Engineering Principles V11

Clive W. Humphris

POWER SUPPLY: Half Wave Rectifier.  

Positive half cycles greater than 0.6V cause the diode D1 to conduct and current flows into the load, also charging C1. During the negative half cycles the diode is cut-off, and C1 discharges via RL. Until the next positive pulse arrives.

One way of increasing the reserve charge stored in C1 is to use a larger capacitance value. This reduces the ripple voltage as there is more energy stored to supply the circuit during off peak periods. Rs limits the diode forward current, especially during switch-on.

The effect of poor smoothing of the voltage peaks can be shown by giving C1 a low value, say 25µF.

There are just two main considerations when selecting a rectifier diode for a particular application. The current flow, both steady and peak values where a smoothing capacitor, for example might need to be initially charged and the Peak Inverse Voltage (PIV). This is the maximum instantaneous voltage that the device can withstand when connected in the reverse direction. If the diode design value is exceeded then the device will break down due to an avalanche effect. This is the same principle as for a zener diode, but this time it will not recover and the diode will be destroyed, most probably inflicting some additional circuit damage.

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