Of course, there are lots of semiconductor testers out there. These range from the handy pocket-sized instruments produced by Peak Electronic Design Ltd in the UK to large laboratory bench instruments made by Agilent and costing many thousands of dollars. The former group are not able to test the range of semiconductors that we would like, while the latter instruments are beyond the reach of all but a few research labs.
So Publisher Leo Simpson set me the task of producing a new design. It had to be easy to drive and would be somewhat similar in concept to the “Test Set For Transistors & Diodes” featured in Electronics Australia magazine way back in the July and August 1968 issues (yes, back in the olden days – and it was my design too!).
It was pretty simple – using a bunch of rotary switches, a 50µA moving coil meter and olde-worlde point-to-point wiring – but it could perform most of the basic tests that were needed on the discrete semiconductor devices of the day.
I took one look at that old 1968 design and shuddered: all that point-to-point wiring – all those switches – no PCB – an analog meter. Gaaakkk! What could Leo be thinking? Not only that, it was designed long before Mosfets were even thought of and we would have to include them, of course.
In the fullness of time (a silly expression glossing over the trials and tribulations – not to mention the blood, sweat and tears – of producing a completely new design), we came up with the SemTest. It’s otherwise known as a Discrete Semiconductor Test Set – which is too much of a mouthful.
It’s around half the physical size of the 1968 design and it’s controlled by a microprocessor, with a 16x2 LCD panel used to display the device to be tested, the test to be run and the test results. There are a minimum of front panel controls: one rotary switch, one pot and five pushbuttons. And the curly problem of catering to all the different semiconductor sizes and pin-outs has been solved by employing an 18-pin ZIF (zero insertion force) socket.
These sockets are normally used for programming microprocessors but they are ideal for this application.
All the parts inside the case are accommodated on two medium-sized PCBs which are connected together by three IDC cables.
However, before we jump into describing the circuitry of the SemTest in detail, we need to discuss the tests it can perform on each type of the most commonly used discrete semiconductors. After all, if you are contemplating building the SemTest, you will want to understand all the tests that it can run.