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Author Topic: How to properly test a transistor  (Read 1628 times)
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the land of sun+snow
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The first thing you think about when seeing low-value Rs [eg, 10 ohms]
is "lotsa power dissipation", meaning the Rs and also the NPN need to
be largish parts.

Full power into the R would mean 9V*9V/10ohms = 8.1 watt. In anything but
a well-designed high power ckt, that's a lot of heat.

Also, in this case, the NPN can conceivably dissipate a lot of heat too. Worst
case is for 9V/2 = 4.5V on the collector, which would mean Ic = 4.5V/10ohms
= .45A, and PD(npn) = 4.5V * .45A = 2 watts. A little TO-92 device can only
dissipate 0.3 watts or so before overheating. Even a TO-220 case will get pretty
warm with PD = 2 watts, and you need to think about heatsinking even here.

Then, the other thing that happens is, when the transistor gets hot, hFE
changes a great deal too. hFE is so variable, all in all, that good ckts are
designed to take this into account.
« Last Edit: June 13, 2012, 12:12:02 pm by oric_dan(333) » Logged

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To test a transistor, you can do simple resistance tests on the various diode junctions, as illustrated here, http://www.learningaboutelectronics.com/Articles/How-to-test-a-transistor. Measure each of the pairs of diode junctions. Collector-emitter, collector-base, base-emitter. Read the resistance of one junction and then read the same junction with the polarity probes switched. One side should read very high resistance, over 1 megohms. And the other should read a moderate resistance, a few hundred thousand ohms. If this is the case for all three junctions, the transistor should be a good working one.

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I have used this circuit for years to quickly sort PNP/NPN transistors acquired from "recycling" or grab bag purchases.  It can also be used "in circuit", provided your hardware it belongs to is powered off.
http://circuitschematicelectronics.blogspot.com/2012/04/transistor-checker-with-555-timer.html

Hm, had a look at that and the instructions are "interesting" to say the least? Not sure I like the idea of the LEDs bursting! But it might be worthe building just for the heck of it.

Jim
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