With TJ Byers
I have been struggling with how to select diodes for the common application of blocking reverse voltage. It seems like, in most cases, the specs are so non-critical that the selection doesn't matter. But in a previous column, there were two schematics that specified two different diode values — 1N4148 and 1N4001 — and I am wondering why. In other simple circuit schematics, I see a mixture of diodes like 1N914, 1N4002, and 1N4003 specified, even though the circuit voltages are less than 14 volts. I have used diodes to block reverse connection at the input of my five-volt power supplies, and as flyback protection next to relays. In these cases, I've chosen the 1N4003 for its higher voltage rating. Is there an advantage to using one of the others? I feel like I am missing something here.
Several factors have to be considered when specifying diodes for an application — not the least of which is cost. As you point out, breakdown voltage is a primary consideration. The breakdown voltage (VR) is the maximum voltage you can apply across a diode in the reverse direction and still have it block conduction. If this voltage is exceeded, the diode goes into the zener mode and conducts current, rendering the diode useless as a rectifier. Exceeding the reverse voltage almost always destroys the diode by shorting it out. There is nothing gained by specifying a breakdown voltage that's greater than needed, that is, a 1N4001 (50 volts) will work equally well in a 12-volt circuit as a 1N4007 (1,000 volts) for about half the price. The reverse EMF of a 12-volt relay coil seldom exceeds 100 volts — the actual voltage is dependent on the size (inductance) of the solenoid. The more metal inside the coil, the bigger the kick.
Of course, the forward current (I/O) rating must be greater than the current demanded by your load. If the current is more than the diode can supply, it overheats and quickly melts down to a shorted device. Diode meltdown in rectifier circuits can also happen if the reverse recovery time is slower than the frequency of the power source. One encounters this situation in power supply switching circuits, like those found in TV high-voltage flyback power supplies. The reverse recovery time is the time it takes a diode to go from forward conduction to reverse-blocking. If the turnaround time is too slow, current will flow in the reverse direction when the polarity changes, which heats up the junction, and eventually results in its demise.
Not all diodes are designed to be power rectifiers. The 1N4148, for example, is a low-capacitance signal diode most often used in switching circuits, like those needed for logic steering. The low capacitance guarantees that the switching edges will be clean, with sharp turn-on and turn-off times. This is why you can see mixed diode types in the same circuit. I hope this helps.