Diodes are electronic one-way valves, letting current flow in one direction but not the reverse. If you’re designing a circuit that uses them, you’ll need to know they have limits. They can handle a rated maximum current, and if you exceed this limit you’ll destroy the part. They also have a reverse voltage limit, past which they will start conducting, possibly with damaging results. And diodes come in several different physical case styles, with leads or in a surface-mount (SMD) case.
Evaluate the schematic at the point where the diode will be located. Determine the maximum current that will be flowing through that point and write the figure down. Determine the maximum voltage that the diode will have to sustain. Voltages are differences in potential between two points, so if one side of the diode is at 25 volts and the other at 5 volts, 25-5 = 20 volts. Write the voltage value down.
Multiply the voltage value by 1.25 and write it down. Do the same for the current value. These will be your minimum ratings. Multiply the voltage value by 2.5 and write it down, and do the same for current. These will be your maximum ratings. The diode you use can exceed the maximum ratings if you cannot find one that’s smaller than it. For example, if your maximum circuit voltage is 15 and the smallest diode value you can find in the catalog is 100 volts, it’s perfectly safe to go with 100. Do not use a part rated for less than your minimum value.
Determine how you’ll be building the circuit. If the diode is rated for much more than 5 amps of current, it may need to be mounted in a metal heat sink. If you’re using surface-mount components, you’ll be looking for that style of diode package.
Open the catalog and search through the diode section, beginning with the lowest voltage ratings that match yours, then look for current ratings that match. The catalog will list diode voltage ratings as either Peak Inverse Voltage (PIV) or Peak Reverse Voltage (PRV). It lists current ratings as Average Forward Current, Recurrent Forward Current and DC Forward Current. If the diode is rectifying 60-cycle AC, use Average Forward Current. If the diode will be handling recurring current pulses, use Recurrent Forward Current. Otherwise, use DC Forward Current to find the right diode. Make sure you select a physical package that fits your overall design in terms of leads or surface mount.
If you select a diode rated much higher than your calculated maximums, it will work in your circuit but it may be expensive and physically large. If you use diodes that match or fit between your minimum and maximum ratings, they will work safely and you won’t waste money.