When measuring breakdown voltage, the meter is in series, from the variable high voltage supply to the collector of the transistor being tested. The metering circuit is designed to provide full-scale with an input of 10V, which matches the current ranges. In 'normal' mode (testing gain), the meter is in parallel with the shunt resistor selected by the 'Range' switch. The highest range was limited to 5A deliberately - even at this current, the transistor will be dissipating up to 20 Watts worst case, so the device under test should be mounted on a heatsink, or the test should be kept to very short duration, otherwise the transistor will overheat and may (will) be destroyed or severely impaired. The meter range extends from the maximum meter sensitivity of 100♚ in decade steps up to 1A. The results are more than acceptable, and because of the design of this unit, it is possible to observe gain droop and other undesirable phenomena up to the maximum current.įigure 2 - The Function Switching For The Tester The basic method for testing the gain of a transistor is shown in Figure 1, and although not the ideal, is far simpler to implement than using a fixed collector current. The 'testers' in some multimeters are even worse - they can only test at very low current. Microcontroller 'all purpose' testers are incapable of testing a transistor to a reasonable current, and while convenient, they cannot replace a dedicated tester. They were once at least available, but a recent search has shown nothing that comes even close. It's very doubtful that you'll be able to buy a transistor tester that matches the specifications and features of the design shown here. Always check the ranges before pressing the test button! For example, if you leave the base current set for 10mA and the collector current range at (say) 1A or more, when you try to test a small signal transistor it will probably fail immediately. The author takes absolutely no responsibility for any damage, whether direct or consequential that may be done to the device under test or the operator due to the use or inability to use the project described. It is entirely the responsibility of the user to ensure that the settings are correct before pressing the Gain switch. Like any similar commercial offering, this tester is just as capable of blowing up a transistor as it is of testing it. Maybe I'll have to make one of these next. Mine (unfortunately) does not have the separate base current and collector current range switches, and is somewhat less useful as a result. Bear in mind that this tester is different from mine (it has more features), so don't try to make a direct comparison of the switching. There are a couple of photos of my unit at the end of this article, so you can get some idea of what it might look like when finished. (I have actually had mine for so long that the variable high voltage source used a valve - it has only recently been replaced with a transistor.) This design is actually better than my existing unit - it has a bigger power supply, and is more flexible in operation. Breakdown voltage (with or without R be - value is selectable)Īs with some of my other projects, this is not particularly cheap to build, but if my own unit is anything to go by will give many years of faithful service.Gain at various collector currents up to 5A.Gain (also referred to as h FE, β or Beta).The design featured here is just what you need, and provides the ability to test: They are less complex than this version, and cannot test breakdown voltage (no high voltage supply), but you may find one of those to be a bit easier to build than the version described here. I suggest that you also look at Project 106 (h FE Tester for NPN Power Transistors) and Project 177 (Constant Collector Current h FE Tester for Transistors). The collector current is usually limited to a few milliamps at most, and that's completely useless for a power transistor which may not show any useful gain until it's conducting somewhere between 10 and 100mA. Don't assume that because your multimeter (or small 'automatic' component testers) can test transistors so are capable of testing power devices, because they are not. When building amplifiers or any other power stages, it is often necessary to test transistors, either to verify that they (still) work, or for some esoteric designs it may even be necessary to match certain characteristics.
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