Quick On-Board Junction TesterCircuit

Posted on Feb 5, 2014

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

Quick On-Board Junction TesterCircuit
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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: A train of one-second lasting beeps spaced by a very short silence (in practice an almost continuous beep) indicates a shorted junction or, on the contrary, a good pcb track 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...

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