SCR (Transistor circuit latches with touch of finger?

Discussion in 'General Electronics Chat' started by Mrdouble, May 16, 2015.

  1. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    I built a circuit that latches and pulls down the base of another transistor but it doesn't work right. I isolated the section from rest of circuit and View attachment 85907 threw in a led. The latch latches seemly at anything, including my finger touching the collector or base of npn transistor. It also seems to be level detecting. Please explain what's going on. Thanks in advance.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    So it's not working "right".
    Then what do you consider right?
     
  3. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    It's a latch... Send a signal to the npn transistor and it latches. But instead it latches on anything, including including turning a led on in another completely isolated part of the circuit
     
  4. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    I have a sneaking suspicion it may be related to PN junction capacitance which would kinda make sense since it seems to latch on a "high to low" transistion
     
  5. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    You need to add resistor between base and emitter for each transistor. So you need two resistors, try 10kΩ first.
     
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  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    What event is supposed to trigger the latch?
    As shown, the LED and middle 1k both allow base current to flow in the PNP, thus triggering the latch.
     
  7. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    Alec_T. I have placement of led wrong (sorry). I'm at boysscout camp with son but will be home and will confirm then. In actual circuit the led is not there. The actual circuit uses A PNP of another transistor to pull latch base on NPN high this latching in.
     
  8. crutschow

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    Mar 14, 2008
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    Remove the 1kΩ resistor from the PNP to ground.
     
  9. ian field

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    Oct 27, 2012
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    The LED is in the wrong place for a start, it should be in series with the 1k in the +6V line - where it is now is switching the PNP on before anything else happens.

    As others have pointed out; a 10k resistor shunting the B/E of each transistor will reduce sensitivity.
     
  10. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    image.jpg Sorry for the delay and sorry about the led in the pic ( it was correct in circuit :). I put the 10k resist between base and emitter and I can no longer latch by touching bases with finger BUT it latches when I try to measure NPN base voltage or plug in small 3v DC motor. The motor is throwing all kinds of transients back into breadboard power even though motor isn't connected to latch circuit other than 6v rail (seen on oscilloscope). Could the transients be the cause of faulty trigger of latch? If so, any idea how to reduce them? I have tried putting .01 uf caps at motor but that didn't help :/

    This just crossed my mind, the transistors are 3904 and 3906. Would it be recommended that I use something more inline with switching like a 2n222. I assumed it wouldn't matter as I'm not passing any current.
     
    Last edited: May 19, 2015
  11. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    remove the two 1K resistors from the bottom.
     
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  12. crutschow

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    Mar 14, 2008
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    The meter draws enough current to trigger the circuit.

    To isolate the motor noise try connecting a 100 ohm resistor between the 6V and the latch circuit with a 100μF capacitor from the latch power to ground.

    I previously suggested you remove the 1k resistor from the base of the top transistor to ground. Why is it in there?
     
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  13. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    You know Crutschow, I really do t know why it's in there. Someone else recommend I put it there because it wouldn't latch. Best guess to keep it from floating
     
  14. crutschow

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    The 10kΩ resistor from base to emitter does that.
    The 1kΩ draws current that tends to start turning on the latch and makes it very sensitive to any external sources of noise.
     
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  15. ian field

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    Oct 27, 2012
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    The 10k resistors were basically a guess as no one knows how much gain in the transistors you're using. They reduce the sensitivity so it doesn't trip just by putting your hand near one of the bases - if its too insensitive, use 22k or more instead.

    Get rid of all the 1k resistors except the one in series with the LED, one of the 1k's in your diagram serves no other purpose than drop a bit of voltage, and the other turns on the PNP whatever.
     
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  16. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    Ok, I removed the 1K and only resisters left are 2 10k across EB (one on each) and 1k led limiter. Still involks latch when motor turns on. Think pic had anything to do with my issue of false triggering of latch? image.jpg
     
  17. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    Thank you everyone for helping me. It's working (mostly). I put a 47uF cap between the power rails of latch and it worked 100% as long as motor wasn't on and dumping transients back on rails. I put a 2uF cap across the motor and it almost worked right for 2 mins without throwing it into latch condition. I still have some work to do on reducing motor noise but it think I got it. Thank you :)
     
  18. crutschow

    Expert

    Mar 14, 2008
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    Did you try adding a small resistor in series with the power to the latch (along with the capacitor to ground) to help isolate the motor transients?
     
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  19. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    image.jpg Just found this and thought I'd post it. Explains perfectly the problem and solution as to why the motor noise was triggering SCR circuit.
     
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