peak inverse voltage

queenstudy

i read this sentence like a 100 times and i still dont get it please can anyone help me??
PEAK INVERSE VOLTAGE (PIV):
PIV is the maximum reverse bias voltage which can be applied safely to a diode without its breaking down. Beyond this voltage a diode breaks down
they say the PIV must be greater than the maximum voltage of the sin form of the ac generator why????

sophiecentaur

This is so that the diode will not break down when the voltage is reversed and it's supposed to be 'off' - which is what diodes are used for.

(PIV refers to the Max PIV that the diode can stand - not the actual voltage you apply)

vk6kro

When a rectifier diode conducts and it charges a filter capacitor, it charges the capacitor up to the full positive voltage of the input waveform (minus a small diode voltage drop).

Now, when the input polarity reverses, the diode gets a peak negative voltage on its anode, but it still has the positive voltage of the capacitor on its cathode.

So, it has roughly twice the peak input voltage across it in reverse. This voltage situation is called the peak inverse voltage and the diode needs to be able to cope with it without breaking down.

queenstudy

what about the formula PIX greater than Vm in both half and full rectification why??
thank you guys for the help

sophiecentaur

You cannot rely on the two reverse biased diodes sharing the applied volts equally.

vk6kro

For the same reason as above, if the diode charges up a capacitor, that voltage will add to the reverse voltage across the diode when the polarity from the transformer reverses.

This applies whether the rectification is half wave, full wave or a bridge. (In a bridge, there are two diodes in series when the polarity reverses and as Sophie mentions, they may not share the voltage equally.)