Quick On-Board Junction Tester Circuit Diagram

Description

Short circuits or broken pcb tracks can be easily recognized by means of a Multimeter, but this tool can give wrong results when testing the efficiency of a transistor or diode, unless the device under test is unsoldered and removed from the pcb. A further shortcoming affecting such way of testing is the necessity to keep firmly the probes on the pins of the device under test and at the same time to turn the head continually to read the Multimeter display.

This device allows the user to concentrate on the (often problematic) pcb probes placement, because a short, a broken track, a good or burnt transistor or diode, will be signaled by a beep, as follows:

Circuit diagram:

Parts:

Circuit operation:

Both inputs of IC1A are connected together by two equal value resistors (R4 and R5) and to half the voltage supply obtained by means of the voltage divider R2 and R3. So, the same voltage should be present at both input pins.

In practice, half the voltage supply (i.e. about 4.5V) will be present at the inverting input (pin #2) of IC1A, but the constant voltage generator formed by R6 and D1, feeding the non-inverting input (pin #3) of IC1A by means of the voltage divider R7 and R8, clamps this pin to about 4.1 - 4.3V: this will cause the output of the op-amp to stay low.

If the circuit input (R2 to R3 junction) is shorted to negative ground (a condition equivalent to a shorted transistor junction) pin #2 of the op-amp will go to 0V and the voltage at pin #3 will decrease to about 0.3 - 0.35V (caused by the constant voltage generator mentioned above): the op-amp output will go high, activating the piezoelectric sounder.

When a real transistor or diode junction is connected to the input of the circuit instead of shorting the input probes directly, the piezo sounder will emit only a short single beep just as the probes will come in contact with a good junction, due to the time delay provided by the discharge of C2 when the voltage at pin #3 is falling from about 4.1V to 0.3V.

To provide a better signaling system, Fet Q1, IC1B and related components were added. This op-amp is wired as a 1Hz square wave generator and Q1 acts as a solid-state switch, going on and off one time per second having the Gate driven by the op-amp output. In this way, the junction of the device under test is connected and disconnected to the voltage sensitive circuit built around IC1A one time per second and the result will be a clearly audible train of short beeps signaling the good condition of the junction or track under test.

Testing directions:

NPN Silicon Transistors:

Place the Red probe on the Base and the Black probe on the Emitter: a train of short beeps should be heard. If not, the junction is broken or the transistor is a PNP type. Always holding the Red probe on the Base, shift the Black probe to the Collector: a train of short beeps should be heard. If not, the junction is broken or the transistor is a PNP type.

Placing the Red probe on the Emitter and the Black probe on the Collector should cause no output from the piezo sounder: the same should occur when reversing the probes. On the contrary, if an almost continuous beep is heard, the transistor is dead.

PNP Silicon Transistors:

Place the Black probe on the Base and the Red probe on the Emitter: a train of short beeps should be heard. If not, the junction is broken or the transistor is a NPN type. Always holding the Black probe on the Base, shift the Red probe to the Collector: a train of short beeps should be heard. If not, the junction is broken or the transistor is a NPN type.

Placing the Red probe on the Emitter and the Black probe on the Collector should cause no output from the piezo sounder: the same should occur when reversing the probes. On the contrary, if an almost continuous beep is heard, the transistor is dead.

Darlington Transistors:

The procedure is similar to that adopted for common transistor types. The main difference is that when testing the Base - Emitter junction, you will hear the train of short beeps even after reversing the probes. This occurs because a couple of resistors is always present across either junction of the two internal transistors forming a Darlington device.

The second difference is due to the fact that an internal diode connected across Emitter and Collector (Anode to Emitter and Cathode to Collector) is always present in these devices. Therefore, with a NPN device, placing the Red probe on the Emitter and the Black probe on the Collector you will hear the usual train of short beeps, but when the probes are reverted there will be no output from the piezo sounder. On the contrary, if an almost continuous beep is heard, the transistor is dead.

PNP devices of this type are tested reversing the probes, as explained above for common transistors. Please note that when testing the Base - Emitter junction the beeps will be shorter compared to common transistors. This is caused by the fact that two junctions in series are to be measured when testing Darlingtons.

FETs:

The testing procedure is the same as that adopted for NPN silicon transistors (N-Channel FETs) or PNP silicon transistors (P-Channel FETs). The only difference is shown when checking Source - Drain connections (corresponding to Emitter - Collector): a faint, blurring sound will be heard if the device is good, even reversing the probes.

MosFets:

These devices cannot be thoroughly tested with this tool, but a MosFet in good condition should cause no beep to be heard when testing all junctions as explained above for common transistors. But the usual train of beeps will be emitted when checking the Source - Drain connection, placing the Red probe on the Source and the Black probe on the Drain of a N - Channel device, because the presence of an internal diode, as explained above for Darlington transistors.

Germanium Transistors:

Use the same testing procedure adopted for silicon transistors. The beeps forming the train will last longer than when testing silicon devices: this is due to the lower junction resistance of germanium devices in respect to silicon types.

Silicon Diodes:

Place the Red probe on the Anode and the Black probe on the Cathode: a train of short beeps should be heard. Reversing the probes no beep will be emitted.

Schottky Barrier Diodes:

The same as above, but the beeps should last longer.

Germanium Diodes:

The same as for Schottky Barrier Diodes.

SCRs and TRIACs:

These devices cannot be tested thoroughly, unless they are shorted: in this case an almost continuous beep will be heard. But this circuit can be useful to distinguish a SCR from a TRIAC. Placing a probe on the Gate and the other probe on the Cathode or, more properly, the MT1 pin of a TRIAC, the Tester will emit the usual train of beeps, even reversing the probes. When testing a SCR, the train of beeps will occur when the probes are placed in one way and not when reversed.