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Topic: How to properly test a transistor (Read 1 time) previous topic - next topic

oric_dan

#15
Jun 13, 2012, 07:09 pm Last Edit: Jun 13, 2012, 07:12 pm by oric_dan(333) Reason: 1
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.

dlhylton

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.


JimboZA


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
My hovercraft is full of eels.

No PMs for help please.
DO NOT power servos from Arduino 5V: give them their own power and connect the grounds.

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