Elna electronic sewing machine control

Discussion in 'General Electronics Chat' started by tinkerman, Oct 31, 2018.

  1. tinkerman

    Thread Starter Active Member

    Jul 22, 2012
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    Can someone perhaps help to identify a transistor?? in the control circuit. I have a circuit worked out, correctly I hope, but the single transistor on the board which controls the current to the motor has NO MARKINGS!! The part is perfectly functional but one capacitor on the board had failed causing the motor to run continually without the foot control depressed. Got everything back to normal but would like to go further with the design.

    Would upload a schematic but can't see how to do it. :(
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    See the 'Upload file' next to Post Reply.
    Max.
     
  3. tinkerman

    Thread Starter Active Member

    Jul 22, 2012
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    Thanks Max. Here is my sketch. Hopefully it is correct. Q1 is NOT and NPN transistor it seems from diode tests with a multimeter. It had a red dot on the top of the can. That's all. :( I can provide photos of the board top and foil side if that helps.

    Thanks
     
  4. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    I would say Q1 is a Triac or Thyristor..

    Best to post a picture of the circuit boards. .
     
  5. tinkerman

    Thread Starter Active Member

    Jul 22, 2012
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  6. tinkerman

    Thread Starter Active Member

    Jul 22, 2012
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    I don’t think triac. Needs a diac as I understand it. SCR never thought of that. There is a diode D1 so motor received only positive pulses. This machine is 1968 vintage.
     
  7. MisterBill2

    Distinguished Member

    Jan 23, 2018
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    Yes, the device could be an SCR, a bit more likely in 1968. but an NPN transistor would also be able to work. and my guess is that the capacitor that failed was C1, and that it failed with either high leakage or a short circuit.
     
  8. tinkerman

    Thread Starter Active Member

    Jul 22, 2012
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    No turned out it was the electrolytic C2 that had failed. 20uF 16V. First thing I tried was an 1A NPN transistor but it heated almost immediately. So then I changed capacitors. I've never seen a SCR circuit before without a diode in the gate. Didn't cross my mind. I think I'll bread board a circuit like it with a SCR and see how that turns out.
     
  9. MaxHeadRoom

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    Jul 18, 2013
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    More likely a Triac, a SCR is uni-directional.
    Max.
     
  10. tinkerman

    Thread Starter Active Member

    Jul 22, 2012
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    There is diode in the circuit that makes it half wave so an SCR would work. I put a scope on it a day or two later. This is lowest speed.
     
  11. tinkerman

    Thread Starter Active Member

    Jul 22, 2012
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    A SCR never crossed my mine as there was no diode like I'm used to seeing SCR circuits. Ruled out a Diac because no triac. After trying to replace with a NPN transistor I was stumped.
     
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Is it a Universal motor?
    Max.
     
  13. pfofit

    AAC Fanatic!

    Nov 29, 2006
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    Is R6, R11, R12, R13, C12, and the zener all connected together?
    Is the emitter connected to the common line and L1 or just to L1?
    Could you put dots on the intersecting lines of your diagram. I like dots. :)
    You have a T2 designation at the top and bottom.
    What is the voltage/ waveform across C2 without touching the foot pedal?
    Can you post a pic of the board?
    Is there a label on the motor?
    thanks
     
  14. MisterBill2

    Distinguished Member

    Jan 23, 2018
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    Yes, an SCR is indeed unidirectional, and looking at the circuit, if it is correct, there is also a unidirectional diode just upstream from the device. I am not certain why it is placed there, but that is what I see.
    And if the part that failed was that capacitor C2, then yes, an SCR is likely. Sort of a different type of circuit, but nothing unbelievable.
    And now, after seeing the waveform, it is more likely an SCR, although a triac would explain the waveform, if D1 is not really a diode.
    And D2, shown as a zener diode, seems to be serving the same purpose as a diac would serve.
     
    Last edited: Nov 3, 2018
  15. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The only problem is if a SCR then you are not getting full wave AC to the Universal motor.
    If SCR I would expect a bridge before it in order to switch each half cycle and run the motor on DC this way.
    Logically the circuit shown would be an Triac.
    Max.
     
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