AC BASICS, GRAPHS & RMS
State the definition of the root-mean-square (r.m.s.) value of an a.c. current.
The r.m.s. value of an a.c. current is the value of the steady d.c. current that would produce the same mean power in a resistor.
What is meant by rectification?
Rectification is the process of converting alternating current into direct current.
What is meant by smoothing?
Smoothing is the reduction of variations in the output voltage of a rectifier to produce a more constant d.c. voltage.
An a.c. supply has a peak voltage of 325 V.
Calculate its r.m.s. voltage.
230 V
Explain how a single diode produces half-wave rectification.
During one half-cycle of the a.c. supply the diode is forward-biased and conducts, allowing current to flow through the load. During the opposite half-cycle the diode is reverse-biased and blocks current. This results in current flowing in one direction only for half of each cycle.
Explain how a capacitor smooths the output of a rectifier.
The capacitor charges to the peak voltage when the rectified voltage rises. When the rectified voltage falls, the capacitor discharges slowly through the load, maintaining the output voltage and reducing ripple.
Explain why doubling the frequency of an a.c. supply does not change its r.m.s. voltage.
The r.m.s. voltage depends on the peak voltage of the supply, not its frequency. Doubling the frequency changes the time period of the waveform but does not change its amplitude, so the r.m.s. voltage remains unchanged.
State one advantage of full-wave rectification over half-wave rectification.
Higher mean output voltage
Higher efficiency
Lower ripple
Ripple frequency is doubled
Explain the role of the time constant τ=RC\tau = RCτ=RC in smoothing.
The time constant determines how quickly the capacitor discharges through the load. A larger time constant means the capacitor discharges more slowly between peaks, so the voltage falls less and the ripple is reduced.
During one half-cycle of a.c. input to a bridge rectifier, how many diodes conduct? Explain why.
Two diodes conduct during each half-cycle. The conducting pair depends on the polarity of the supply, but in each case the diodes are arranged so that the current through the load flows in the same direction.