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Diode and transistor tester

This gadget allows us to test diodes and transistors very easily. When testing diodes, it tells us which terminal is the cathode and for transistors it shows whether it's an NPN- or PNP-transistor.

The circuitry around opamp U1.A make an oscillator. Directly after connecting the power supply, C1 is still empty and thus the voltage across it will be 0V. The output will carry the supply voltage. The output is also said to be 'high'. C1 is charged via R4. When the voltage over C1 exceeds the voltage at pin 3, the output voltage will be 0V, or 'low'. C1 will now discharge via R4 until the voltage is less than the voltage at pin 3. Now everything starts all over again. Resistor R3 provides the hysteresis.

U1.B's invertering input is connected to U1.A's non-inverting input and vice versa. So the voltages at pin 7 and at pin 1 are in antiphase. That is: if pin 1 is 'high', pin 7 is 'low' and vice versa.

Next, we connect our test diode to pins 1 and 3 of connector J1, making sure the cathode is connected to pin 1. If U1.B's output is 'high' (and U1.A's output is 'low'), a current will flow from U1.B via the test diode, D2 and R7 to U1.A. LED D2 comes on. After the oscillator flipped the output of both opamps, there is no path for the current to flow. After all, the test diode is in reverse. So actually D2 blinks, but the frequency is so high that you won't notice that. When we swap the terminals of our test diode, only D1lights. So the LEDs show us which side of the diode is the cathode. They also show if the diode is working properly: if both LEDs light, the test diode apparently conducts current in both directions. If neither of the LEDs light, the diode blocks current flow in both directions and we can trash it as well.

Let's now try connecting a NPN-transistor to J1. The diagram above shows where the collector, base and emitter go. If U1.A is 'high', current will flow via R5, the base and emitter of our transistor to U1.B which will be 'low' of course. If the transistor functions properly, it will now switch on, allowing current to flow from U1.A, via R6, D1, the collector and emitter of the transistor to U1.B. So, in case of a NPN-transistor, D1 comes on. If we connect a PNP-transistor D2 lights. And again: if both LEDs come on of remain off, the transistor is busted.

To make it easier for you to assemble this nice device, a PCB has been disigned. The design can be downloaded in JPEG, EPS and HPGL format.

The picture below shows which component goes where.

J1 is a 5 pin socket. Of course 3 pins are enough to test a transistor, but this design allows us to test any transistor no matter if the base, collector or emitter is in the center. If we use a 3 pin socked, we may need to bend the terminals of some transistors, which may result in shorts.

The power supply can be a 9V block battery of 4 AA batteries in series.

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