC voltage is applied to a discharged capacitor, the capacitor draws a charging current and “charges up”, and when the voltage is reduced, the capacitor discharges in the opposite direction. Because capacitors are able to store electrical energy they act like small batteries and can store or release the energy as required.

The charge on the plates of the capacitor is given as Q = CV. This charging (storage) and discharging (release) of the energy of a capacitor is never instant but takes a certain amount of time to occur with the time taken for the capacitor to charge or discharge to within a certain percentage of its maximum supply value being known as its Time Constant ( Ï„ ).

If a resistor is connected in series with the capacitor forming an RC circuit, the capacitor will charge up gradually through the resistor until the voltage across the capacitor reaches that of the supply voltage. The time also called the transient response, required for the capacitor to fully charge is equivalent to about 5 times constants or 5T.

This transient response time T is measured in terms of Ï„ = R x C, in seconds, where R is the value of the resistor in ohms and C is the value of the capacitor in Farads. This then forms the basis of an RC charging circuit were 5T can also be thought of as “5 x RC”.