Using the Fault Finding Chartįollow the instructions in boxes 1, 2 and 3 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), and it may be faultyįig.7.3.3 Identify Transistor Pinouts and Findįaulty Transistors. Plan C - Testing Transistors With Unknown Pinouts.Īnother useful way to find the base however is to measure the resistance between the various pins. So if Plan B has not solved the puzzle, no worries, there is always Plan C. There are variations even within the same type of package. Study the common package types illustrated in Fig.7.3.2. 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. In transistor packages such as the TO39 that's easy the emitter is nearly always next to the metal tab and the collector is connected to the can. What we really need to find however is the base. This means that if you measure the resistance from the case or the metal mounting area to each pin in turn, the one pin that measures zero ohms is the collector. A metal case or heat sink area is almost always connected to the collector so that the heat is dissipated easier. If the transistor you are testing has a metal case, such as in the layout plans for the common encapsulations TO18, TO3, TO126, TO202 TO72 etc., that's helpful. What if you can't find the vital transistor identity number on the transistor itself? All is not lost you can still find the pin functions by a little detective work. Plan B - Finding The Pin Functions by looking at the transistor package information. In the off chance that you can't find the right information it will be necessary to resort to Plan B.įig.7.3.2 Common Transistor Packages. You also should find a diagram in the datasheet showing the transistor pin connections (pinouts) where the collector, base and emitter pins and any variants are shown. Just type the transistor number in internet search bar and you should find a selection of sites publishing the data you need. The best way to check the functions of the pins is to use the manufacturer's datasheet, just about every transistor ever made has its own datasheet on the Internet. Plan A - Use The Manufacturer's Data Sheet However to do this it is first necessary to find out which pin is which. So to test a transistor you just need to check the forward and reverse resistance of each of these junctions. For the purposes of testing, these can be thought of as simply as two diodes with one common connection i.e. Fig.7.3.1 The Two Diode Transistor Model.Īs shown in Fig.7.3.1 whether a transistor is (a) an NPN type or (b) a PNP type bipolar transistor, it consists of two diode junctions, the base-emitter junction and the base-collector junction.
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