Diode Network - is that a Darlington Transistor

Discussion in 'General Electronics Chat' started by onpiopah, Oct 19, 2012.

  1. onpiopah

    Thread Starter New Member

    Jan 27, 2012
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    Hi,
    I am trying to get an amp to work and just checking everything.
    I am checking the SK B1647 transistors. My atlas DCA55 say it is a SERIES DIODE NETWORK , yet the datasheet says it is a darlington PNP Epitaxial Planar Transistor - Are they the same thing?

    thanks.
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    It is a PNP darlington transistor.
    The full name is 2SB1647 from sanken.
    On the parts the 2S is often not printed.
    See attached datasheet.

    Bertus
     
  3. onpiopah

    Thread Starter New Member

    Jan 27, 2012
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    Thanks, excuse my lack of knowledge but is a "series diode network" something different from a PNP Darlington transistor. I am using a semiconductor component analyser which says it is a "series diode network." I know it is a PNP darlington but wonder if they are the same thing, or is the component broekn so it is reading as a diode network.
    thanks.
     
  4. bertus

    Administrator

    Apr 5, 2008
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  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    The analyzer probably thinks it is a series diode network, becasue the same way as a bipolar transistor can look at it as if it were two diodes connected back to back, a darlington can look like three diodes connected together - for an NPN you have one diode drop between B and C and two diode drops between B and E. But it still remains a (double) transistor.

    Wonder why, but on the webpage of that analyzer they specifically show it measuring a darlington, and they also claim it has automatic lead identification. I dont understand why it doesnt recognise yours as a darlington.
     
  6. onpiopah

    Thread Starter New Member

    Jan 27, 2012
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    So a 'series diode network' and a 'Darlington transistor' are not the same thing. I will contact the company and ask them for some clarification on the specifications of their machine.

    thank you for helping me out with that, it was unusual beacuse i could not find anything searching the internet under 'series diode network'
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    If you connect it incorrectly, it could possibly look like a "series diode network" to your analyzer.
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Yes, but the meter in question claims automatic lead identification.
     
  9. onpiopah

    Thread Starter New Member

    Jan 27, 2012
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    I have tried all six ways of connecting the transistor up to the machine but it always comes out as the same "series diode network." I also have another B1647 in the Amp which I have taken out and this also reads as a "series diode network." I am assuming either the machine cannot read this Transistor (it does read other transisitors as Darlington pairs) or BOTH my B1647's transistors are broken. Can anyone give me any advice please?
     
  10. onpiopah

    Thread Starter New Member

    Jan 27, 2012
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    the meter is an Atlas DCA55 which states on the manual auto lead detection.
     
  11. BSomer

    Member

    Dec 28, 2011
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    I assume he saw
    and did a little search and found this. But then I am just guessing. :)


    To the OP, if those transistors are not too expensive just buy a couple if you can find them. Then you should have some known good transistors of the same type. What kind of problems are you experiencing with your amp? There are a few people here that can probably help you fix it.
     
    Last edited: Oct 21, 2012
  12. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    And I found this.:D
    I think the culprit is the 70Ω resistor that shunts the base-emitter junction of the 2nd transistor. The C-E current would have to be at least ≈9mA to turn on the 2nd transistor, and I doubt that the analyzer does this.
    However, I'm a little surprised that the analyzer doesn't see the first transistor as a simple PNP. I wouldn't think the 70Ω resistor would prevent it from doing so.
     
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  13. Jez Siddons

    New Member

    Oct 23, 2012
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    Jez Siddons from Peak Electronic Design Ltd here, I've just joined, Hi all :)

    Apologies if I'm repeating anything already mentioned here, I'm putting in the details that I recently sent to the OP in an email but I found this question here so I thought it would make sense to put it here too.

    I've downloaded the datasheet for the 2SB1647 which I *think* is equivalent to the transistor you're using.

    It's a slightly unusual part in that it has a resistor shunt between one of the internal base-emitter junctions. Resistor shunts aren't unusual, but usually if they are present then they are present across both base-emitter junctions rather than just one of them.

    Anyway, if that type of part is confusing the DCA55 then the DCA55 may be falling back onto the analysis of the internal PN (diode) junctions.

    Please see the attached picture that I've put together that shows how the DCA55 could interpret the PN junctions within this part.

