Which diode to prevent "backflow"?

Discussion in 'General Electronics Chat' started by kgstewar, Dec 28, 2012.

  1. kgstewar

    Thread Starter Member

    Apr 5, 2012
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    Hi all,

    I have a couple of CD40192 counters that get reset to zero via two separate switches. S1 should only reset 40192A while S2 should reset both 40192A and 40192B. I tried using a 1N4001 diode in the circuit shown below but it got fried and now conducts in both directions. Is there an appropriate diode that would work in this circuit, or should I try a different approach?

    Thanks!

    Kevin
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    If a 1N4001 got fried you have some serious trouble in there, as it can handle 1A without a sweat. Probably the actual circuit is not connected the way you show, my bet is that S1 is a SPDT between ground and +V.
     
  3. kgstewar

    Thread Starter Member

    Apr 5, 2012
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    Perhaps I've done something different than what I've shown, I'll certainly check. It actually worked for awhile so maybe I'll replace the 1n4001 with another and see what happens. Reverse voltage of 12V is not too much for this diode?

    Kevin
     
  4. Ron H

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    Apr 14, 2005
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    The diode will not be hurt by 12V.
    I would use 1N4148, but 1N4001 will be OK .
    You need to add a 10kΩ pulldown resistor from each reset input to ground.
     
  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    1N4001 is good for 50V peak, 1N4007 is for up to 1000V peak. These diodes don´t simply die unless severly abused. Don´t put another one in until you find the problem, you may damage other components in the process.
     
  6. kgstewar

    Thread Starter Member

    Apr 5, 2012
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    Ah yes, already done, left that off the schematic by mistake.
     
  7. kgstewar

    Thread Starter Member

    Apr 5, 2012
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    You all convinced me that the 1n4001 should be bomb-proof, more or less, so I pulled the diode and checked it out of the circuit. It's totally fine, so the problem lies elsewhere. Unfortunately, I have discovered that the problem is more mysterious.

    The schematic I showed is a small, small part of a much more complicated scoreboard circuit. The reset features work fine when the circuit is first powered up and continues to work fine for about 15 minutes. After that, S1 resets both counters, instead of just 40192A. If I switch the circuit off and then back on, the problem remains. If I power it off for about 15 minutes or so, then power it back on, it again works well for about 15 minutes.

    I checked all the ICs and transistors to see if anything is getting hot, but all seems cool, although it seems like something must be warming up to cause the problem to arise after 15 minutes....

    Anyway, I realize this is hard to diagnose without a schematic, so perhaps this is my motivation to put together a complete schematic now.

    Kevin
     
    Last edited: Dec 31, 2012
  8. Ron H

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    Apr 14, 2005
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    The zero bias capacitance of 1N4001 is about 30pF. See attachment from Diodes Inc. datasheet. Depending on the stray capacitance at the S2 reset, S1 switching could cause a transient at S2 which is large enough to initiate a reset.
    Try adding a 1nF cap from the S2 reset to ground.
    My personal choice for signal diodes is 1N4148. 1N4001 is a rectifier. Sure, it's also a diode.
     
  9. kgstewar

    Thread Starter Member

    Apr 5, 2012
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    Thanks Ron, I'll try adding that cap.

    I can pick up some 1n4148s at my local radio shack. Do these have lower capacitance than the 4001? Is that their advantage?

    Many thanks!

    Kevin
     
  10. Ron H

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    1N4148 is 4pF max @0v. Typical is 1pF or less.

    EDIT: What is the value of your pulldown resistors?
     
    Last edited: Dec 31, 2012
  11. kgstewar

    Thread Starter Member

    Apr 5, 2012
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    Thanks Ron, and forgive the basic question, but which side of S2 does the 1nF cap go on? The side closest to the IC?

    Kevin
     
  12. kgstewar

    Thread Starter Member

    Apr 5, 2012
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    Pulldown resistors are 100K. Too much?

    Kevin
     
  13. Ron H

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    The cap goes from the reset pin to ground.
    You may have addressed this already, but do you have power supply decoupling caps (0.1uF) from each IC's power supply pin to ground? You can get erratic performance if you don't.
    100k pulldowns should be OK.
     
  14. kgstewar

    Thread Starter Member

    Apr 5, 2012
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    Yes, decoupling caps on all IC power pins (10uF and 0.1uF to ground)
    Thanks!

    Oh, and I just realized I have a 1 uF cap from the reset pin on 40192A to ground already so that it goes to zero at power on. Perhaps this is the problem?

    I realize that I am making this difficult by not providing an accurate schematic from the get go. So sorry!

    Kevin
     
  15. kgstewar

    Thread Starter Member

    Apr 5, 2012
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    Actually, let me put together a proper schematic. I hate that I'm taking your time without having given you a complete picture!

    Many thanks, as always.

    Kevin
     
    Ron H likes this.
  16. Ron H

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    Apr 14, 2005
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    The 1uF caps should prevent crosstalk across the diode, but they might be hard on your switches. The current spike can be on the order of amps for a very short time.
    I would think you would want to reset the ICs on power up. To do that, the caps need to connect to vcc, not ground.
     
  17. kgstewar

    Thread Starter Member

    Apr 5, 2012
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    Of course you are right, the 1uF caps I have now go to Vcc, not ground. I'll add the .001uF cap as you suggested and report back.

    Thanks!

    Kevin
     
  18. Ron H

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    With the 1uF caps in place, the 1nF cap shouldn't do anything. I doubt that the problem is cap coupling across the diode.
     
  19. timescope

    Member

    Dec 14, 2011
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    You can also make sure that all unused inputs of the 40192 are either connected to ground or the supply, Vdd. Floating inputs can cause weird problems.

    Timescope
     
  20. kgstewar

    Thread Starter Member

    Apr 5, 2012
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    I swapped out the 1n4001 for a 1n4148 and the problem seems to have gone away! I did not add the 1nF cap as suggested but if the problem reappears I will give that a try as well. Many thanks to all and especially Ron.

    Kevin
     
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