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How to identify an LED

Discussion in 'Troubleshooting and Repair' started by Pascal666, Sep 7, 2012.

  1. Pascal666

    Pascal666

    18
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    Aug 20, 2012
    I need to replace the bad battery LED on an APC Smartcell XR. The lens managed to get cut in half. It is a 5mm LED with a red lens. These are widely available, but there appear to be several voltages to choose from. Anyone know which LED this unit takes? They were manufactured in 1995.

    I have another unit with a good LED on it, but since the LED only lights when there is a fault, I can't simply measure the voltage on the good unit. I can easily disconnect the good LED from its electronics to make measurements upon it, but I don't know what to measure. How do I determine what type of LED I need?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I would recommend a LED of the same colour that physically fits.

    Don't worry to much about "voltage ratings" unless they become an issue later.
     
  3. thebluepuppy

    thebluepuppy

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    Sep 7, 2012
    actaully if its a camera the replaced led if not properaly selected like the proper current rating and voltage rating can fry things later on down the road then you might have to replace more then just the led. best way to check is to get a (digital multi meter) and test it. if its blown complelty try finding the schematic for the device online in a manual.
    best of luck.
    ps watch out for them capacitors they can kill you if you try and operate on it if it still has a charge in it.
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    I disagree: This LED is (from the OP's description) just a fault indicator LED. Even if the LED is open or short circuited this should have no influence on the operation of the APC (uninteruptable power supply) at all. Any standard 5mm red LED should do the job. The voltage of a red LED is on the order of 1.6V, give or take a few 100mV. The tolerance is typically irrelevant if the LED is used as an indicator,

    What can happen is that a replacement LED has a slightly different hue of red or a slightly different brightness than the original LED. But then again: who cares? If the fault LED is turned on, you have other problems than the hue or the brightness of the indicator.

    I'd be surprised if they had used any fancy special LED in this application.
     
  5. Jotto

    Jotto

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    Aug 24, 2012
    Wouldn't it be easier to see what the current limiting resistor is and that way you know about what the VF and IR of the LED you require? That would eliminate a lot of LEDs.

    Also, can't you make it fault to allow you to see the voltage?

    It would at least put in the ballpark.
     
    Last edited: Sep 7, 2012
  6. CocaCola

    CocaCola

    3,635
    4
    Apr 7, 2012
    What if there is no resistor?

    Also it's overkill to be so anal in most applications, as LEDs don't have a dead set value they operate at, instead there is a 'safe' range and they can even operate outside that range but it shortens their life... But, in many cases does it matter? Do you need an LED that last 100,000 hours for an indicator like that might in it's entire existence be on for 1000 hours? A little over drive and life shortening won't hurt most applications... Thus if you choose an LED the same color and size chances are REAL good that the operating range is plenty close to work for an acceptable number of service hours... And has been said because it's just a visual indicator if it's not the same 'brightness' or color, it's likely not a concern...
     
  7. Pascal666

    Pascal666

    18
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    Aug 20, 2012
    Thank you for all of the replies. I don't normally work with low voltage parts like this, so I'm not real knowledgeable about LEDs. Please forgive any stupidity.

    These battery packs are known for causing batteries to overheat and leak or explode. This LED is practically the last line of defense. I'd rather not find out the hard way I put the wrong one in.

    I tried several different ways to get the LED to light on the good unit before posting here. Putting a set of bad batteries in didn't work because the UPS just rejects the pack if its voltage is too low. Connecting two packs together, one with good batteries and one with bad made the UPS accept it, but when I removed the good pack the LED still didn't light on the bad one. I don't know what it would take to get it to light.

    Both of my household multimeters have a setting to "test the voltage drop in diodes" in mV. They both just read "1" when hooked up backwards (as the manual states). Forward they both cause the LED to light dimly and flash a number between 1200 and 1900 for a split second and then read "1" again. I'm assuming this means they are only designed for normal diodes not LEDs.

