Bipolar Junction Transistor Testing Basics

Discussion in 'DIY' started by EchoWars, Jul 15, 2005.

  1. EchoWars

    EchoWars Hiding in Honduras

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    Get a lot of Q's on this, so I thought I'd put together a post.

    First you need a decent digital meter with a diode test function. Forget about using the ohmmeter part of your meter, it is way too unreliable for this. Almost all digital meters nowadays have a diode test included, and it is 100% necessary for even the rudimentary reliable results that we are shooting for here. If you need to buy a meter, remember that you'll get what you pay for. Don't cheap out with a crappy $10 meter from Harbor Freight or something, when from eBay you can get a good used Fluke 77 for $30. You can buy a decent new meter for less than $100.

    Just so you understand the basics of the diode function, the reading you get when you use the diode test is the voltage necessary to overcome the depletion layer at the P-N junction of the diode. Don't sweat the meaning of that, I just want you to understand the units of what you will be measuring.

    If you grab a plane-Jane 1N4004 diode, for instance, set your meter for a diode test, and connect the positive lead to the anode and the negative lead to the cathode (the side with the band). You should read about 0.45V to 0.65V or so, depending on the amount of current that your meter feeds through the diode. Reverse the leads with positive on the cathode and the negative on the anode, and you should read an 'OL' or 'Overrange'...check the documentation on your meter to better understand how it will indicate an open circuit, but essentially there should be no conduction with the leads reversed. A shorted diode will show '0V' with the leads in either orientation.

    Many meters give a short 'beep' to audibly indicate conduction, and a continuous 'beeeeeeeep' to indicate a short, or a very low depletion layer voltage. Handy.

    Now that you know the basics of how a diode reads with a meter, you can test a transistor. The pic below shows simplified equivalent circuit of a NPN and a PNP transistor, as well as anode and cathode identification of a diode. Of course, you cannot 'build' a transistor like this, but it is a good visualization to help you understand how to check one.

    Lets assume you have a transistor to test. Of course, this will require that you identify which lead is the base, collector, and emitter, and also determine if the transistor is an NPN or a PNP. Check the schematic, or the part number.

    (hint: with Japanese transistors, all 2SAxxxx and 2SBxxxx transistors are PNP, and all 2SCxxxx and 2SDxxxx are NPN transistors. Often, the '2S' part of the transistor part number is omitted, thus a 2SA733 transistor is usually labeled 'A733', and the '2S' prefix is assumed)

    Large TO-3 metal output transistors always have the outer case as the collector. Once you know that, you can figure out which of the other two pins are the base and emitter. Smaller transistors come in every configuration, EBC, BCE, ECB, so you need to either view the schematic to see which transistor leg connects to where, or find a data sheet for the transistor which will identify the leads for you. Of course, you can also figure it out for yourself.

    Lets say you have found that you have a NPN transistor, and have identified the base, emitter, and collector. Set your meter for diode test, and place the positive lead on the base. Place the negative lead on the emitter. You should read a diode drop voltage of about 0.45 to 0.65V. Now place the negative lead on the collector. You again should read a diode drop of about 0.45 to 0.65V (these values are not written in stone...the 0.45 to 0.65V is the most common range however). Now place the negative lead on the base, and the positive on the emitter (called 'reverse biasing'). There should be no conduction. Now move the positive to the collector, and again, there should be no conduction. Lastly, move the negative to the emitter. Again, there should be nothing.

    (note: most transistors fail with a dead-short from the emitter to the collector, especially in the later power stages of an amplifier. Knowing this can allow you to check high-power stages quickly for obvious failures)

    For PNP transistors, the leads are reversed while doing the same checks as above (negative on the base to begin, positive on the emitter, and then on the collector etc. etc. to read the proper diode drops)

    I must warn that when testing transistors in-circuit you are very likely to get voltage readings from collector to emitter, or when reverse-biasing the base-emitter or base-collector diode, all due to the multiple conduction paths in the circuit(s) that allow the meter current to flow around the component under test. Again, understand that most failed transistors short from collector to emitter, so if you read a voltage where you should not, that does not mean you have found a bad transistor. Yes, this complicates troubleshooting, but if this was easy, people would not be making a living fixing electronics. :scratch2: Bottom line...when in doubt, remove the transistor from the circuit and test it.

    Last note...this type of test is generally worthless for finding transistors with 'leaky' junctions, as the voltage applied by a meter is very low and won't induce the failure. In a case like this, you will be reduced to measuring voltages with the circuit powered on to determine what is pulling excessive current, or just 'shotgunning' a whole section of the amp (replacing multiple components in the hopes of getting the bad one). Also, many old transistors will test just fine, but through age will have such low current gain that they can no longer do the job they were chosen for in the circuit. Relay driver transistors are especially bad about failures like this, where the transistor has spent hundreds of hours in a saturated state and gain eventually droops to the point that it can no longer close the relay. For this reason I replace relay driver transistors in amps as a matter of course, whether they are acting up or not.

