Testing transistors

Two Junction Transistor Model

A bipolar transistor has two junctions, which can be considered simply as two diodes for our testing purposes. To test a transistor you just need to check the forward and reverse resistance of both junctions. To do this we must first find out which pin is which.

 

 

 

Use The Manufacturer´s Information

The best way to check the functions of the pins is to use the manufacturer´s information. If you have a circuit diagram for the circuit containing the transistor, you should also find a "layout diagram" showing you what the circuit board looks like. Pins are often identified on this drawing. Another way manufacturers use to illustrate pin outs is a separate diagram showing the pin connections for each semiconductor used in the equipment.

Refer To A Supplier´s Catalogue

What happens if you have no manufacturer´s information? Next best is to refer to a supplier's catalogue, and using the number stamped on the device itself, look up the data for that device, which will usually include the "package type". This is given as a number referring to a standard package with a standard layout of pins. The semiconductor section of the catalogue will normally have at least one page devoted to drawings of transistor pin out diagrams.There are many supplier´s catalogues now on-line, most of which contain links to semiconductor data sheets. When looking for transistors or diodes these are usually listed in the "semiconductor" section. See our Transistor Web section

Finding The Pin Functions By Measurement.

If the transistor you are testing has a metal case, such as in theTO18, TO3, TO126, TO202 TO72 etc., that's helpful. A metal case or heat sink area is almost always connected to the collector so that the heat is dissipated easier. This means that if you measure the resistance from the case to each pin in turn, the one pin that measures zero ohms is the collector. What we really need to find however is the base. In transistor packages such as the TO18 or TO39 that´s easy; the emitter (or the source in FETs) is nearly always next to the metal tab and the collector is connected to the can.

Notice that often this makes the base the centre of the three connections - but this is not always the case; don't rely on the base being at the centre. Study the common package types illustrated in Fig 3.1.26c. There are variations even within the same type of package. The best source for transistor pin outs is normally a supplier´s catalogue or manufacturer´s data sheet.

If you have no pin out data available, you can still identify the pins by measuring the resistance between the various pins as the next page shows. Imagine to begin with that we suspect an unknown transistor may be a NPN type (these are far more common than PNP in modern circuits).

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