Diodes in Parallel to Increase Current Rating,
Plus Surge Current Protection.

Created "June 21 2004"

Some Persons believe two diodes in parallel Doubles the Current Rating. However just
putting two diodes in parallel may or may not increase the current rating at all.

All diodes have a foreward voltage drop and if you measure a bunch of a particular
diode, (even all being in the same batch) you will find each is slightly different
at a given current. And this can change in a Non-Linear way for other currents.
When diodes are placed in parallel, the diode with the Lowest Voltage drop will
conduct the most Current.

This could result in No Advantage, Partial advantage or High Advantage.
But Extremely Doubtful it will ever Double the current.

One way to help fix this is to put a Low Value, Series Resistor in front of Each Diode.
The voltage drop across this series resistor tends to Equil Out the differences in
the diodes. The resistance of each series resistor should provide a voltage drop,
somewhere around 0.3 to 0.6 volts at the current draw of the particular circuit it
is used in.

Another Problem that can occur sometimes: Is when a large filter capacitor is used after the diode and If the Power transfomer has a low impedance and is capable of driving large currents. "This Inital High Surge Current, Can Damage the Diodes".

In these cases it is advisable to place a low value, series resistor. Sometimes it is called a "Fusing Resistor". This resistor Raises the Effective Impedance and Lowers the High In-Rush Current to a safer level, thus protecting the diode from damage.

In Low Voltage Supplies, Resistor Values between 0.1 ohm to 10 ohms are quite effective. And in High Voltage Supplies, Values between 47 ohm to 100 ohms are used to fix this. In the case of a Diode Bridge Circuit, a resistor is placed on Both Inputs.

A bit of Wisdom, from my many years experience.

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*Copyright © 2004*