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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 20th Jun 2009, 3:14 pm   #1
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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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Old 20th Jun 2009, 3:43 pm   #2
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Default Re: Testing Transistors.

Moving on to transistors the device being tested here is an NPN silicon device. As with most, but not all, transistors the base lead is in the centre with the emitter and collector leads on either side.

The test procedure is to connect the black lead to the base and the red lead to the collector and emitter in turn. Then repeat with the red lead connected to the base and the black lead connected to the collector and emitter in turn. Note that in one direction we see a forward volt drop and in the other an infinite resistance.

As long as you can identify the base lead it doesn't really matter which is collector and emitter, so long as you see volt drop one way and infinite resistance the other.
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Old 20th Jun 2009, 3:52 pm   #3
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Default Re: Testing Transistors.

A final test is to test between collector and emitter as shown here. Expect to see a high resistance in both directions.
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Old 20th Jun 2009, 3:57 pm   #4
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Default Re: Testing Transistors.

If you see results as shown above the transistor is generally good. On very few occasions I have seen transistors which are OK on a diode test, but have no gain.

Faulty transistors will generally have open circuit or short circuit junctions. The former will read high resistance both ways and the latter will show a very low or zero volt drop both ways.
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