How to Use a Transistor As a Diode

Transistors are sometimes described as "two diodes back to back." This is not strictly true, but it is true enough that you can use a transistor as a diode in an emergency. A diode consists of two dissimilar chips of semiconductors. A transistor consists of three semiconductor chips where the middle chip is different from the two similar end chips. There are some differences in the chips -- for example, the middle chip of a transistor is thinner than the other two chips -- but the path through the center chip and either end chip is similar to the path through a diode.

• 1

  Find out if you have a PNP (positive-negative-positive) transistor or an NPN (negative-positive-negative) transistor. If it has more than three wires, it is neither and you can not use it as a diode. You can tell the difference between PNP and NPN transistors with an ohmmeter. The leads of the transistor will be labeled B (base), E (emitter) and C (collector). Zero-adjust the ohmmeter, then put the positive lead of the ohmmeter on the base lead. If it is an NPN transistor, the resistance will be high, but possibly different, when the negative lead of the ohmmeter is placed on either the emitter or collector leads. If it is a PNP transistor, the the resistance will be low, but possibly different, when the negative lead is placed on either the emitter or collector of the transistor.

• 2

  Verify the transistor type by placing the negative lead of the ohmmeter on the base lead of the transmitter. If it is an NPN transistor it will have a low, but possibly different, resistance when the positive lead of the ohmmeter is placed on either the emitter or collector lead. If it is a PNP transistor, it will show a high, but possibly different, resistance when the positive lead of the ohmmeter is placed on either the emitter or collector leads of the transistor.

• 3

  Use either the emitter-base or collector-base of the transistor for your diode. Remember that the lead that goes to the N type semiconductor will be the anode of the diode and the lead that goes to the P type semiconductor will be the cathode of the diode. You should also remember that one of the two possible connections (emitter-base or collector-base) might make a better diode than the other. Either connection will not be a perfect diode, because a diode consists of two similar-sized chips (N for cathode and P for anode) and each chip will have a similar number of current carriers. Transistors always have a thinner base slice and the number of carriers of the three slices are different.