LED Array protection

Discussion in 'The Projects Forum' started by Rocco, May 19, 2008.

  1. Rocco

    Thread Starter New Member

    May 19, 2008
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    I have 81 red leds in a series parallel configuration, (9x9). I am driving them with a current source setup using an op amp & transistor. How can I protect the array from burning up... if one led dies, the current source increases the voltage to maintain current, this will eventual cause self destruction of the panel as the current will stay the same regardless of how many leds are still active. Can I put a zener in parallel with the array?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    It would help a lot if you posted the circuit so we didn't have to guess. The first guess is that the zener would need current limiting in order not to burn out - what than?
     
  3. dileepchacko

    Active Member

    May 13, 2008
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    Instead of going for transistor driver , go for ULN2003 output driver. Actually it is a Darlington transistor array output driver. It is more stable and economical. 6 Darlington array is inbuilt in this IC. Either you can regulate the power supply or put a zener for better protection for your LED's
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    This is precisely the problem one runs into when paralleling LEDs and feeding them a controlled current. Your best defense is to put current limiting resistors on each series string, and supply them with a controlled voltage instead. The larger the resistors, the more power will be expended in waste heat - but the more tolerant the array will be of voltage fluctuations.

    Another option would be to control the current to each string individually - but resistors certainly are cheaper.

    Not unless you're using current limiting resistors on each string in the array.
     
    Last edited: May 20, 2008
  5. Rocco

    Thread Starter New Member

    May 19, 2008
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    Attached is a schematic of the circuit. I thought about using a voltage controller earlier but for some reason opted against it. I have a 0-5 vdc control voltage... D/A output or a digital pot in manual mode. So I would need to use a voltage controlled voltage source?

    Schematic
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Oh, you DO have current limiting resistors in each string!

    So, change it from a constant current source ito a constant voltage source.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    I can't read the values. How much current are you thinking per string, and what is the voltage of the power supply?

    If the current values low enough there are other types of current regulators that are simple, cheap, and low parts count.

    I agree the resistors are the simpliest way. Your real problem is if an LED burns out it will generally open, which will darken the entire chain it is part of.

    What is this, some sort of IR flood or something?
     
  8. Rocco

    Thread Starter New Member

    May 19, 2008
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    It will be used for a machine vision application, I'm using the shut down bit on the op amp to sync with ccd frame rates. Typical current per leg is about 60mA with a kHz pwm. Leg resistors are 180 ohm and LED p/s is 24vdc. I already had a bad LED which caused a panel to self destruct with the constant current source. What the resistor arrangement using an op amp for a constant voltage source?
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    So you're modifying an existing design, or is this something you built yourself?

    Figuring each LED drops about 2.5VDC (a bad assumption BTW) then they should actually be way under 60ma. If they were dropping 1.5 VDC then the 180ohm resistor would be just about right (I calculated 200ohm), with each resistor dropping just under a watt.

    Something doesn't jive. You're schematic is right on the edge of being unviewable, so it is hard to see or say anything with certainty, but the current limiting resistors should do the trick. The op amp circuit is more for on and off than anything, from what I can see. Have you looked at your power supply to make sure it is regulated at 24VDC, and has no AC riding on it? You are either feeding excessive voltage to the strings, or the resistors are a lot less than 180ohms.

    Here is my take on what you have said, minus the op amp section.

    [​IMG]

    This is a stable configuration, and should work indefinitely.

    The way I interpret it, the op amp current limiter is a safety feature as well as a switch, and doesn't actually regulate the current but protects the wires from shorts. The 180ohm resistors regulate LED currents quite nicely.

    BTW, by measuring the voltage across the 180ohm resistors you can get a nice measurement of actual current going through each string.
     
    Last edited: May 20, 2008
  10. Rocco

    Thread Starter New Member

    May 19, 2008
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    I posted a better image of the schematic below. It is currently running in DC mode with 25mA across each leg. It is my design, but I need to correct to the self destruct "feature" if LED's start to fail which is where I'm at now. In pulsed mode (later design) I will change the leg resistors to increase current / intensity, but for now I need to come up with a voltage controlled voltage source rather than a current source or some way to prevent the voltage across the led panel from changing if / when LED's start to fail. I had read an article about using a zener diode across led panels run in current mode to limit voltage but haven't modeled it yet. Ideally minimal changes to the design would be desired!


    Schematic
     
    Last edited: May 20, 2008
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Ok, so if you're putting 25mA through a 180 Ohm resistor, you should see a 4.5v drop across it.

    What's the voltage on the collector of Q1?

    Take that voltage, and add it to 4.5. Divide that total by 25mA (0.025) to get the new value for your current limiting resistors. Don't forget to calculate the wattage. Then pop out Q1 and your op amp (whatever it is, I can't read your schematic) and ground where Q1's collector is.
     
  12. Wendy

    Moderator

    Mar 24, 2008
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    Your new schematic is exactly the same size as the old one, which means it is exactly as legible.

    LEDs have a wide range of drop voltages across them, although within a family they are very predictable. This is why Sarge is telling you what he is. We're both saying something similar, in that using the voltage across the 180ohm resistor will show you the real current you are sourcing the diodes. Given the number of diodes in each chain small variations can add up.

    Lets say you've got the values you're planning on. V*V/R is the wattage, so 4.5VDC * 4.5VDC / 180 = 0.1125W or around 1/8 W. If this is true then a 1/4W resistor should suffice.
     
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