Power Supplies

Reservoir capacitor

C in Fig 6.4 is called a 'reservoir capacitor'. Its purpose is to store energy during the positive half-cycle and to supply the load during the negative half-cycle while the diode is non conducting. The diode only conducts during the time VAC exceeds the voltage across the reservoir capacitor (see Fig 6.5). If the value of C is made large, say 10,000µF, then a large pulse of current will be needed to charge it, because the time during which the diode conducts is short.

Fig 6.4. The reservoir capacitor

The reservoir capacitor charges up to a voltage which equals the peak value of Vac, ie  √2 times Vac. On the negative half cycle, polarities are reversed but the magnitude is the same. Thus the maximum voltage across the diode is now 2 x √2 Vac. This value is known as the 'peak inverse voltage' (PIV). The maximum PIV allowable across a rectifier diode is a very important characteristic of the diode.

Fig 6.5. Output waveforms of rectifier circuit using a reservoir capacitor