In the previous Interface article we considered the basics of using a digital to analogue converter to provide analogue to digital conversion. In particular, the use of this method in an "improved" transistor tester was considered. Most transistor testers use a fixed base current which produces a collector current that varies widely depending on the gain of the device. The method outlined previously uses a "suck it and see" approach that steps-up the base current until a certain collector current is achieved. In other words, it uses a fixed collector current and a variable base current. This ensures that the gain reading is always taken at a reasonably high collector current where the test device will operate quite efficiently. This avoids having low gain but serviceable transistors being erroneously indicated as duds.
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