    Sadly there is the odd part that causes difficulties for the DCA55, particularly parts that contain fairly low resistances (70 Ohms across one of the base-emitter junctions in this case). Low resistances can dominate the current path when the test currents are low (as the DCA55 uses less than 5mA). When that happens, the DCA55 falls back to looking for PN junctions, and in this case the PN junctions resolve down to a simple series network (as far as the DCA55 is concerned).

    Finally, I am a little surprised that the DCA55 hasn't managed to exercise the first transistor within that darlington. For most darlingtons, even ones with resistor shunts, the DCA55 can still identify the full pinout and "see" the transistor action. It is *possible* that your particular example is not a working darlington, do you think that could be the case? If that is the case then maybe the DCA55 can only see the PN junctions anyway, hence the details you're getting.

    As a matter of interest, I've just tested a handful of various darlingtons here and the DCA55 can identify them fine, all except one part that has a very low gain and has a low value resistor across the first base-emitter junction.

    I hope that helps to explain your results. If you have any further questions then don't hesitate to ask.

    Kind regards,
    Jez Siddons
     
  14. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I made those same points (but without your helpful detail and graphics) in post #12.:)
     
  15. Jez Siddons

    New Member

    Oct 23, 2012
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    Hi Ron, You are quite correct, apologies for any duplication. I decided to put in the entire contents of my email to the OP so as not to miss any elements. But that has led me to saying some things that you've already mentioned!

    Interestingly, I found this thread when searching for the datasheet for the OP's transistor as a result from me receiving his email query about this subject yesterday.

    I'm glad however that I'm not the only one to come up with the conclusion. It's tricky to ensure all eventualities are dealt with by the DCA55, but we're always looking at ways it can be improved.

    Cheers,

    Jez
     
  16. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Hi Jez,

    It just dawned on me that your company makes the DCA55.
    Welcome to the forum!
     
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  17. Jez Siddons

    New Member

    Oct 23, 2012
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    Cheers Ron. I hope I can be of use while I'm here :)
     
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  18. onpiopah

    Thread Starter New Member

    Jan 27, 2012
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    I am very impressed Ron, I hope I will be as knowledgeable one day about electronics. Unfortunately I have a very long way to go. I was hoping tech like the DCA55 would give me a head start but it has caused me some confusion. I was hoping to just connect it to the Transistors and know right away if they were broke or not – too easy. I have for the past few days been reading and researching about PN junctions and transistors to try and make sense of the readings from the DCA55 but I have to admit I am lost.
    I have tested 2 of the Transistors and the reading from the DCA55 are as follows
    Transistor number 1
    B1647 – Diode network 1 Vf 0.88 & If 4.51ma
    Diode network 2 Vf 0.56 & If 4.85ma

    Transistor number 2
    B1647 – Diode network 1 Vf 0.88 & If 4.51ma
    Diode network 2 Vf 0.58 & If 4.83ma
    Both sets of values appear to be so similar that my assumption is that the Transistors are still functioning . Am I right?
     
  19. Jez Siddons

    New Member

    Oct 23, 2012
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    Generally speaking, if the DCA55 comes up with proper transistor details then you can be confident that it's a good transistor, it's then up to you if you're happy with the measurements in terms of suitability for your application.

    However, if DCA55 comes up with just diode junctions then it's possible that the part you're testing is faulty OR the DCA55 is having difficulties with that part for some reason.

    By all means, pop a suspect part in the post to me and I'll look at it in detail here and get to the bottom of the results you're seeing.

    Basically the results you're seeing are just the diode (PN junctions) that all transistors are made of being detected, rather than the transistor itself being detected. Sadly the diode message you're getting does not necessarily mean that the transistors are working, it just means that the PN junctions are intact.

    Cheers,

    Jez
     
  20. Jez Siddons

    New Member

    Oct 23, 2012
    9
    2
    I forgot to mention, the user guide (Pages 11-17) describes in a little more detail what to expect from your DCA55 when testing transistors, in particular page 14 discusses "Faulty transistors". I've included a link here if you haven't still got your user guide handy:

    http://www.peakelec.co.uk/resources/dca55_userguide_en.pdf

    If you have any other questions, don't hesitate to ask. I'll try to help where I can, but there are lots of knowledgeable people on this forum too of course.
     
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