Universal BJT, JFET, MOSFET Tester Circuit

This useful transistor tester allows the user to quickly check the functionality of an NPN/PNP transistor, JFET or (V)MOSFET as well as determine the orientation of their terminals, or the pins appropriately.

A three-pin BJT or FET provides an overall 6 feasible correlated configurations, however just a single will likely be the right one.

This universal transistor tester circuit offers a easy and foolproof recognition of the appropriate transistor configuration as well as creates a practical examination of the transistor simultaneously.

How the Circuit Works

The tester circuit on its own includes a transistor that collectively with the transistor-under-test (TUT) forms an astable multivibrator circuit.

The tester features 5 testing slots in close proximity with each other, determined by their respective labeling:

E/S - B/G - C/D - E/S - B/G
This arrangement makes it possible for the below shown devices to be examined through the mentioned configurations:
• Bipolar Transistors: EBC / BCE / CEB, and reversed: BEC / ECB / CBE.
• Unipolar Transistors (FETs): SGD / GDS / DSG, and reversed: GSD / SDG / DGS.

The astable multivibrator stage of the circuit oscillates and blinks a bright white LED (Figure 1) whenever the transistor under test is connected the right way. The LED could also flash if the E and C pins of the transistor are swapped, however the blinking speed is going to be faster.

This demonstrates the truth that a few varieties of BJTs can function even when their emitter and collector leads interchanged although with a performance characteristics that may be lower than in the normal configuration.

Testing JFETs

While testing JFETs having a symmetrical source and drain structure, it may be only feasible to distinguish the gate pin with any level of assurance, and the source and drain pins could be interchanged.

The load resistance of the transistor-under-test is constructed like a potential divider circuit with half the supply voltage by using resistors R3/R4. This enables an ordinary switch (S1) to swap from N(PN) to P(NP).

Using an LED Indicator

A flashing LED reveals proper positioning of the device under test! If the LED remains shut off or remains ON constantly indicates an incorrect configuration or dead, blown BJT.

This situation can additionally indicate that the unit being tested may be simply not a transistor.

The item could possibly, for instance, be a 3-pin voltage regulator, an SCR or a triac and so on.

Using a Buzzer Indicator

The next variant of the universal transistor tester exhibited in the figure below employs a piezo buzzer instead of the LED indicator. The frequency determining capacitor value in this design can be seen much reduced compared to the LED version in order to increase the oscillation frequency and make it audible.

A low volume buzzing sound from the buzzer signifies that the transistor is rightly inserted and is perfectly doing the job.
If there's no sound from the buzzer indicates that the BJT or the FET under test is either inserted incorrectly or it may be completely dead.

The push button allows you to switch the circuit on and check the transistor simultaneously as soon as it is hooked up. The entire circuit can without any difficulty accommodate over a tiny piece of veroboard. Power supply can be obtained from a standard 9 V PP3 battery.