Led failure detection

Discussion in 'The Projects Forum' started by b_p4tel, Jun 16, 2011.

  1. b_p4tel

    Thread Starter New Member

    Jun 16, 2011
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    hi

    I have a basically a 12v supply with 14 Green LEDS in pairs with a resistor in-between each LED. Each pair is in parallel.

    I need to add a further ciruit which has a RED LED that can detect when a green LED fails (open circuit or short circuit) the RED LED will light up.

    Is there an easy way to do this?
     
  2. b_p4tel

    Thread Starter New Member

    Jun 16, 2011
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    any body?!
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    Can you provide a schematic?

    Be patient - we are all volunteers, and this is not a chat room environment. You may have a wait of some hours before anyone decides to help.
     
  4. John P

    AAC Fanatic!

    Oct 14, 2008
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    You could install op-amp adder circuits measuring the voltage across each of the 7 resistors, with each output driving a pair of comparators that would register whether the current was too high (short) or too low (open). But it's a lot of hardware.

    It would be easier if you could place the LEDs in series off the 12V supply, with the resistor connected to Gnd. Then you could send the voltage across it to the comparators directly. Or you could put in a voltage-follower buffer for each circuit, with all the outputs connected together via diodes and a shared pair of comparators. I'm not positive on how that would work, but I think it could be done.
     
  5. b_p4tel

    Thread Starter New Member

    Jun 16, 2011
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    Sorry thanks for the inputs, Ill wont get a chance to upload the schematic till tomorrow.

    Thanks john P interesting idea with the adder circuits Il go over this idea tomorrow Thanks
     
  6. b_p4tel

    Thread Starter New Member

    Jun 16, 2011
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  7. #12

    Expert

    Nov 30, 2010
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    It looks like your second post can give an output for your first post that is proportional to the current used. That should tell you if any LED has gone open or shorted. You will need a window comparator for the output of the 1NA193 and the comparator circuit can turn on the red LED.
     
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  8. SgtWookie

    Expert

    Jul 17, 2007
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    LEDs are so reliable that it probably is not worth the extra circuit complexity to check whether an LED failed or not. An LED that is operating on less than it's typical rated current can last a half-million hours; which is around 57 years of continuous operation; it's where they are operating at or near maximum current where more rapid failures occur.

    If you are operating from a 12v lead-acid battery and your green LEDs have a typical Vf of 6v @ 20mA, you should really be using higher value resistors, as with 270 Ohm resistors, you would wind up with ~26mA current flow, or even more when the battery is being charged.

    For example, if you are charging the battery at 14v, then:
    Rlimit = (Vsupply - Vf_LED_total) / DesiredCurrent = (14v-2*3v)/20mA = (14-6)/0.02 = 8/.02 = 400 Ohms. With the 270 Ohm resistors, your LED current would be ~ 8v/270 = 29.63mA.

    But, only you know the LED specifications right now.
     
  9. TBayBoy

    Member

    May 25, 2011
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    Oh I was worried there for a moment that image shack stopped their mega spamming policy, but wham o, away it went... anyone wand discount Viagra?
     
  10. #12

    Expert

    Nov 30, 2010
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    Along Wookie's line of thinking, I could also suggest that you look at the LEDs with your eyes and you can see if any aren't working, but only you know why this is important to do with an electronic circuit.
     
  11. b_p4tel

    Thread Starter New Member

    Jun 16, 2011
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    thanks for your help! could you help me a little on the window comparator bit please
     
  12. b_p4tel

    Thread Starter New Member

    Jun 16, 2011
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    Also I know how good LEDs are, but its just a "little" LED monitoring circuit "incase" one of the LEDs somehow fail
     
  13. #12

    Expert

    Nov 30, 2010
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    Show me your drawing of how you have the current monitor chip connected to a window comparator and I'll help you a little.
     
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