In-Circuit Tester for SCR, Diodes, and Transistors

The circuit has been constructed to provide an in-circuit system that will test the integrity of electronic components such as Silicon Controlled Rectifiers, diodes, and PNP or NPN transistors.

• 4093 – a quad 2-input NAND with Schmitt trigger inputs integrated circuit, generally characterized by small fluctuation in voltage supply, very high impedance, outputs that can sink and source, one output can drive up to 50 inputs, high speed gate propagation time, high frequency, and low power consumption

The tester works by indentifying the reliability of electronic components which are not connected live to the circuit as well as their polarities and the transistor type, if unknown. Using only a single CMOS IC CD4093, a frequency of 2 Hz is produced as the IC1a & IC1b operates as a square wave oscillator while the inversion of the 2 Hz signal is performed by IC1c & IC1d. The power of Device Under Test (DUT) is supplied by the two complementary square wave voltages while biasing the transistor through the 1K Ohm resistor.

Across the output are two opposing mode red LEDs which are connected in parallel. The 470 Ohm resistor limits the current flow through either LED with either LED having their peak inverse voltage (PIV) limited to about 1.7 V. This value serves as the ON-state or conducting voltage of the other LED. When the push button for testing is pressed without the presence of DUT, an alternate flashing will be exhibited by the LEDs.

Upon connecting a good working transistor, one LED will remain dark and be effectively short circuited, during the half cycle the biasing is turned ON. The alternate half cycle will not cause the DUT to switch ON due to the polarity of the applied signal. This will then result to the other LED to illuminate due to the current that will flow. The illumination and flashing of the LED signifies the 2 Hz frequency. If the DUT is shorted, both LEDs will remain dark while flashing will appear on both LEDs if the DUT is open.

The only allowed current pathway through the DUT is the primary function of the two strings of series connected silicon diodes that are connected in series with the DUT. This is only made possible when the diodes are fully saturated or turned ON which can be proven when an insufficient current flow through parallel circuit resistors which will cause one or both LED to be dark. The design of this circuit can be referred to as an in-circuit tester where no messy soldering done to be able to isolate a faulty component in case.

For the testing of diodes and SCRs, the switch S1 is placed to the proper position wherein one of each two series pair of diodes is switched out. Doing this operation is essential due to the forward ON-state voltage of a good diode or SCR being about 0.7 V. this will then cause the three conducting series junctions to have about 2.1 V across the normally dark LED.

The in-circuit tester is primarily used for indicating if a transistor fails or passes by using the combined result of bvceo and false oscillation tests. It can also use the alternating line current in phase between emitter-base and emitter-collector for testing power transistors. This apparatus can be used to determine the operating characteristics and defects of silicon controlled rectifiers and turn-off controlled rectifiers under dynamic conditions.

Source:users.otenet.gr/~athsam/tester_vk6fh.htm