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SemTest: A Discrete Semiconductor Test Set; Pt.3

Now that we have looked at the full circuit of our new Discrete Semiconductor Test Set, it's time to describe its construction and the setting-up procedure. We also describe how to fit a crowbar circuit to quickly discharge the HT after making high-voltage measurements.

By Jim Rowe

Click for larger image
ABOVE: this handy table shows the pin connections for many discrete semiconductor devices. The ZIF socket on the front of the SemTest makes it easy to connect devices for testing.

AS SHOWN IN the photos, the SemTest is built in an ABS enclosure measuring 222 x 146 x 55mm. Apart from VR10 (the Mosfet VGS pot) and the five pushbutton switches (which mount directly on the front panel), all the components are mounted on one of two PCBs.

Both boards are double-sided, so there is no need to fit any wire links. Incidentally, our prototype photos in the February & March 2012 issues showed numerous link positions on both PCBs. These have now been incorporated into the copper patterns on the top layers of both boards so that is one less tedious task needing to be done.

The main board (coded 04103121) mounts in the bottom of the enclosure, while the display board (coded 04103122) sits behind the front panel and is spaced 18mm from it. The two boards are linked via three flat ribbon cables fitted with IDC connectors.

Rotary switch S2 is mounted on the lower PCB. Its control shaft is 42mm long, so that when the case is assembled, it passes through clearance holes in both the display PCB and the front panel.

Power switch S1 and 12V input connector CON1 are both located on the righthand end of the main board, towards the rear, and pass through holes in the righthand end of the enclosure. A small hole nearer the front of the enclosure provides access to trimpot VR2, which is used to set the micro’s 2.490V reference voltage.

Six similar holes along the top edge of the righthand end of the enclosure are used to access various trimpots mounted on the righthand end of the display board.

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