    Just dawned on me I should try the multimeter from my car. It has a setting to "determine the forward voltage for diodes." It did not make the LED light (that I could tell) but it reads 1.587v across the good LED.

    Is forward voltage the same as voltage drop? How do these relate to the typical and maximum voltage ratings for a LED? RadioShack appears to carry three 5mm red diffused LEDs. They say "Typical voltage: 1.8, maximum of 2.4V", "Typical Voltage is 2.25, with a maximum voltage of 2.6V", and the third just says "12V". I don't understand how these labels relate to each other. Do all of these light at 1.6V and just have different points at which they burn out? So the 12V one has the largest operating range?

    I pulled out the circuit board the LED connects to. One leg of the LED connects to a 110 ± 2% ohm resister, the other to one leg of a small black half cylindrical thing and then the trace continues but I can't follow it (I'm assuming this is ground just going to other components). Just out of curiosity, what are the small black half cylindrical things on the circuit board?
     
    Last edited: Sep 9, 2012
  8. CocaCola

    CocaCola

    3,635
    4
    Apr 7, 2012
    They are related but no the same...

    The LED is almost certainly not being used as a safety or defense feature...

    Review the stick on LEDs here, and Radio Shack should be shot for calling any LED a '12V' LED... LEDs are current driven, after reading Steve's sticky post a lot of your questions will or should be more clear...

     
  9. Pascal666

    Pascal666

    18
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    Aug 20, 2012
    Smartcells are wired in parallel. The UPS continues to charge all of them even after one of them goes bad. The LED is there to alert someone that one of the Smartcells has suffered a failure and needs to be removed from the string. Failure to do so can result in the batteries overheating causing them to leak or explode, depending upon the failure mode.

    I believe this is the thread you are referring to. I found and read it before posting here. It does have a lot of good information, but some of it is a bit over my head. What I did understand did not appear to answer my underlying problem. It appears to be more targeted toward planning a circuit using LEDs. It would be nice if a section were added on replacing a broken/burned out LED. As circuits using LEDs age, more and more people will find it necessary to replace them.
     
  10. CocaCola

    CocaCola

    3,635
    4
    Apr 7, 2012
    To be blunt you are over making this more complicated than it needs to be and over analyzing and over thinking the replacement, for all practical purposes...

    You were told the basics of what to do in the first reply, just get an LED of the same size, color and type... The specifications of LEDs are not 'identical' across the board but in almost all cases they are plenty close enough for interchangeability as long as you match size, color and type (and color/size is not even all that important in many cases)... This variance in specifications is true with most electronic components and LEDs are not an exception, you rarely ever need 'exact' specification replacement components...

    You said it was a "5mm LED with a red lens" I'm going to take that as being a red colored diffused translucent lens, meaning that it's a 'standard' LED not a super bright that almost always has a clear, tinted transparent or milky lens...

    Like this?

    [​IMG]

    If so it's just a 'standard' 5mm red LED, and there is a 99.99% probability that any 'standard' 5mm red LED you pick up will work just fine... Do mind the polarity though when installing, look for the flat side at the base of the LED...

    Like I previously asked "Measure the voltage on the other side of the resistor (not the LED side), from that we can take a very educated guess at the LEDs perimeters..." the guess we could make with that value and the resistor value you have already given, will enable us to confirm the compatibility of a new LED without doubt...

    As I suspect the LED is just a visual minder, it does nothing to actually 'protect' or 'provide' a line of defense, it just indicates a failure and leaves the rest up to you...
     
    Last edited: Sep 10, 2012
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,690
    Jan 21, 2010
    Failures of LEDs are typically a design or mechanical issue. The next most common cause would be (probably) random failure. Last would come worn out.

    If you have to replace a LED, the reason almost always has nothing to do with the LED.

    I will note that I have a string of LED Christmas lights that hang around our front door. They have been turned on for about 5 hours every night for the last 12 years. For the first few years they were on 24 hours a day. They're looking a bit dim these days. This is the sort of lifetime you expect. (way over 20,000 hours so far).
     
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