    Best of luck to ya!!
     

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  2. macaltec

    macaltec Metal Master Subscriber

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    Thanks EW. I've been wanting to get my head into this process in an attempt to start troubleshooting some of my broken gear. You just made it alot easier.

    mac
     
  3. Kevin's Rack

    Kevin's Rack Technics-aholic

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    You should make this a sticky!!
     
  4. EchoWars

    EchoWars Hiding in Honduras

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    As many times as people ask me how to test, well, maybe not such a bad idea.

    Thatch?
     
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  5. Kegger

    Kegger Anything can be S "MODed" Super Mod Subscriber

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    Agreed I bet you get a ton of questions regarding how to test a transistor!
    By the way cool little simple diagram that anyone can follow.
    Keep up the good work!
     
  6. Strawman

    Strawman Moderator

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    Thanks echo, my son is getting into electronics and I am going to print this out for him. Instead of a Sticky, I vote they set up an "Echowars Corner" section or something so we can all get learned up properly. :naughty:
     
  7. txsviking

    txsviking Active Member

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    Hey EchoWars, I hope you didn't copyright that yet or I'll
    be in trouble!
     
  8. EchoWars

    EchoWars Hiding in Honduras

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    Bump. :yes:
     
  9. markthefixer

    markthefixer On Hiatus, dealing with Dad's estate full time Subscriber

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    EW: perhaps someday a similar post on JFETS would be appreciated, especially with respect to differentiating source/drain on an unknown jfet.

    (just a thought)
     
  10. EchoWars

    EchoWars Hiding in Honduras

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    The problem with most FET's is that there is essentially no difference between the source and drain that can be found with a meter. Need to locate data sheets on these.
     
  11. markthefixer

    markthefixer On Hiatus, dealing with Dad's estate full time Subscriber

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    ouch... and thank you.
    ---------- --------- -------- -------
    Edit:here's some stuff I worked up. using EchoWars's post:
    Here are the results of multimeter diode tests on TWO GOOD bipolar transistors, a 2sa725 PNP transistor and a 2sc945 NPN transistor:

    first the PNP:
    black lead to base, red lead to collector, result = 675
    black lead to base, red lead to emitter, result = 688
    red lead to base, black lead to collector, result = OL = No conduction
    red lead to base, black lead to emitter, result = OL = No conduction
    black lead to emitter, red lead to collector, result = OL = No conduction
    red lead to emitter, black lead to collector, result = OL = No conduction

    next the NPN:
    black lead to base, red lead to collector, result = OL = No conduction
    black lead to base, red lead to emitter, result = OL = No conduction
    red lead to base, black lead to collector, result = 677
    red lead to base, black lead to emitter, result = 680
    black lead to emitter, red lead to collector, result = OL = No conduction
    red lead to emitter, black lead to collector, result = OL = No conduction

    Here is a graphic:
    echowars_transistor1.jpg

    Now for Jfet testing, EchoWars posted this elsewhere, I'm adding a link to it
    link doesn't work, doesn't like .doc, so I'm copying the text:
    ------------------------------------------ under construction -----------------------------------------------------------
    JUNCTION FIELD-EFFECT TRANSISTOR (JFET) TESTS
    The junction field-effect transistor (JFET) has circuit applications similar to those of a vacuum tube. The JFET has a voltage-responsive characteristic with a high input impedance. Two types of JFETs that you should become familiar with are the junction p-channel and the junction n-channel types, as shown in figure 2-16. Their equivalent circuits are shown in figures 2-17 and 2-18, respectively. The only difference in your testing of these two types of JFETs involves the polarity of the meter leads.



    (2SK) (2SJ)
    Fig. 2-16



    N-Channel Test
    Using an ohmmeter set to the R ? 100 scale, measure the resistance between the drain and the source; then reverse the ohmmeter leads and take another reading. Both readings should be equal (in the 100- to 10,000-ohm range), regardless of the meter lead polarity. Switch the meter to read for a diode test, and connect the positive meter lead to the gate. Using the negative lead, measure the voltage between the gate and the drain; then measure the voltage between the gate and the source. Both readings should indicate a diode drop plus a bit more to account for the substrate resistance of the drain or source, and be approximately the same. Disconnect the positive lead from the gate and connect the negative lead to the gate. Using the positive lead, measure the voltage between the gate to the drain; then measure the voltage between the gate and the source. Both readings should show infinity, or an open circuit depending on how your meter functions.



