Transistor latching circuit problem

Discussion in 'General Electronics Chat' started by TheNight Silence, Apr 3, 2016.

  1. TheNight Silence

    Thread Starter New Member

    Feb 4, 2016
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    Hi all
    I've made a latching switch using a transistors but it's not working as it supposed, and i don't know why
    When pressing the push button the led is lit but it's not latched since it turns off when depressing the switch
    pls help why it's not latching the output voltage?
    translatch.png
     
  2. #12

    Expert

    Nov 30, 2010
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    Add a diode from the top of R2 to the left of R4.
    When the push button applies Vcc to R2, Q2 has no voltage across it and thus doesn't get involved in the current flow.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    To avoid any possibility ambiguity, the diode #12 suggested you use should be in series with the wire going from output to input and the diode anode should be towards R2.
     
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  4. #12

    Expert

    Nov 30, 2010
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    You know how it is. I assume a person that can make this circuit would know a diode in parallel with a wire won't change anything.
    But, there I go assuming again.:oops:
     
  5. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Couldn't make it latch either :oops:

    Maybe I did something wrong with the sim. V2 delivers a short pulse through D3, thus simulating a switch, IMHO. But current flows through the LED at all times, 5.774mA when V2=0 and 5.775mA when V2=5

    Capture01.JPG



    However, if I simply cut out D3, and leave V2's positive side floating, the circuit does latch... what's going on????

    Capture02.JPG

    What am I doing wrong?
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    It's look like some kind of "simulation artefacts". To fix this try in "Edit Simulation Command" window just turn on the option "Start external DC supply voltages at 0V".
     
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  7. crutschow

    Expert

    Mar 14, 2008
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    You are doing nothing wrong. It's just that, for whatever reason, the circuit goes into the latch mode when LTspice is doing its initial DC bias calculation before doing the transient analysis
    If you start the supply voltage at 0V, it works.

    Edit: Jony beat me to it.

    upload_2016-4-3_11-32-16.png
     
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  8. TheNight Silence

    Thread Starter New Member

    Feb 4, 2016
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    Thanks for you my friends ,
    i will try your suggestions and i will come back with the results
    B.R
     
  9. TheNight Silence

    Thread Starter New Member

    Feb 4, 2016
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    Hi friends
    I've put a diode as your suggestion,but sorry it doesn't work,even the led didn't lit when i press the switch
    when releasing the button
    The voltage across the diode= -5V,Vc(Vout Q2)=-.094 V(with respect to ground),Vbe(Q1)=4.98V
    when pressing the switch
    The voltage across the diode= -4..65V ,Vc(Vout Q2)=-0.91 V(with respect to ground),Vbe(Q1)=4.98V
     
  10. TheNight Silence

    Thread Starter New Member

    Feb 4, 2016
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    the diode is reverse biased in the two state
     
  11. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    How about changing R1 to 1.2k?
     
  12. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    I also use a voltage source and a series diode to simulate a switched input, mainly because I've never figured out the LTS switch component. But I make the voltage source a pulse with a visible delay and relatively slow rise and fall times so I can see how the transition affects each circuit element. For this circuit, a 1 second delay and 1 second rise time makes the circuit latch every time, with or without D2.

    ak
     
  13. cmartinez

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    Wouldn't that be the equivalent of adding a capacitor in series with a resistor?
     
  14. crutschow

    Expert

    Mar 14, 2008
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    Vbe(Q1) cannot be 4.98V if it's properly connected. It should be no more that 0.8V.

    Below is the simulation with the LTspice switch model. I added a resistor between the base and emitter of Q1 to take care of any leakage currents and help insure that it powers up in the OFF state.

    upload_2016-4-3_15-48-33.png
     
  15. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Maybe, but I'm not suggesting it as a circuit mod. It is a way of seeing what the simulator thinks the circuit is doing.

    ak
     
  16. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    This seems easier unless the OP needs a ground connected LED.

    image.jpg
     
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  17. cmartinez

    AAC Fanatic!

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    It does indeed... I bet (not my life though ;)) that there's an equivalent circuit for a grounded LED
     
  18. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Probably, I just didn't bother designing one. The one in above post was my best memory of an SCR from discrete transsotres. PNPN.

    In simulation, swapping the current limiting resistor and LED cause the circuit not to latch. In both cases below. They work fine as shown.

    image.jpg
     
    Last edited: Apr 4, 2016
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  19. TheNight Silence

    Thread Starter New Member

    Feb 4, 2016
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    really i need the circuit to be controlled by single pulse from my PC serial port which is generated by a peace of software,
    so i need the circuit work from single push button, since i will remove it and apply the control signal in stead;
    i hope you could help
     
  20. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    If the PC output pin is making a positive pulse, you will need to connect it to the left side of R4 through a diode (1N914, etc.) to prevent a transient conflict between the PC output and Q2 collector.

    ak
     
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