Test for the Determination of Transistor is NPN or PNPThis simple Circuit helps you to distinguish between PNP or NPN type transistors.
This circuit consisting of two diodes 1N4001 and two LEDs and a Heavy series resistor.
Connect the two leads of transistor as shown in circuit diagram.
The current passes through both the diodes hence both the LEDs glow alternately. But it appears as both the LEDs are glowing continously due to the 50Hz supply frequency.
Now let the emitter base junction of a PNP transistor is connected across the end terminals as shown in figure. In This case current doesnot flow through the circuit during the half cycle when the Emitter Base junction is reverse biased. During the half cycles in which Emitter Base junction is forward biased, current flows through LED1 series Resistor Emitter Base junction and diode D2. Thus only LED1 glows. This test indicates that the terminal connected to the test led 1 is P-type and that connected to test led 2 is N-type. Thus P-type and N-type terminals of a P-N junction is identified, LED 1 will glow if COLLECTOR-BASE junction of PNP transistor is connected across test leds 1 and 2. However, none of the LED glows when test leds are connected to the collector and Emitter of the transistor because in this case Base is open circuited.