Triode good or bad tester

Test - lowercase jpg

Kaixin micro test

Test probe P100-M3

LED light-emitting diode

This straightforward circuit has proven extremely useful for various applications. It allows me to assess the quality of transistors efficiently. In the circuit, whether it's the BC junction with a resistance as low as 40 ohms or the BE junction, I can use it to check the performance of power output stages in amplifier circuits. The transistor plays a critical role here.

The underlying principle of this circuit is quite fascinating:

In this setup, the 555 timer (IC1) functions as a 12 Hz oscillator, driving the 4027 (IC2) bistable circuit. By dividing the signal by two, the outputs at pins 14 and 15 drive LED1 and LED2 via current-limiting resistor R3. These LEDs are configured such that when transistors of different polarities are connected to the circuit, only one LED lights up at a time. If no transistor is inserted, the two LEDs alternate flashing. The voltage from pins 14 and 15, divided by resistor R4, connects to the B terminal of the tested transistor via R5. When a good transistor is connected, it conducts, lighting one of the LEDs. For example, if an NPN transistor is connected, LED1 will flash; if a PNP transistor is used, LED2 will illuminate. If an open-circuit transistor is attached, both LEDs will blink alternately. Conversely, if a short-circuit transistor is used, neither LED will light up.