Testing Transistors Beta in circuit and out of circuit

Discussion in 'General Electronics Chat' started by SamEricson, May 3, 2015.

  1. SamEricson

    Thread Starter Member

    Apr 25, 2015
    196
    0
    When testing a transistors beta gain in circuit using a transistor tester. Is there any difference between testing a transistors beta in circuit compared to an out of circuit beta test?

    When is a transistors beta to low for it to be a bad transistor?
    I get beta readings that are 50. a good transistor will be 200 beta when it's new

    When a transistor is used as a switch, how low can the beta be before considering its a bad transistor?
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    There is a simple rule of thumb, use 1/10 of the current on the BE that you will be using on the CE. If it can't go into saturation in this state then I would assume it is bad.
     
  3. SamEricson

    Thread Starter Member

    Apr 25, 2015
    196
    0
    But a transistor can go into saturation even if the beta is getting low or on the borderline. What happens is a few months later the transistor in the circuit won't be switching off and on.

    I'm not sure the range of beta that is considered bad
     
  4. SamEricson

    Thread Starter Member

    Apr 25, 2015
    196
    0
    Is it ok to measure a transistors beta in circuit or it's a big difference?
     
  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    How do you plan to measure beta in circuit? And in which circuit?
     
    absf likes this.
  6. SamEricson

    Thread Starter Member

    Apr 25, 2015
    196
    0
    I use a transistor test that measures the beta, i just measure the beta in circuit when the circuit board is turned OFF
     
  7. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    That will give you completely uselsess results.
    Also, why do you think that testing for beta is the best approach?
     
  8. SamEricson

    Thread Starter Member

    Apr 25, 2015
    196
    0
    Because if the beta is to low it will cause the transistor not to switch on or off , when a transistor is function as a switch
     
  9. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Well of course, but you could also check the B-E and C-E voltage and/or current to see if the transistor works as it should.
     
  10. dl324

    Distinguished Member

    Mar 30, 2015
    3,246
    622
    In general, you can't measure components in-circuit and expect to get accurate results.
    It depends. I have never replaced a transistor because it's beta dropped. Usually the transistor dies.
    Where does that come from. Many power transistors have beta < 50 when new, as if that makes a difference; most that I've used are closer to 10.
    When the transistor won't go into saturation...
     
    absf likes this.
  11. SamEricson

    Thread Starter Member

    Apr 25, 2015
    196
    0
    Yes true, but the transistor still will work at a very low beta or it's on the border line

    Because when transistors are functioning as a switch it will be intermittenly not turning on or off or having switching problems because the beta is to low or drifting
     
  12. #12

    Expert

    Nov 30, 2010
    16,298
    6,811
    Intermittent low gain..I've never seen that happen. I don't think you will either. You merely had an idea with no basis in reality and you want somebody else to explain how to measure it.
     
    absf likes this.
  13. WBahn

    Moderator

    Mar 31, 2012
    17,748
    4,797
    You are thinking that the beta of a transistor is some constant under all conditions and that a "low beta" is the result of some change in the transistor.

    Why don't you spend some time learning what "saturation" is and how it is defined by most transistor manufacturers?
     
  14. SamEricson

    Thread Starter Member

    Apr 25, 2015
    196
    0
    I thought when a transistor is having intermittent switching problems its because the beta was to low
     
  15. WBahn

    Moderator

    Mar 31, 2012
    17,748
    4,797
    Why not go look at a manufacturer's data sheet and see what the beta is when the transistor is in saturation.

    Since this is something that we know you resist doing, here is the relevant section from the Fairchild 2N3904 datasheet.

    upload_2015-5-3_16-9-1.png

    What is the beta for this transistor in saturation?
     
  16. SamEricson

    Thread Starter Member

    Apr 25, 2015
    196
    0
    Base current is 1 milliamps
    Collector current is 10 milliamps
    Beta should be 10
     
  17. WBahn

    Moderator

    Mar 31, 2012
    17,748
    4,797
    So what's the takeaway from this?
     
  18. SamEricson

    Thread Starter Member

    Apr 25, 2015
    196
    0
    you measure the CE voltage and BE voltage and compare it to the transistors datasheet
     
  19. WBahn

    Moderator

    Mar 31, 2012
    17,748
    4,797
    Interesting takeaway.

    The point I was (hoping) you might get is that a transistor's beta is not some constant. It varies with operating point. Just look at a traditional transistor characteristic and ask what the beta is at various points and you will see this.

    The very question of what it means for a transistor to be "saturated" is not well defined since it is a continuous process. So, rather arbitrarily, most transistor manufactures define a transistor to be in saturation when the Vce at a given base current results in the beta of the transistor being 10. This also means that the Vcesat is not a constant value for a given transistor (as the 2N3904 datasheet snippet reveals). For power transistors the beta=10 criterion is often used at relatively low collector currents but for relatively high collector currents an even lower beta value, such as 3, is used.
     
    absf likes this.
  20. SamEricson

    Thread Starter Member

    Apr 25, 2015
    196
    0
    How do you track down when a transistor is causing switching glitches or turning off and on at random or turn on and off at different times?

    If a circuit board has many transistors as switches , how about you find the one that is causing the switching problem?

    I was measuring the beta of each transistor
     
Loading...