P–N Junction Diodes:testing P–N Junction Diodes

testing P–N Junction Diodes

A diode can be tested by checking the forward-to- reverse-resistance ratio with an ohmmeter. The resistance ratio indicates the ability of the diode to pass current in one direction and block current in the other direction.

A germanium diode has a low forward resistance of several hundred V. The reverse resistance is high, greater than 100,000 V. Silicon diodes have a higher forward and reverse resistance than germanium. An ohmmeter test of a diode should show a low forward resistance and a high reverse resistance.

c A u t i o N !

Some ohmmeters use high-voltage batteries that can destroy a P–N junction.

The polarity of the terminals in an ohmmeter appears at the leads of the ohmmeter: Red is positive and black is negative or common. If the positive ohmmeter lead is connected to the anode of the diode and the negative lead to the cathode, the diode is forward biased. Cur- rent should then flow through the diode, and the meter should indicate a low resistance. If the meter leads are re- versed, the diode is reverse biased. Little current should flow, and the meter should measure a high resistance.

If a diode shows both low forward and low reverse resistance, it is probably shorted. If the diode measures both high forward and high reverse resistance, then it is probably opened. An accurate diode test can be made with most types of ohmmeters.

The forward resistance voltage must be larger than the barrier voltage of the diode (0.7 V for silicon and V for germanium) for conduction to take place.

An ohmmeter can also be used to determine the cathode and anode of an unmarked diode. When there is a low reading, the positive lead is connected to the anode, and the negative lead is connected to the cathode.

Questions

1. How is a diode tested with an ohmmeter?

2. What precaution(s) must be taken when testing diodes with an ohmmeter?

3. How does the ohmmeter indicate that a diode is shorted?

4. How does the ohmmeter indicate that a diode is opened?

5. How can an ohmmeter be used to determine the cathode end of an unmarked diode?