    Fig. 2-17 N-Channel FET

    P-Channel Test
    Using an ohmmeter set to the R ? 100 scale, measure the resistance between the drain and the source; then reverse the ohmmeter leads and take another reading. Both readings should be the same (100 to 10,000 ohms), regardless of meter lead polarity. Switch the meter to read for a diode test, and connect the negative meter lead to the gate. Using the positive lead, measure the voltage between the gate and the drain; then measure the voltage between the gate and the source. Both readings should indicate a diode drop plus a bit more to account for the substrate resistance of the drain or source, and be approximately the same. Disconnect the negative lead from the gate and connect the positive lead to the gate. Using the negative lead, measure the voltage between the gate to the drain; then measure the voltage between the gate and the source. Both readings should show infinity, or an open circuit depending on how your meter functions.







    Fig. 2-18 P-Channel FET




    http://audiokarma.org/forums/showpost.php?p=764244&postcount=2


    here is a hopefully useful graphic:


    [​IMG]
     
    Last edited: Jan 28, 2017
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  12. bigphil

    bigphil Stop the Addiction

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    bump... how many more threads are there like these? I only know about DC offset, and maybe something on capacitors.... they should all be stickied together somewhere...
     
  13. Haoleb

    Haoleb Super Member

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    Thanks for taking the time to write this up EW. As usual, Another priceless post. :thmbsp:
     
  14. mbates14

    mbates14 Well-Known Member

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    well, especially mosfets, Since the gate is high impedence, you can charge up the gate with your multimeter, and you should read almost a direct short both ways from drain to source channel. the only thing youll read is Rds. if you discharge the gate, you will get no conduction one way, but you will get a .5 to .6v drop the other way, because of the parasitic diode. If these conditions are true, the mosfet is good. but if you get .014 or something like that with the gate charged/discharged, its bad. There should be NO leakage from the gate to source/drain. since on a mosfet, the gate is ISOLATED from the source/drain channel. if you get a medium to low reading, the mosfet is blown out.

    jfets are another story. You have to apply a voltage to turn the jfet off, as its normally on. mosfets are the opposite. D-mosfets are like jfets, but with the gates isolated.
     
  15. rulerboyz

    rulerboyz AK Member

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    Let's see you if you guys can help walk me through testing a transistor. The transistor is numbered: C536. Based on the original post that would maked it an NPN transistor (2SCxxxx). Based on the readings I'm getting on diode mode it appears that the lead orientation is TO92A in the diagram below:

    [​IMG]

    From Base (+) to Collector (-) = 1034 (conducts only one way)
    From Base (+) to Emitter (_) = 1039 (conducts only one way)
    From Collector to emitter (doesn't conduct either way)

    When I set my multimeter to test the current gain I am not exactly sure what to do. I set it to hFE and put the three leads into their respective holes (NPN side) and I get a reading of 0. If I switch it around I get a reading of 336.
     
  16. markthefixer

    markthefixer On Hiatus, dealing with Dad's estate full time Subscriber

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    The reading of zero means that the way the leads were inserted is wrong, and the reading of 336 means that it is the correct lead layout. This type of test is actually the best and final arbiter of pinouts and whether the transistor is working or not. NO hfe reading means the transistor circuit for hfe test is NOT working and if there is a gain reading it means everything works...

    congrats....
     
  17. rulerboyz

    rulerboyz AK Member

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    How do I find replacements for transistors that I can't seem to match.

    How do you find an approximate match when you can't locate what you are looking for?

    The service manual for Rotel RTC-850 preamplifier seems impossible to find. I was able to find the owners manual, but that doesn't have a schematic or anything detailed in it.
     
    Last edited: Oct 1, 2005
  18. Morden2004

    Morden2004 Relaxin'

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    You look it up in the Semiconductor Cross Reference Data Book (Sams).

    Paul

    PS: As usual, EW has crafted yet another great thread! Thanks, EW.
     
  19. rulerboyz

    rulerboyz AK Member

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    I just found something that appears to give me the answers. It is the cross reference search on NTE website. I seem to recall that NTE wasn't a very popular on here though.

    Another question: When you measure hFE for a transistor with a multimeter does this value correspond the Maximum DC current gain which is listed on the data sheet?

    thanks
     
  20. Jim H B

    Jim H B Active Member

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    I'm just posting some "keywords" here so that I can find this excellent post easily from the Search:

    How to test a diode
    How to test a transistor
    Testing diodes
    Testing transistors
    How to check diodes
    How to check transistors

    Jim
     
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