Low Current Blue LED?

Discussion in 'General Electronics Chat' started by tracecom, May 4, 2012.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    I built an amplifier kit; it worked, but I realized that I forgot to trim the leads on the last component I put in - a 3mm LED. I disconnected the power and touched my cutters to the LED and got a spark. After trimming the LED leads, I reconnected power and the amp still worked, but the LED doesn't light. Probably, I discharged a 1000 μF filter cap through the LED.

    The kit claims that the maximum voltage is 14 VDC, and power runs through a reverse polarity protection diode, so call the voltage 13.3. The LED was blue, so the Vf is maybe 3.5, and the current limiting resistor is 10k. Thus, the current through the LED is about 1 mA.

    I don't find any blue 3mm LED's at that low current. Does anyone know of a source? Thanks.
     
  2. Audioguru

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    Dec 20, 2007
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    Some LEDs are made to be very bright. Then they still have plenty of light when the current is only 1mA.

    Other bright LEDs have a case that focusses the light into a narrow beam.

    We don't know if your blue LED was made to be very bright or if it simply has a narrow beam.

    Your math is wrong. If the supply is 13.3V and the LED current-limiting resistor is
     
  3. tracecom

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    R = (Vs - Vf) / I
    RI = (Vs - Vf)
    10000I = 13.3 - 3.5
    10000I = 9.8
    I = 9.8 / 10000
    I = .00098A = .98mA

    Is that not right?
     
  4. Audioguru

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    Your math is correct. I was interrupted in the middle of my reply last night. 1mA makes an LED appear dim unless its light is focussed into a narrow beam. But then it cannot be seen if it does not point directly at you.
     
  5. THE_RB

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    Feb 11, 2008
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    AG is right about some LEDs being ok at low currents, I regularly buy "high intensity" LEDs of different colours, generally over 6000mCd (their brightness spec) and usually run them about 1mA which is plenty bright enough as an indicator.

    In some battery powered stuff I have run the LED at 0.2mA or so and still got a good visible indicator, at least with indoor conditions.

    Ebay often has bulk packs of high intensity LEDs, just test them as some LEDs in the pack might be less bright, I think they sometimes sell "iffy" batches of LEDs that come off the production line.
     
  6. #12

    Expert

    Nov 30, 2010
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    Bottom line: You aren't looking for an LED rated at 1ma. They don't exist. You're looking to run 1 ma through an LED that is bright enough to show some sufficient amount of light at 1 ma.
     
  7. tracecom

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  8. Audioguru

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    The very old Hewlett Packard LEDs (HLMP in the part number) have a brightness at 2mA of only 1mcd to 2.3mcd. Many modern LEDs have a brightness of thousands of mcd's at 20mA and hundreds at 2mA. They do not include blue because the green is very old technology. The forward voltage of the green LED is only 1.8V to 2.2V. Modern very bright green LEDs have a forward voltage of about 3.5V like blue LEDs.
     
  9. takao21203

    Distinguished Member

    Apr 28, 2012
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    From my experience this voltage is 2.5 volts, for blue LEDs, a little less for green LEDs. Most LED now sold having clear case are high efficiency. You can run them using 2.2k or 4.7K instead of 470 Ohms/1K.

    If you use regular vendors, look at the mcd rating, and if they have clear case.

    On ebay nearly any LED with clear case will do.
    Blue LEDs with diffused case are rare! These also should be high-efficiency these days.
    I have rectangular blue LEDs here, diffused, while the yellow one's of the same type are low brightness, disappointing, need full 20mA.

    If you are explicitely looking for a blue LED rated for low current, and having low brightness, eventually you will only be able to find these as vintage products.
     
  10. bertus

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    Apr 5, 2008
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  11. SgtWookie

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    Jul 17, 2007
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    Most of what you will find on auction sites have a very narrow focus; < 30° - they do this to get inflated MCD ratings. Unfortunately, this makes the LEDs much less useful for indicator lights due to the narrow focus of the beam.

    However, you can make the beam far wider by doing a little re-shaping of the nose of the LED, or even just roughening up the surface. A nail file/nail board works pretty well for this, and the price is right.
     
  12. #12

    Expert

    Nov 30, 2010
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    One of my favorite tools is a diamond coated fingernail file. They work for electronics projects long after they are not proper for use on fingernails. You can use them to reshape an LED, remove solder, sharpen a probe, shorten the ends of component wires after they have been soldered into a board, knock the burr off a hole in a metal box, etc.
     
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