SCR ground reset not working

Discussion in 'The Projects Forum' started by c5f8, Feb 16, 2014.

  1. c5f8

    Thread Starter New Member

    May 3, 2007
    6
    0
    I come to this website regularly but usually just search. This time I have a question. I'm using an SCR as a regenerative feedback switch in a medical simulation.

    Right now I have an SCR model from Helmut.
    http://forum.allaboutcircuits.com/newsgroups/viewtopic.php?t=1200

    I want to see the SCR turn on and off using LTspice. I can make it turn on but I thought grounding the anode reset the SCR. That doesn't work.

    I uploaded the asc and a txt but the txt should be renamed lib.
     
  2. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    It would work in the real world, so I must conclude this is a simulation problem, not an electronics problem.

    I don't sim. Next helper, please.
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    It's not a simulation problem. All SCRs are sensitive to dv/dt at the anode and can turn-on if that limit is exceeded. The limit for the 2N2326 is 1.8V/μs. You simulation has a much higher dv/dt then that at the anode when S2 turns off after the reset, which causes the SCR to immediately turn back on. If you add a 7nF or larger capacitor between the anode and ground to reduce the dv/dt to below 1.8V/μs when S2 turns off then the SCR also stays off.

    What does you actual regenerative feedback circuit look like? What will be S2?
     
  4. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    Maybe I should have asked for a schematic instead of assuming the OP knew about dv/dt limits.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    You really should join the digital world and download LTspice. You might even like it. ;)
     
  6. burger2227

    Member

    Feb 3, 2014
    190
    24
    Sure download LTSpice and spend half your time troubleshooting that! I like real world a lot better than theory thank you.
     
  7. c5f8

    Thread Starter New Member

    May 3, 2007
    6
    0
    Thank you for the information about SCR dv/dt at the anode that can turn-on SCR if limit is exceeded. I installed a 7nF capacitor between the anode and ground and now simulation works.

    When I get back tonight I'll post it.
     
  8. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    You may spend half your time troubleshooting Spice when you first start, but not after you become familiar with it. Learning to effectively use Spice is rather like learning a new Video game. It does take a fair amount of practice to become proficient.

    I would never build a circuit without simulating it first. It's found a lot of design errors for me and allows me to easily try design variations to determine which is likely best before I ever built the circuit. It also allows easy examination of all the voltages and currents in the circuit (as many at a time as you like), and plot things like circuit frequency response and examine the effect of component tolerances that can be difficult or time-consuming to do with the real circuit.

    But to each his own. :rolleyes:
     
  9. burger2227

    Member

    Feb 3, 2014
    190
    24
    Well I don't design circuits, I sometimes put more than one together though. If that doesn't work, I find another way. Not much of a video game player either.

    I work with the mistakes learned by others... :D
     
  10. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Well, you certainly have an endless source of information...:rolleyes:
     
  11. c5f8

    Thread Starter New Member

    May 3, 2007
    6
    0

    Heart beating is caused by a resting state then depolarization followed by repolarization back to resting. Depolarization opens sodium channels which causes more depolarization. The attached asc file is a model of the heart beating process.

    The pair of bipolar transistors connected as an SCR is my regenerative switch in which T2's base current is derived from the collector T1, whose base current is in turn derived from the collector of T2. This SCR models the depolarization.

    Run the simulation and observe the voltage at port "inside cell". This is the action potential as seen in an ECG. The model also includes the cell membrane capacitance and resistance.

    In my first file, SCR_Helmut02.asc, the switches are only there to aid understanding how the SCR operates. Thanks for your help.
     
Loading...