Test a Bi-polar Transistor (out of Circuit)




Introduction: Test a Bi-polar Transistor (out of Circuit)

About: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying posting things I have learned and done since I got my first ...

You built a one transistor project and it worked great, but now it has stopped working. You decide the transistor may be faulty. But, you are not sure how to test it.

This Instructable is for testing a transistor after it has been removed from the circuit. When removing it from the circuit, always use a heat sink to protect the diode junctions from failure due to too much heat.

Pictured is an ordinary 2N2222 NPN low voltage bi-polar switching transistor. The order of pins from left to right is collector-base-emitter. The flat front provides the proper orientation for viewing the transistor. The order of the pins can vary, but the scheme used on this transistor is pretty common.

Step 1: Bias the Transistor for Testing

Use a 470 ohm resistor and a volt-ohmmeter with a diode check feature to check the transistor. As you can see a 470 ohm resistor has a yellow (4)-violet (7)-brown (x10) color band code.

The red lead from the meter connects to the positive socket on the meter. The black lead connects to the negative or common socket on the meter.

Normally, I just hold the resistor in one hand with the leads bent so I can touch two legs of the transistor at the same time. But, I needed one hand to operate the camera, so I used a breadboard to set up these photos. One lead of the resistor connects to the collector. The other lead of the resistor connects to the base. The positive lead (red) from the meter is connected to the collector. The negative or common lead (black) is connected to the emitter.

If this were a PNP transistor, rather than an NPN transistor, the red and black leads from the meter would be reversed in their positions.

Step 2: Turn on the Meter and Look for a Reading

Set the meter to the diode check position and turn the meter "on." If the transistor is good, there will be a reading similar to what you would expect across a diode junction. A word of caution: a weak or leaky transistor can show "good" with this test and still be faulty.

If you do not have a diode checker function on your meter, you can use the ohmmeter function, but the scale will have to be set to a very high range. I do not know why. It just works that way with my meter. Be careful, though. The reason you want a diode checker function is that it limits the current to the diode junctions in the transistor and protects them from overloads that could damage them. An ohmmeter could send too much current through the transistor and damage or destroy it.

I first became aware of this method of testing a bi-polar transistor through Using Your Meter by Alvis J. Evans. It was sold through Radio Shack and has a 1985 copyright.

Step 3: Testing Power Transistors

The same process for testing a small bi-polar transistor can be used for testing large power transistors. The metal case is the collector. The two pins are a bit above the center of the transistor, as you can visualize here. The left pin is the base. The right pin is the emitter. Use a 100 ohm resistor to bias the transistor instead of a 470 ohm resistor.



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      11 Discussions

      I did see your Instructable. I believe it was published after I had published this one. I have needed to test a transistor only on rare occasions and the method I described here (not original with me) has worked well. But, someone may find yours more suitable to their needs.

      The reason some multimeters have to be set to a higher ohmmeter rangesetting when checking diodes and transistors is because the voltage usedwithin the meter isn't high enough on the low ohms setting to overcomethe barrier voltage within the diode or transistor junction (usually 0.7volts for silicon) to allow it to conduct current.  On the higherohms ranges a higher voltage is used to allow enough current to measurethe ohms value. The ohmmeter must have current flow into and out of thedevice you are measuring to convert that current to an ohms reading.

      It is a neat procedure, if you can use it. I am glad it will be helpful to you. I may have the pinout wrong, though. I am trying now to get a circuit I built to work. It uses two 2N2222 transistors. Contrary to notes I made in a book, the pinout is E-B-C, not C-B-E as I gave it. As I have seen in a reference book I have, there is more than one way to bias a transistor to make it conduct.

      It's extremely useful no matter how you bias the Q. The EBC varies a bit to from model to model. Never really bothered about what number it uses, I just make sure EBC is in the right place and the transistor and resistor looks sturdy enough. Half the time I have to use several highpowered transistors in parallel anyway and the only resistors not yet blackened are the ones for 10W or more. Luckily I desolder components as a form of meditation :) What's the circuit you're working on for, amplifier?

      The circuit I have been trying to make work is supposed to fire an auxiliary photo flash in synchronization with the open shutter on a digital camera when the flash on the camera fires. The circuit causes the auxiliary flash's trigger cube to ignore the first or pre-flash from the camera and fire on the second flash that coincides with the open shutter. A description and a schematic can be found at: http://www.dulcemelos.ca/personal/inventions/digiflash.asp Last night I used my own Instructable to find and replace a bad transistor, but it still does not work. My flash is a Vivitar 283 just like the one the author used. The auxiliary flash seems to fire immediately, rather than after a slight delay. I have checked my circuit layout to be sure I followed the schematic. None of the capacitors show a short. It would be a useful device to have, since I have a couple of working Vivitar 283 flashes. I am thinking of trying a two stage time delay circuit built from two 555 timers or one 556 timer. The first stage would trigger from the pre-flash and start the second timer. It would close an open switch on the auxiliary flash's trigger circuit and the auxiliary flash would fire when the flash sensor cube "saw" the real flash from the camera.

      Cool circuit. I'm just guesstimating here you understand .. assuming the transistors are good and you've got the circuit hooked up properly, resistors rarely ever fail, unless the caps are different (looks like c1 & c2 are ceramic, c3 could be electrolyte) I suppose it could be a malfunction in the flash itself but failing that or the caps I can only suggest you try building the entire curcuit using all new components. 555-delay sounds like a good idea. I noticed a flaw in the transistor-testing. Sometimes when I overload a transistor it doesn't go all dead but still works a bit, this only shows up in the test as a higher resistance between C & E and isn't always noticable unless you know the previous value.

      All three capacitors are ceramic. The seemingly large value number on C3 is picofarads. An equivalency converter gives 1000 pf as 0.001 mf. I had to gang resistors together in series to get the values given. They are all new resistors and I checked the number of ohms after ganging them together. The URL I gave borrowed the circuit from another man, and some of the values are a little different in his circuit, but they are not supposed to be critical. I am tempted to build the circuit again laid out just like the schematic, in case I made an error I do not see. Thanks for the mention of the flaw. I think the test procedure was offered in the book I have as a simple "go/no go" test adequate for many normal circumstances. It does not cover every eventuality. Somewhere, as I mentioned, I read it may show a leaky Q as good. I have, however, used this little test on Q's in circuit. It works as long as something is not providing a parallel path. The author of the circuit says 5 volts is available at the flash contacts, yet flash trigger voltages are usually high enough that they will fry digital camera circuitry, if an old flash is fired from a new digital camera. I am not sure what the actual voltage is at the contacts and hesitate to risk my meter to find out. Thanks.

      The test is really great for figuring out what kind of Q I've unsoldered.