Multiple LED's, Multiple Switches, Single Power Supply

Discussion in 'The Projects Forum' started by kelphead3, Sep 30, 2014.

  1. kelphead3

    Thread Starter New Member

    Sep 30, 2014
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    [​IMG]

    I have this scenario but have a total of 15 LED's and 15 switches. I am getting shorts across my open switches. I can disconnect the power supply from the circuit and the short goes away. These shorts are causing some LED's to light when it is unwanted. I have tried moving the individual resistors to a common resistor and putting a RL257 diode in place of resistors. The power supply with all output wiring removed shows dead short between positive and negative. Link to power supply. Any ideas on how to isolate the legs of circuits would be greatly appreciated. This is a small part of locking control system for a small prison. This shows on the control board if the door is open or not. I tried to keep it simple for ease of repair and less points of failure.

    Thanks
     
    Last edited: Sep 30, 2014
  2. wayneh

    Expert

    Sep 9, 2010
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    What do you mean by that? Can you show us a picture?
    That comment also baffles me. What do you mean by "shows dead short"? HOW does it show that?
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    There is nothing wrong with that circuit at all.
    If the light is on, the switch is closed or being bypassed by something else. Now YOU have to find out what that is or find your wiring mistake..
    Could be anything from shorts in the wiring from mice or the prisoners messing with you.

    Is this for that youth detention center that all those kids have been escaping from lately :)
     
  4. #12

    Expert

    Nov 30, 2010
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    Wrong move. The schematic is proper as posted. Leave it alone and fix the wiring or the switches.

    What mcgyvr said, only he was faster than I was.
     
  5. kelphead3

    Thread Starter New Member

    Sep 30, 2014
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    [​IMG]

    With the switch open I read a short where the red lines are.


    [​IMG]

    I have checked a few other DC power supplies on my workbench and they read a short across the output terminals too.
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Then the switch is NOT open.. maybe it has failed.. it happens.
    Part number of switch?

    I'd also suspect you don't know how to use a multimeter either..
     
  7. kelphead3

    Thread Starter New Member

    Sep 30, 2014
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    I thought it was wiring or a switch as well. I have been backwards a forwards through these circuits. It's only affecting three out of the fifteen LED's. I have checked for ground faults. Once I remove the power supply from the circuit the switches and wiring all test good.
     
  8. kelphead3

    Thread Starter New Member

    Sep 30, 2014
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    I know the switch is open because I have it disconnected. The switch is open, that is why I am asking this question. I know how to read open and shorts with an Ohmmeter. That's why it is weird to me.

    Thanks again
     
  9. wayneh

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    Sep 9, 2010
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    Once again, I don't think anything short of a good photo and an extra set of eyes is going to solve this. There's a detail missing.
     
  10. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Then its a short from the cathode side of the LED to ground.. I'd suspect thats in your "indicator" enclosure then.
    And you must have negative of the PS tied to chassis ground?
    That PS has each lead capacitively coupled to ground for EMI reasons..
     
  11. #12

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    If that is true, why would you even try to measure the ohms of a power supply output? It's supposed to supply voltage. The ohms across its output terminals are undefined, change from one model to the next, and is irrelevant to this exercise.
     
  12. kelphead3

    Thread Starter New Member

    Sep 30, 2014
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    I do not have the negative of the PS tied to chassis ground. The cathode side of the LED is directly wired to the negative of the power supply. I have a ground going to the input side of the PS.


    I wasn't actively measuring the ohms of a power supply output. I was following shorts in my wiring and traced it back to the power supply. I was surprised to read .000 ohms on my meter. I had never done it before and found it odd. I went back to my workbench to see if other dc power supplies did the same thing, and they do. I am just trying to provide all the info I came across in my own troubleshooting.
     
  13. #12

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    OK. You just chased a short to a dead end. No worries.
     
  14. mcgyvr

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    Oct 15, 2009
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    If the LEDs lightup then you have a "path" to ground from the negative side of the LED.. Start tracing :)
     
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