Testing Switching Transistors

Discussion in 'General Electronics Chat' started by bwilliams60, Sep 15, 2014.

  1. bwilliams60

    Thread Starter Active Member

    Nov 18, 2012
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    I am currently working on a Samlex SEC-1235 switch mode power supply and I have no DC voltage output. It should be in the area of 13.6 VDC. I am to the point that I believe I have a bad switching transistor. I have tested it like a bipolar but it does not make sense to me. Is there another way to test them? The two readings are different from each other which would confirm my suspicions but would like to know more about them.
     
  2. ISB123

    Well-Known Member

    May 21, 2014
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    Post the schematics if you can.
     
  3. bwilliams60

    Thread Starter Active Member

    Nov 18, 2012
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    Sorry I haven't been able to find a schematic for this supply. If someone has one and would like to share. I would be grateful. Thanks in advance. I will try to take pictures of it tomorrow and post them to the page.
     
  4. b1u3sf4n09

    Member

    May 23, 2014
    115
    14
    Did you already investigate the possible causes as laid out by the product manual?

    "PROBLEM : The output voltage is 0 V or very low

    PROBABLE CAUSE SUGGESTED REMEDY:
    Input voltage is very low Check that the input voltage is 120VAC

    The unit is in current limit condition Check the output terminals are not
    due to overload caused by large reactive shorted. Remove the load. If the
    loading or by the output being short output voltage gets restored, the
    circuited load is shorted or is offering large
    reactive impedance.

    Unit is shut down due to over Check that the fan has not failed or
    temperature. the vent openings are not blocked

    PROBLEM : Output voltage drops as soon as the load is switched on

    PROBABLE CAUSE SUGGESTED REMEDY:

    The unit is going into current Reduce the load current
    limit protection mode to less than the current limit value.
    Motors, pumps, compressors, relays,
    incandescent and halogen lamps and
    large capacitors in the input section of
    the DC devices draw very high inrush
    or starting currents of up to 10 times
    their normal operating currents.
    Ensure that these inrush/starting
    currents are below the current limit
    value of the power supply."
     
  5. to3metalcan

    Member

    Jul 20, 2014
    228
    23
    The transistor is likely to be a MOSFET. It won't diode test the same way a BJT will...but you can still probably tell if it's blown if any if the terminals are shorted to one another.
     
  6. bwilliams60

    Thread Starter Active Member

    Nov 18, 2012
    722
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    There is no load hooked up to this unit. I have traced voltage to the rectified side of the bridge rectifier and found 345 VDC, which puzzles me but nonetheless, it's there. I seem to lose it going through these switching transistors but unsure how to test. They do have different readings on diode test and one looks like it has a small pinhole where smoke escaped but hard to tell with the naked eye. I am going to put it under magnification tomorrow to see if I am correct. I will post the pics.
     
  7. gerty

    AAC Fanatic!

    Aug 30, 2007
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    I believe you'll have to connect a load to the power supply to test it properly
     
  8. tom_s

    Member

    Jun 27, 2014
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    Last edited: Sep 16, 2014
  9. bwilliams60

    Thread Starter Active Member

    Nov 18, 2012
    722
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    Tom_s, this looks very similar to mine. The unit does not have to have a load to have an output gerty. I have a similar unit and it sits at 13.7 vDC with no load attached. I am almost certain that the transistors, or at least one, is shot.
     
  10. bwilliams60

    Thread Starter Active Member

    Nov 18, 2012
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    Here are the pics of the supply and transistors. Notice the spot on the one on the left.
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    If its a bipolar transistor, it should test like 2 back to back diodes. The B/E junction could show a little bit of reverse leakage if your meter's test voltage is more than about 5V - there should be no leakage whatsoever from C/E.

    A MOSFET can be a little confusing because testing for gate leakage can leave a charge on the gate capacitance (The longer it takes to leak away - the better the MOSFET). Charge on the gate capacitance can switch the MOSFET on or off, so drain conduction can depend on the polarity of your last gate test - if you leave a negative gate charge (on a N-channel MOSFET) there should be no drain conduction.

    The MOSFET has a parasitic "body diode" from drain to source that is not forward biased with the correct polarity on the drain.
     
  12. Cal Rico

    New Member

    May 1, 2012
    15
    1
    It is an NPN bipolar. High Speed, High voltage. I just Goggled on DuckDuck go the part number. came up with
    the link: http://www.alldatasheet.com/view.jsp?Searchword=SC3320
    and below is one of the datasheets (there are always multiple versions of the same transistor part these days). Despite the variations, for testing the specs are usually the same or very similar.
    Be sure to desolder at least one leg. before testing.
     
  13. bwilliams60

    Thread Starter Active Member

    Nov 18, 2012
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    If they are both NPN bipolar, then they are both bad. Time to order up some new ones and that could be a chore. Not finding a lot of matches online. Anybody know where to buy these in North America?
     
  14. ISB123

    Well-Known Member

    May 21, 2014
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    Search for HIGH VOLTAGE HIGH SPEED SWITCHING NPN and get one that has similar values.
     
  15. bwilliams60

    Thread Starter Active Member

    Nov 18, 2012
    722
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    Thanks ISB123. Will begin my search tomorrow am. The number on them doesn't come right up so looking for similar transistors. I have a newer power unit from same company, might check and see if they are using something new and improved.
     
  16. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    3,807
    2SC4140 seems to be a fairly good match. Confirm Hfe of the transistor you have.
     
  17. ian field

    Distinguished Member

    Oct 27, 2012
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    Sounds like its a half-bridge circuit, that means the 2 transistors in series across the rectifier/reservoir capacitor - even if one tested good, it would be advisable to replace both together.
     
    bwilliams60 likes this.
  18. bwilliams60

    Thread Starter Active Member

    Nov 18, 2012
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    Ian, if you look at post #8, you can see how they are hooked into the circuit. I would say they are in parallel, is that correct? I'm still trying to understand what the left side of the circuit does and how it does it. I am fairly new to electronics as far as understanding how the components react with each other. Is it normal for there to be 345 VDC after the rectifier? What is the purpose of this?
     
  19. ian field

    Distinguished Member

    Oct 27, 2012
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    They're not parallel, one has its collector to positive and the other emitter to negative.

    Its pretty standard as switchers go.

    The driver transistors are single-ended push pull with a center tapped primary on the driver transformer.
     
  20. bwilliams60

    Thread Starter Active Member

    Nov 18, 2012
    722
    88
    Thank you Ian, appreciate the feedback and the lesson. Slowly learning, very slowwwwly.

    Since these transistors are blown, I like cause and effect so what might have been the cause of their destruction and what else should I be looking at?

    Does anybody have a good source for these transistors? What is the Chinese company that everyone on here talks about?
     
    Last edited: Sep 18, 2014
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