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How many amps to light a led

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large_ghostman

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Ok not Amps but parts of. Often we see and use the standard advice of 20mA, but seriously is that much needed? Personally i dont use anywhere near that these days. What got me thinking was two things mainly, on one micro board dev board i use alot it measures total current from the board.

The IDE and sensing on the board is capable of reading uA range easily. For this microbe cell i need really low power so i am using a ultra low current micro, i had the IDE current screen up and was measuring sleep states and current draw, the board has a tiny smd led on it thats seen easily when its lit, when i looked the entire board was running at around 1mA with the led included.

So i did some searching and you tube shows a photon meter and a led being lit at very low current ~500-600uA. The led was a normal 5mm cheapo red led, it wasnt bright by any standard but you could see it was on (just).

So looking at some the other boards i use i measured those, all use smd leds and there are various colours. Voltage is under 5V and the leds are plenty bright at 1mA.

So i swapped out some the smd leds and used 3-5mm leds (red,green,blue,white and RGB) all at 4V, i got decent brightness from all of them at just under 2mA (~1.7mA average). So generally what current do you aim for?

For most apps i admit it dosnt make much difference if run at 25mA or 2.5mA, but when your using really low power micro's or want/need long battery life or have low current sources, every mA counts, for the microbe cell i can get decent current density but the linear tech boost chips have pretty low max current inputs.
So this has given me a problem, the energy board i am using from linear tech has a max input of 20mA on the input i am using so i dont want to waste most of that on a led!

It does have a cap bank for storage and it allows the different harvesting sources to be diode Ored, so i am splitting the cathodes from the cell into several of the boards inputs and adding bigger cap storage bank. But i still think its worth saving what i can.

The other thing is the pic ports, some the newer chips have 20mA max output per pin and fairly low max port current, so why use 20mA on a led when 2mA seems plenty.

So just out of interest i wondered what current most design there leds to use in circuit and why. For those interested the photon counter video is a EEV blog one if you want to go check it out, shame they didnt use different leds though.
 

JonSea

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I think LEDs from years ago needed 10 or 20mA to be visible, and even then they are kind of dim. Efficiencies have improved so much other the years.

My pet gripe. Well, 2 really. No non-diffused LEDs for indicators. Some of them practically burn a hole through your retina and light up the room at night. And blue LEDs for indicators have to die. Rhey are always too bright and swamp out other LED colors.
 

large_ghostman

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Oops i often use blue smd leds (sorry), but at ~1mA they are pretty good, i dislike the water clear leds that are coloured!! I dont see them very well from certain angles, 5mm red leds seem to be the most dim at low mA. Working on this really low power source has been a eye opener, good practice for long lasting bat apps. The Energy micro boards are great for environmental experiments or Daqs when you can use energy harvesting boards and the very low power transceiver chips to grab the data.

I have one board thats been running off a 3 square inch solar panel (tiny one), into a harvesting board for over a year. At night it runs the board in ultra low power mode from the cap bank, then during the day once the cap bank is recharged it sends all the data when the solar cell has enough light. From now on i think i will stick with ~1-2mA for lighting leds. 18x less power is a good saving :D.

Once i manage to get pics working again, i will also adopt the clock switching and sleep modes more. In the past i have always run pics flat out with the onboard OSC, but really have often do you need a pic to run more than 1-4 meg? Not that often in reality.
 

OBW0549

Active Member
So i swapped out some the smd leds and used 3-5mm leds (red,green,blue,white and RGB) all at 4V, i got decent brightness from all of them at just under 2mA (~1.7mA average). So generally what current do you aim for?
For ordinary LEDs used as indicators, I rarely run them at more than a milliamp or two. Blue and white ones seem especially bright, and running them at any higher current they become painful to look at.

LEDs have gotten much brighter over the years. I still have some ancient (≈ 1978) red and green indicator LEDs, and at 20 mA they're barely visible in ordinary room lighting.
 

large_ghostman

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For ordinary LEDs used as indicators, I rarely run them at more than a milliamp or two. Blue and white ones seem especially bright, and running them at any higher current they become painful to look at.

LEDs have gotten much brighter over the years. I still have some ancient (≈ 1978) red and green indicator LEDs, and at 20 mA they're barely visible in ordinary room lighting.
Are these watr clear blue leds (if you see what i mean) or the smd blue, blue leds? Looks like i will be using blue for indicators from on :D. Looking way back on the forum most the advice for resistors for leds puts the current at 20-25mA, so i guess technology moved on but the 20mA thing didnt.

Even in the last 5-6 years leds have changed alot, look at the really high powered ones and the vast array of colours you now get
 

be80be

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I use 1mA the leds look good to me I still have 100 or so red ones that 10 years old there happy at 1 mA I have some blue ones that are fine a 1 mA too there maybe 3 years old I buy them in a big bag from https://www.taydaelectronics.com/ there the WATER CLEAR ULTRA BRIGHT blue ones and red ones
 

MikeMl

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I got a bag full of the ultra-bright, water-clear, dome lens type. I have used them as a "power-on" indicator in battery powered equipment, where I am using about 400uA of current. They are bright enough to see in direct sunlight.
 

be80be

Well-Known Member
I really never seen the need to run a led a 20 mA These old red leds are maybe 17 years old I got 1000 for a dollar the shipping was like 10 bucks from some where can't remember.
A whole 3 mA maybe You can see them good tho
 

RODALCO

Well-Known Member
2018-06-04_20-45-56.jpg

White Led indicator running at 1 mA of 240 Volts mains, series R is 270 k.Ohm 1 Watt.

2018-06-04_20-46-35.jpg

Classic Red led (2005) running at 1 mA @ 240Volts with also a 270 k Ohm series R.
 
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