To test an NPN transistor using this circuit, simply reverse the ohmmeter leads and carry out the procedure described earlier. A 10-to-1 resistance ratio in this test between meter readings indicates a normal gain for an audio- frequency transistor. Current now flows in the emitter-collector circuit, which causes a lower resistance reading on the ohmmeter. When the switch is closed, the emitter-base circuit is forward biased by the voltage across R1 and R2. Therefore, the ohmmeter should read a high resistance, as indicated on the meter. base so that it is able to supply majority charge carriers to the base. The emitter is always forward biased w.r.t. With the switch in figure 2-18 in the open position as shown, no voltage is applied to the PNP transistor's base, and the emitter-base junction is not forward biased. Emitter It is left hand section (or region) of the transistor and its main function is to supply majority charge carriers (electrons in case of NPN transistors and holes in case of P-N-P transistors) to the base. Figure 2-18.Testing a transistor's gain with an ohmmeter. Any internal battery may be used in the meter provided that it does not exceed the maximum collector-emitter breakdown voltage. The only precaution you should observe is with the ohmmeter. The principle behind the test lies in the fact that little or no current will flow in a transistor between emitter and collector until the emitter-base junction is forward biased. The test circuit can be made with just a couple of resistors and a switch, as shown in figure 2-18. ≺ basic transistor gain test can be made using an ohmmeter and a simple test circuit. Tests of a transistor's junction resistance will reveal leakage, shorts, and opens. Testing Transistors with an Ohmmeter Two tests that can be done with an ohmmeter are gain, and junction resistance. The measurement is taken with specified values of IC and IB. Test both the base- collector and the base-emitter. Since it is impractical to cover all the different types of transistor testers and since each tester comes with its own operator's manual, we will move on to something you will use more frequently for testing transistors-the ohmmeter. What It Is: Collector-emitter saturation voltage is the VC below which an increase in IB does not cause an increase in IC. 2N2219 you expect to find a forward voltage of 0.6 or 0.7 volts. For maintenance and repair, however, a check of two or three parameters is usually sufficient to determine whether a transistor needs to be replaced. The Base region triggers and controls the amount of current flows through the Emitter to Collector. The Collector region collects most of all charge carriers emitted from the Emitter. There are four basic tests required for transistors in practical troubleshooting: gain, leakage, breakdown, and switching time. Regarding the three terminals of the NPN transistor, the Emitter is a region is used to supply charge carriers to the Collector via the Base region. With most transistor testers, it is possible to test the transistor in or out of the circuit. 2-34 Transistor testers are nothing more than the solid-state equivalent of electron-tube testers (although they do not operate on the same principle).
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