Testing Transistors using an External power supply and DVM

Discussion in 'General Electronics Chat' started by Calton57, May 8, 2015.

  1. Calton57

    Thread Starter Member

    May 4, 2015
    33
    0
    When troubleshooting transistors i have found they test good using the DVM meter diode check mode. No opens or shorts and I get .7 volts from base to emitter and base to collector. So i use to think the transistor was good and would resolder it back in the circuit. The problem with these transistors is they pass at low test voltage but not at high test voltage. I'm guessing the transistors are conducting at lower voltages but not conducting at a higher voltage.

    I used an external power supply set at +28volts and used my DVM meter set to measure DC volts. The Negative of the power supply goes to the N-junction and the Positive of the power supply goes to the P-Junction. I measure the voltage from base to emitter and from base to collector and from emitter to collector. I should get .7 volts from base to emitter and base to collector if the transistors good even if the external power supply is set at +28volts?

    When testing a Diode is the same way, just connect an external power supply across the diode and measuring the DC voltage "across" the diode using a DVM meter set to measure DC voltage. If the diode is good you will measure .7 volts even if the external power supply across the diode is set at +5 thur +12volts?
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
    3,249
    626
    Measuring the junction voltages is only checking the junctions. There are other transistor parameters that are critical to correct function (e.g. beta, breakdown voltage, etc). If you had the transistor out of the circuit, a curve tracer could measure more important parameters. Don't know what you mean by not conducting at a higher voltage. Can you post a schematic with voltages for each terminal on the transistor?
    If you didn't current limit the external supply, the transistor should be dead.
    As with transistors, there are other diode parameters that are important. If you didn't current limit the external voltage, the diode should be dead.
     
    cmartinez likes this.
  3. Calton57

    Thread Starter Member

    May 4, 2015
    33
    0
    dl345 SAID:
    What should I set the current limit on the external power supply to when testing transistors out of circuit?

    dl345 SAID:
    Is the curve tracer supplying a low voltage or high voltage to the transistor when curve tracing?

    What parameters can a curve tracer test for? the forward current, beta, reverse bias breakdown voltage

    A transistor tester can test these parameters but it test the transistor at a low voltage not at a high voltage

    dl345 SAID:
    I mean that the transistor passes using the DVM meters diode checker mode test voltage. So the transistor is conducting from the DVM meter test voltage. The transistor doesn't conduct when you apply a higher test voltage.

    The transistor conducts when applying a low test voltage
    The transistor doesn't not conduct when apply a high test voltage

    dl345 SAID:
    Why do I need to current limit the power supply, i thought a component will just draws the current it needs from the power supply without damaging it.
     
  4. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,574
    2,543
    The simplest way of limiting current to a device is by placing a resistor in series with it. And no, if you apply enough voltage any device will draw too much current, exceeding its designed capacity ... for instance, a light bulb is nothing more than a filament that has an inherent resistance, and when you apply the rated voltage, it will light up drawing the amount of current that it was rated for... imagine what would happen if you were to connect a flashlight's light bulb designed for 12 VDC to 120 VAC mains...
     
  5. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    Calton57
    I have been repairing electronics for 45 years. It is rare that a transistor passes the Vbe and the Vbc test, but fails the Vce breakdown test. Perhaps you have a circuit that does something nasty to the transistors.

    A typical breakdown test would be to short the base to the emitter, put a high resistance in series with the collector, and apply a variable high voltage from the emitter to the other side of the resistor. Observe the collector-emitter voltage as the voltage is increased. You will see when it starts to zener.

    Do you have a part number for the transistors??
     
  6. dl324

    Distinguished Member

    Mar 30, 2015
    3,249
    626
    Examine the I-V curve for a diode and you'll have the answer.
    It depends on the curve tracer. Mine will give me 50+ volts for CE/DS; enough to breakdown most transistors.
    Google is your friend... They can measure beta at different currents, breakdown voltage, ...
    A curve tracer is a transistor tester.
    A diode check isn't definitive. Can your DVM also measure beta? If it can, at least you know the transistor has some function.
    Post a schematic showing how the low and high voltages are being applied.
    Without some form of current limiting, the diode will be destroyed.
     
  7. Calton57

    Thread Starter Member

    May 4, 2015
    33
    0
    2N2907A they pass the DVM meter diode check mode no opens or shorts and i get the forward bias voltage but fail when +28 volts is applied to them
     
  8. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    Do you have a schematic? Is this a circuit that was working and failed, or a circuit you are building??
     
  9. Calton57

    Thread Starter Member

    May 4, 2015
    33
    0
    How can I connect an external power supply and set it to +28 volts to test each junctions of a 2n2907A transistor? each junction should still be at .7 volts dc even if the power supply is set at +28volts?
     
  10. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,574
    2,543
    Those transistors are rated for 60V, but will definitely fail if you applied 28V without a series resistor to limit their current, which is only 0.6 Amps.
     
  11. Calton57

    Thread Starter Member

    May 4, 2015
    33
    0
    CMARTINES SAID:
    Where do I put the series resistor on the base or collector and how do I find out the resistance value?

    0.6 amps is from the datasheets of the 2N2907A? only 0.6amps the transistor draws the current?
     
  12. dl324

    Distinguished Member

    Mar 30, 2015
    3,249
    626
    R=V/I. Put the resistor in series with the junction you're testing.

    I don't mean to be rude, but do you have any training in electronics?
     
  13. Calton57

    Thread Starter Member

    May 4, 2015
    33
    0
    The junction i'm testing should always be at .7 vdc volts , it doesn't matter which supply voltage i set it to?
     
  14. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,574
    2,543
    Why don't you setup a simple circuit on a breadboard to test your transistor? Say, make it light a LED in series with the current limiting resistor... you will also need another resistor at the gate.
     
  15. Calton57

    Thread Starter Member

    May 4, 2015
    33
    0
    Yes i want to make a simple circuit to test transistors at higher voltages but i don't know how to make a circuit to test transistors at a higher voltage
    how can I make one please?
     
  16. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,574
    2,543
    Check this page, the two figures at the bottom show a simple circuit using a PNP transistor just like yours. To use it with a +28V power source, just change the resistor in series with the LED for a 3K one. And the resistor at the base of the transistor for a 6K, or a 6K8.
    The LED will light up when you connect the base resistor to ground, and will turn off when you connect it to +28V.
     
  17. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    See post #5
     
  18. Papabravo

    Expert

    Feb 24, 2006
    10,148
    1,791
    Why do you guys keep feeding the trolls?
     
    OBW0549 likes this.
  19. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,574
    2,543
    Am I really that naive???? how can you tell????
     
  20. Papabravo

    Expert

    Feb 24, 2006
    10,148
    1,791
    The question continues to be asked as if the TS/OP never read your response. It's the BillyMayo pattern.
     
Loading...