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Q&A Driving LED with NPN transistor from I/O pin

If you choose suitable ultrabright LEDs , there is no need to use 40 mA. I have over 10 thousand LEDs all over 16,000 mcd at 20 mA which is painfully bright for close indicator but works well 100 ...

posted 1y ago by TonyStewart‭  ·  edited 1y ago by TonyStewart‭

Answer
#3: Post edited by user avatar TonyStewart‭ · 2023-10-07T00:25:13Z (about 1 year ago)
  • If you choose suitable ultrabright LEDs , there is no need to use 40 mA. I have over 10 thousand LEDs all over 16,000 mcd at 20 mA which is painfully bright for close indicator but works well 100 m away. They are also all matched brightness and Vf within 10%, but I ordered these special for a high volume user. Otherwise, you can get very wide tolerances from batch to batch from unknown sources.
  • Most uC now are using advanced 74ALCxx type CMOS which has a nominal driver impedance of around 22 ohms +/- 50%.Ro= Vol/Io
  • This is more than the internal resistance of a small 5mm (15 ohm) or 50 mW SMD LED so that you can drive it directly from the uC with a series R if you know your LDO is 1 or 2% accurate.
  • I would choose ultrabright white 5mm LED's 16 Cd @ 20 mA and run them at 5 mA and add a series R something like 50 ohms. But I would do a tolerance stackup calculation.
  • Most good 5mm white LEDs these days will be 2.9V at 11 mA, so I don't know what you have. Then 3.0 to 3.1 at 20mA rating.
  • If you choose suitable ultrabright LEDs , there is no need to use 40 mA. I have over 10 thousand LEDs all over 16,000 mcd at 20 mA which is painfully bright for close indicator but works well 100 m away. They are also all matched brightness and Vf within 10%, but I ordered these special for a high volume user. Otherwise, you can get very wide tolerances from batch to batch from unknown sources.
  • Most uC now are using advanced 74ALCxx type CMOS which has a nominal driver impedance of around 22 ohms +/- 50%.Ro= Vol/Io
  • This is more than the internal resistance of a small 5mm (15 ohm) or 50 mW SMD LED so that you can drive it directly from the uC with a series R if you know your LDO is 1 or 2% accurate.
  • Using your LED measurement of 3.24@11mA for [ this simulation](https://tinyurl.com/ywplsape).
  • I would choose ultrabright white 5mm LED's 16 Cd @ 20 mA and run them at 5 mA and add a series R something like 50 ohms. But I would do a tolerance stackup calculation.
  • Most good 5mm white LEDs these days will be 2.9V at 11 mA, so I don't know what you have. Then 3.0 to 3.1 at 20mA rating.
#2: Post edited by user avatar TonyStewart‭ · 2023-10-07T00:19:45Z (about 1 year ago)
  • If you choose suitable ultrabright LEDs , there is no need to use 40 mA. I have over 10 thousand LEDs all over 16,000 mcd at 20 mA which is painfully bright for close indicator but works well 100 m away. They are also all matched brightness and Vf within 10%, but I ordered these special for a high volume user. Otherwise, you can get very wide tolerances from batch to batch from unknown sources.
  • Most uC now are using advanced 74ALCxx type CMOS which has a nominal driver impedance of around 22 ohms +/- 50%. This is more than the internal resistance of a small 5mm or 50 mW SMD LED so that you can drive it directly from the uC if you know your LDO is 1 or 2% accurate.
  • I would choose ultrabright white 5mm LED's 16 Cd @ 20 mA and run them at 5 mA and add a series R something like 50 ohms. But I would do a tolerance stackup calculation.
  • Most good 5mm white LEDs these days will be 2.9V at 11 mA, so I don't know what you have. Then 3.0 to 3.1 at 20mA rating.
  • If you choose suitable ultrabright LEDs , there is no need to use 40 mA. I have over 10 thousand LEDs all over 16,000 mcd at 20 mA which is painfully bright for close indicator but works well 100 m away. They are also all matched brightness and Vf within 10%, but I ordered these special for a high volume user. Otherwise, you can get very wide tolerances from batch to batch from unknown sources.
  • Most uC now are using advanced 74ALCxx type CMOS which has a nominal driver impedance of around 22 ohms +/- 50%.Ro= Vol/Io
  • This is more than the internal resistance of a small 5mm (15 ohm) or 50 mW SMD LED so that you can drive it directly from the uC with a series R if you know your LDO is 1 or 2% accurate.
  • I would choose ultrabright white 5mm LED's 16 Cd @ 20 mA and run them at 5 mA and add a series R something like 50 ohms. But I would do a tolerance stackup calculation.
  • Most good 5mm white LEDs these days will be 2.9V at 11 mA, so I don't know what you have. Then 3.0 to 3.1 at 20mA rating.
#1: Initial revision by user avatar TonyStewart‭ · 2023-10-07T00:18:22Z (about 1 year ago)
If you choose suitable ultrabright LEDs , there is no need to use 40 mA.  I have over 10 thousand LEDs all over 16,000 mcd at 20 mA which is painfully bright for close indicator but works well 100 m away.  They are also all matched brightness and Vf within 10%, but I ordered these special for a high volume user. Otherwise, you can get very wide tolerances from batch to batch from unknown sources.

Most uC now are using advanced 74ALCxx type CMOS which has a nominal driver impedance of around 22 ohms +/- 50%. This is more than the internal resistance of a small 5mm or 50 mW SMD LED so that you can drive it directly from the uC if you know your LDO is 1 or 2% accurate. 

I would choose ultrabright white 5mm LED's 16 Cd @ 20 mA and run them at 5 mA and add a series R something like 50 ohms. But I would do a tolerance stackup calculation.

Most good 5mm white LEDs these days will be 2.9V at 11 mA, so I don't know what you have. Then 3.0 to 3.1 at 20mA rating.