The tester can be used to test virtually any kind of semiconductor device, ranging from switching diodes to power transistors. ln addition. it provides a rough indication of the gain of bipolar transistors, and, more generally, can be a useful aid in finding functional, short-circuited and internally open devices in semiconductor batches.
The tester is based on a single CMOS IC and a bi-color LED as a visual indication. Gate IC1a forms an RC oscillator. The oscillator signal is buffered and made available in true and inverted form by the three remaining gates in the IC.
The bi-color (red/green) LED indicates the direction of the current that is allowed to pass through the test probes or the device under test. Resistor R1 functions as a current limiter.
The signals at the input and the output of gate IC1, are applied to a pair of test probes. a two-terminal test socket for diodes, and a three-terminal transistor socket. The base current for the transistor on a test (TUT) can be set with preset The preset may be calibrated with the aid of known, functional transistors to give an approximate gain scale.
Only one LED lights when a semiconductor is functional. The LED color then indicates the polarity (n-p-n or p-n-p, or cathode/anode). When the component is internally open, no LED lights. A semiconductor with an internal short-circuit is easily recognized by the green and red LEDs lighting simultaneously at about equal intensity. Transistors must be connected with the base, collector and emitter pins to the indicated socket terminals. so check the pinout before running the test.
The circuit may also be used as a simple continuity tester. It draws a current of about 300 without a DUT (diode on a test) or TUT (transistor on a test) connected. and about 7.5 mA with the probes short-circuited.
