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LED Optimum Voltage

Discussion in 'The Projects Forum' started by roscirc, Apr 30, 2012.

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  1. roscirc

    Thread Starter New Member

    Apr 30, 2012
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    Hi All,

    I need to calculate the resistors for an LED with the following parameter as per datasheet:

    -Optimum Voltage 24V dc
    -DC current 21 mA (max)
    -Reverse Voltage 5V

    I will have to use a power supply of 110V , this and LED type as above come from the design of an existing project.
    In the LED datasheet there are no indication about forward voltage but as far as I understand it should be always in the range 1,5/3.8V.

    So my question is , what does the Optimum Voltage stand for? would it be possible it is the forward voltage I will have to use to calculate the resistors?



    Thanks,

    Ros
     
  2. #12

    Expert

    Nov 30, 2010
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    9,691
    Without seeing the document that said, "optimum voltage" I don't have the context to work with. Can you post it?
     
  3. roscirc

    Thread Starter New Member

    Apr 30, 2012
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    Here is the original datasheet
     
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  4. #12

    Expert

    Nov 30, 2010
    18,076
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    That one already has a resistor in it to compensate for 24 volts.
    Just take your 110 volts DC and subtract 24, then calculate the added resistance for .021 amps.

    110-24/.021 = 4095 ohms. Next closest value is 4200 ohms, 5%, 4 watts

    I sincerely hope you are talking about 110 DC volts! This isn't built for AC.
     
  5. roscirc

    Thread Starter New Member

    Apr 30, 2012
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    Thanks #12.

    The strange thing though is that in the original design they connected in series a 150kOhm resistor with which the LED works fine.
    The reason why I was trying to calculate it, is that this LED keeps failing over time and I wanted to check whether they sized the resistor correctly but with 150kOhm there is actually a current of 0.57 mA far less than the 21mA.

    Another reason for the failing I suspect is a transient reverse voltage of about 200V for few milliseconds which was recorded with a data logger in some case. Would this short time be enough to cause a breakdown?

    thanks,

    Ros
     
  6. #12

    Expert

    Nov 30, 2010
    18,076
    9,691
    Oh yes. You should put a diode in parallel with the LED to bypass the reverse voltage spikes around it.
    or a series diode worth about 400 volts. 1N 4005 or better.

    You know how?
     
  7. roscirc

    Thread Starter New Member

    Apr 30, 2012
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    That's interesting but not sure how I should connect it..
     
  8. roscirc

    Thread Starter New Member

    Apr 30, 2012
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    I guess the Diode in parallel should be connected as "inverse parallel" but what type should I be looking at?

    Would you have some internet resources about protecting LED this way?

    Thanks,

    Ros
     
  9. #12

    Expert

    Nov 30, 2010
    18,076
    9,691
    No. I never even looked for a web page about paralleling a common rectifier with an LED to protect from reversed voltages.
     
  10. roscirc

    Thread Starter New Member

    Apr 30, 2012
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    Thank you!!
     
  11. Wendy

    Moderator

    Mar 24, 2008
    21,838
    3,047
    Please read our ToS, if I interpret this correctly powering LEDs from mains is directly mentioned. Am I mistaken about where the power is coming from?

    Power anything like this from mains is a major safety hazard. If I am you may contact the mods via PM or email.
     
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