Measuring the speed of a diode

The speed of a diode is a measure of how fast it can switch from being on to being off. This is known as the reverse recovery time (trr).

The PN junction acts like a small capacitor. Then the voltage is reversed across the PN junction, the capacitance will resist the change as it discharges.

Power rectifier diodes are generally slow, but are optimized for handling large current and substantial reverse voltage. "Switching" diodes or "high speed signal" diodes are more optimized for speed, like the common 1N4148.

Measuring the reverse recovery time (“trr”)

To measure the “trr”, we first need to charge the capacitance by applying a forward voltage to the diode.

In this circuit, The IC1A will apply a forward voltage and a reverse voltage 50,000 times a second. When Q is LOW and ~Q is HIGH, the Diode Under Test (DUT) will be forward biased and charge any capacitance across the PN junction. The base of TR1 is at a lower voltage than its emitter so the transistor is OFF. This causes the output of IC2A to be LOW.

When Q goes HIGH, ~Q is LOW, the Diode Under Test (DUT) will be reversed biased and any capacitance across the PN junction will discharge. The “delay” in switching off results in current still flowing in R2. Because the emitter of TR1 is now at 0V, the voltage drop across R2 is enough to switch on TR1 until the capacitance across the PN junction discharges. This causes the output of IC2A to be HIGH while TR1 is on. The amount of time that the output of IC2A is HIGH is the “trr” time.

The “trr” time is used to charge a capacitor. The higher the “trr”, the higher the voltage on the charged capacitor. This voltage is then buffered and amplified and feed into a VU style meter consisting of 5 LEDs.

Build

I have a number of cheap 85mmx50mmx20mm cases which I bought a while ago on eBay. They were less than a $1 each.

The Eagle files for the PCB have been included should you wish to get them commercially made or you can choose to make them yourself. I used the Toner method to make mine.

Conclusion

Discreet electronics without microprocessors is still fun. It's a pity in some-ways that it is a dying art but if you are interested in looking at old electronic magazines, the best source that I have found is https://worldradiohistory.com/