Problem with SCR's driven by 4017b

Discussion in 'The Projects Forum' started by Nickage, Jan 3, 2013.

  1. Nickage

    Thread Starter New Member

    Jan 3, 2013
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    All,

    I am building a sequencer control board, using a 4017b decade counter. I built the circuits in steps to verify progress. The problem is, once I setup the 2n5060 SCRs to latch whe nthe outputs are triggered, the test LEDs chase as if the SCRs are not there.

    The 4017b outputs are directly wired to the gate, 13.8VDC going to the anode, and the cathode is wired to the LED/resistor array. Am I missing something? The voltage at the gate when the output is on is 13.6VDC.

    Any help would be appreciated!
     
  2. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Can you post a schematic of your circuit ??
     
  3. Nickage

    Thread Starter New Member

    Jan 3, 2013
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    It's a rudimentary sketch, but should give you an idea of what's going on.
     
  4. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Look at this - circuit with motor and battery
    http://www.allaboutcircuits.com/vol_3/chpt_7/5.html

    The Led's need to be between V+ and the anode, cathode to ground.
    Be aware that once the scr's are triggered they latch on and must have the DC voltage removed before they shut off.
    What do you want this circuit to do ??
    Transistors would be better if you just want to flash the led's

    Look at the datasheet for the 4017. You should connect pin 13 either to gnd or V+... But which ???
     
    Last edited: Jan 3, 2013
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  5. Nickage

    Thread Starter New Member

    Jan 3, 2013
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    13 is grounded.
     
  6. Nickage

    Thread Starter New Member

    Jan 3, 2013
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    I made the switch you suggested, and it works. What I'm trying to do is get a row of LEDs to light up like chaser lights, only I want them to stay on.
     
  7. Sensacell

    Well-Known Member

    Jun 19, 2012
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    Also note that SCR's are current controlled devices, not voltage controlled devices. You need a current limiting resistor in series with the gate lead.

    The cathode should be tied to GND. with the load connected to the anode, it's then a "sinking driver"

    Then, of course, the SCR's will NEVER turn off again unless you interrupt the current flow by an external means. Using full-wave rectified AC as the power source will allow the SCR to turn off every 1/120th of a second, when the voltage crosses zero.
     
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  8. Nickage

    Thread Starter New Member

    Jan 3, 2013
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    Does the current limiting resistor need to be between the 4017 and the gate?
     
  9. vrainom

    Member

    Sep 8, 2011
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    There is a way to turn off scrs after they're latched on, and that's forcing the gate to a voltage equal or lower than the cathode's so the current is cut off. That's why your scrs were turning off when the output that drives the gate changed states. I think that in order to make your previous configuration latch you could have used a diode and a limiting resistor in series between the 4017 ic and the gate. Or maybe just the limiting resistor if it's large enough so it won't be able to cut the gate's current.
     
    Last edited: Jan 3, 2013
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  10. tubeguy

    Well-Known Member

    Nov 3, 2012
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    You could connect a PNP transistor between V+ and the LED's supply, connected to the reset thru a 10k resistor to base. Emitter to V+, collector to the LED's anodes.

    When reset is low the transistor is on supplying voltage to LED's.
    Reset going high will turn off LED and SCR voltage. Should reset the SCR's (maybe not the last --try it.)
    You might put a 2.2k resistor from base to emiiter also.
    Current limiting resistors in series with SCR gates could be 4.7k-10k.
    And might need a 47k resistor from SCR gate to Gnd,
     
    Last edited: Jan 4, 2013
  11. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
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    Connect the load in the Anode side like this...
    BUT the thyristor will latch on when it is triggered and can only be switched off when the Anode current is broken, or the Anode and Cathode are shorted out, so if you want to use it as a chaser, you need to use transistors.
     
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