Triode good or bad tester

Test - lowercase jpg

Kaixin micro test

Test probe P100-M3

LED light-emitting diode

This straightforward circuit proves incredibly useful in various scenarios, especially for checking the quality of transistors. It can be used to measure the resistance between the BC pins, which should ideally be around 40 ohms, or between the BE pins. Additionally, it’s handy for examining the power output in amplifier circuits.

The underlying principle of this circuit is quite simple:

In this setup, the 555 timer (IC1) functions as a 12 Hz oscillator, with its three-pin output driving a 4027 (IC2) bistable circuit. After dividing the signal by 2, the output appears at pins 14 and 15. These outputs are then connected to LEDs (LED1 and LED2) via a current-limiting resistor R3. The arrangement of the two LEDs ensures that only one LED lights up when a transistor of a specific type is connected. If no transistor is inserted, both LEDs will alternate flashing.

Pins 14 and 15 also connect through a voltage divider resistor R4 to the base (B) of the tested transistor via R5. When a functional transistor is attached, it turns on, allowing one of the two LEDs to illuminate. For example, when an NPN transistor is connected, LED1 will flash; if it's a PNP transistor, LED2 will light up instead. If the transistor is open-circuit, the two LEDs will flash alternately, while a short-circuited transistor will leave both LEDs unlit.

Overall, this little device is a great tool for diagnosing issues related to transistors and understanding their behavior in different circuits. It’s compact, easy to use, and provides quick feedback, making it indispensable for hobbyists and professionals alike!