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HOWTO: BJT sanity check tutorial

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HOWTO: BJT sanity check tutorial

Postby amb » April 9th, 2010, 5:18 am

Introduction

This tutorial is about how to do sanity check of a BJT (bipolar junction transistor). You can use this method to test "loose" transistors (i.e., not installed in a circuit). Most of the time you could also do this with the transistor still mounted on the circuit board (with some caveats, see "Exceptions" below). This tutorial also assumes a silicon transistor, but the same applies for a germanium transistor except replace all occurences of "0.6V to 0.7V" mentioned below with "about 0.2V".

The method outlined below cannot be used to test a FET (field effect transistor), whether it's a JFET (junction field effect transistor) or MOSFET (metal oxide semiconductor field effect transistor).

Background

This tutorial uses the "diode check" function of the DMM (digital multi-meter). This function is normally used to check the forward voltage drop of a diode, but it can also verify basic diode integrity. This function is selected by setting the meter's dial to the "diode check" mode, which is usually indicated by a diode symbol on the dial marking Not all meters have a diode check function, if yours does not, then you cannot use the procedures outlined below.

A diode is made up of a P-N semiconductor junction:

Image

The image below shows the DMM being used to test a diode's forward voltage drop:

Image

The meter display shows the forward voltage drop of the diode. If you reverse the probes, the readout should show "over-range". Some meters display "OL" (overload) and others simply shows "1" (with no decimal point) to indicate this condition. This checks the reverse voltage of the B-C junction "diode", which (if the diode is sane) should not conduct.

If the forward voltage is what you expect (for most silicon diodes this is between 0.6V and 0.7V), and the reverse voltage is over-range, then the diode is ok.

Bipolar Junction Transistors

There are two basic types of BJTs, "NPN" and "PNP". These refer to the "polarity" of the device, and stems from the basic construction of the transistor:

Image

As its namesake suggests, the NPN transistor is comprised of N-P-N junctions, and the PNP is P-N-P. Since each P-N junction is a diode, you can treat the BJT as the equivalent of the following:

Image

Given this, we could then also check the basic sanity of a transistor by using the DMM's "diode check" function.

Checking NPN transistor

1. First, you should read the pertinent transistor's datasheet to identify the pin-out (i.e., which pins are the B (base), C (collector) or E (emitter).

2. If the transistor is in a circuit, make sure the power is turned off and all capacitors are discharged. Set your DMM to the "diode check" mode.

3. Touch the DMM's + probe (red, if you have the probe wires plugged in to the meter correctly) to the transistor's B pin, then touch the - probe (black) to the transistor's C pin. You should read between around 0.6V to 0.7V on your meter display. This checks the forward voltage of the B-C junction "diode".

4. Keeping the red probe on the B pin, now touch the black probe on the E pin. You should also read around 0.6V to 0.7V. This checks the forward voltage of the B-E junction "diode".

5. Now touch the black probe on the B pin, and the red probe on the C pin. Your meter should read "over-range".

6. Keeping the black probe on the B pin, now touch the red probe on the E pin. You should also read "over-range". This checks the reverse voltage of the B-E junction diode.

7. Lastly, put the red probe on the C pin, and the black probe on the E pin, note the reading. Then reverse the two probes and check the reading again. Both of these should show "over-range".

If any readings are not as expected, then the transistor might be damaged (see "Exceptions" below). In particular, if any measurement is far below the expected 0.6V to 0.7V, then there might be an internal short in the transistor. A higher-than-expected forward voltage reading might indicate that the junction has become open-circuit. Both are possible failure-modes of a BJT.

Note that this only verifies the basic integrity of the transistor. Passing this test does not guarantee that the transistor would meet all of the published specifications. More advanced testing would be required for that.

Checking PNP transistor

Checking the PNP transistor is identical to the procedures for NPN above, except the red and black probes must be reversed for each of the measurements.

Darlington transistors

A Darlington transistor is a compound BJT pair internally connected as follows (NPN device shown):

Image

Note that the external B-E junction consists of two actual P-N "diodes" in series, so the measured forward voltage should be 1.2V-1.4V instead of an ordinary BJT's 0.6V-0.7V. The external B-C junction is only one "diode" (on the first internal transistor), so it will have a forward voltage of 0.6V-0.7V. Other than that, the sanity check procedure is the same as for an ordinary BJT.

Exceptions

When checking a transistor in-circuit, other components in the circuit may affect the test and produce false results.

Some circuits intentionally use only one junction of the transistor and "shorts out" the other one explicitly. An example is the BJT current mirror:

Image

Note that Q1's B and C pins are connected in the circuit. Thus, if you use the DMM diode check function to measure the B-C junction of this transistor, you will get 0V readings both forward and backward. AMB project examples of this are:


Another example is if there is another diode or transistor's diode junction connected in reverse-parallel with the particular junction you're measuring, then you would read 0.6V to 0.7V in both directions.

Thus, you should always look at the schematic diagram of the circuit to determine if any such scenario exists.
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Will this work for 2SK170/2SK370?

Postby sheya » April 27th, 2010, 10:34 pm

I have checked the data sheet for the 2SK170 and it did not say that these are bipolar. I'm wondering if your test applies to them as well. I have some that I might have blown in my Pass Labs B1 from reversing voltage, blowing the caps without realizing it, and then possible blowing the transistors from blowing the caps. It would be great to be able to test the JFETS to see if they are working to track down my problem with the circuit.

Thank you for your time, and thank you for putting together this forum and for all the work you do. I've built 3 Mini3's, and I really enjoy building your designs.

Best,
Aaron.
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Re: HOWTO: BJT sanity check tutorial

Postby amb » April 28th, 2010, 12:29 am

2SK170 is a JFET. The method outlined here does not work on any kind of FET.
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Re: HOWTO: BJT sanity check tutorial

Postby sheya » April 28th, 2010, 11:46 pm

Thank you, sorry for having to ask. I appreciate your taking the time reply.
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