How to choose a diode in a rectifier circuit ?

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Hi

On diodes datasheets there is a rating value called "the rectified average current (Io), what this value mean ? how to calculate it ?
- How to choose the diodes for use in a rectifier circuit depending of Io ?
- How to calculate the value of (Io) if i use for example a simple power supply : a transformer with 9v rms secondary voltage + bridge rectifier + 6800uF capacitor + load (maximum load current 250 mA)

sorry for my english..
 

The selection of the diode is based on the peak current and average.
For a full wave rectified sinusoidal id 0.637 the peak value.
Bot the average and peak current must be below the values indicated in the datasheets of the diode.
For the transfoemer you need to specify the it's power .
 

thx but anyone can tell me How to calculate the value of (Io) ? if i use for example a simple power supply : a transformer with 9v rms secondary voltage + bridge rectifier + 6800uF capacitor + load (maximum load current 250 mA)

is it equal approximately to Uc/Rload ?

(Uc = Vpeak = 9 *√2]
 

In operation the average current through each diode will be your load current. But that's average.

In detail... during a cycle each diode must carry a brief pulse which is much higher than your load current. So your diodes really ought to be rated higher than your load current.

Not only that, power-up causes large surges to go through the capacitor because it is like a dead short when it is not charged. So your diodes must endure overcurrent for a few cycles.

You probably can get by with 1A diodes rated for reverse 50V. This is a common 1N4001. Or use a bridge rectifier with the same rating.

If one diode burns up, then you should replace all with 3A rating.
 
thx BradtheRad..i knew all that but what i really want is how to calculate this average current ?
any formula or link that show calculation will be great
 

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