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Old 20th Jun 2009, 3:14 pm   #1
Station X
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Default Testing Transistors.

Transistors are basically two diodes connected back to back. A PNP Transistor is like two diodes connected cathode to cathode. An NPN Transistor is like two diodes connected anode to anode.

If we can find a simple way to test diodes we have a simple way of testing transistors. Diodes pass current on one direction only, so by measuring their resistance in both the forward and reverse directions we can tell whether a diode is good, open circuit, or short circuit. An analogue meter set to the ohms range can be used to test diodes in this way, but it doesn't work so well with Digital Multimeters (DMMs).

Many DMMs have a diode test facilty. With the diode connected one way (forward biased) they display forward volt drop and with it connected the other way round (reverse biased) they display reverse resistance.

The pictures illustrate this. Note that the meter leads are connected to the correct sockets. The meter's internal battery's positive terminal is connected to the positive lead of the meter. As far as transistors and semiconductor diodes are concerned current flows from positive to negative. Being a silicon device this diode has a forward volt drop of about 0.6 volts. If testing a gemanium device expect to see about 0.2 volts. The 0L reading denotes infinite resistance.

The markings on the diode don't really matter when it comes to testing. Note however that the marked end is the cathode, as if used in a rectifier circuit this would be the positive side of the supply.
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