Search for a LED option to work with 5v and under 4mA

Thread Starter

cawensel

Joined Nov 4, 2023
2
Hello,

I'm trying to find an extremely bright LED option for a replica prop I'm working on. The PCB is completed and the connector for this light provides 5V and has a 100Ω resistor connected to it. It also warns of not to draw more than 40mA from this connector. I'm looking to find something super bright white that will work in place of just a standard 5mm white flat top LED. Any help is appreciated.
 

SamR

Joined Mar 19, 2019
5,019
Depends a lot on ambient light. The 5mm super-bright round tops I use typically are used below 20mA and even at 10mA are plenty bright unless in a very brightly lit environment. So 5V/10mA=500Ω or so. I typically use a 560Ω resistor and even a 1kΩ give good indoor indication.
 

MrChips

Joined Oct 2, 2009
30,448
You can either change the 100Ω resistor or you can add another resistor in series.
150Ω total resistance will give about 20mA.
Experiment with resistances from 100Ω to 1000Ω and select the one that best suits your taste.
 

eetech00

Joined Jun 8, 2013
3,820
Depends a lot on ambient light. The 5mm super-bright round tops I use typically are used below 20mA and even at 10mA are plenty bright unless in a very brightly lit environment. So 5V/10mA=500Ω or so. I typically use a 560Ω resistor and even a 1kΩ give good indoor indication.
Hi SamR.
Don't forget to account for the Vf drop of the LED...
 

dl324

Joined Mar 30, 2015
16,665

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Tonyr1084

Joined Sep 24, 2015
7,766
Welcome to AAC.

An LED works on current. SuperBright (SB) LED's I have in white have a forward voltage drop of 2.94 average. I have 100 of them and tested them at 20mA. They vary in Vf (forward voltage) from a minimum of 2.92 to 3.01 max, with a standard deviation of 30mV.

You calculate the current by taking the starting voltage then subtracting the Vf to come up with a final voltage. That final voltage needs to be dropped via some sort of current limiting device. Most often it's a standard resistor. You select the resistor that will give you the amount of current you require for your LED. 40mA is quite a bit of current. With a SB LED you're going to see quite the blinding light. Even at 20mA they're still painfully bright. I've seen them operate as low as 5mA. That's not to say 5mA is their minimum, that's just what I've seen.

So with my SB LED's at 5mA the math works out like this:
(5V - 2.94Vf) ÷ 0.005A (that's 5mA) = 412Ω of resistance. That will limit the current to 5mA. But as the LED heats up the current draw will start to climb. As it climbs it can get even hotter. As it gets hotter, current continues to climb. This is a runaway thermal failure. But you're starting with 5mA (that is to say MY example is) the likelihood of a thermal runaway condition is unlikely. At 40mA ? ? ? Yeah, you're talking about a short life LED. One that won't live long. It will begin to burn out and in a short order of time it will completely fail. Seconds to minutes, possibly hours or even days. But not likely to live for weeks or months let alone years. Running it at 20mA will extend its lifespan to somewhere around its rated lifespan of possibly 10,000 hours continuous. At 10mA - even longer. A lot longer. And with SB, you won't be needing a whole lot of brightness.
 

Thread Starter

cawensel

Joined Nov 4, 2023
2
Tonyr1084,
Where did you get your SB LED's from?

Welcome to AAC.

An LED works on current. SuperBright (SB) LED's I have in white have a forward voltage drop of 2.94 average. I have 100 of them and tested them at 20mA. They vary in Vf (forward voltage) from a minimum of 2.92 to 3.01 max, with a standard deviation of 30mV.

You calculate the current by taking the starting voltage then subtracting the Vf to come up with a final voltage. That final voltage needs to be dropped via some sort of current limiting device. Most often it's a standard resistor. You select the resistor that will give you the amount of current you require for your LED. 40mA is quite a bit of current. With a SB LED you're going to see quite the blinding light. Even at 20mA they're still painfully bright. I've seen them operate as low as 5mA. That's not to say 5mA is their minimum, that's just what I've seen.

So with my SB LED's at 5mA the math works out like this:
(5V - 2.94Vf) ÷ 0.005A (that's 5mA) = 412Ω of resistance. That will limit the current to 5mA. But as the LED heats up the current draw will start to climb. As it climbs it can get even hotter. As it gets hotter, current continues to climb. This is a runaway thermal failure. But you're starting with 5mA (that is to say MY example is) the likelihood of a thermal runaway condition is unlikely. At 40mA ? ? ? Yeah, you're talking about a short life LED. One that won't live long. It will begin to burn out and in a short order of time it will completely fail. Seconds to minutes, possibly hours or even days. But not likely to live for weeks or months let alone years. Running it at 20mA will extend its lifespan to somewhere around its rated lifespan of possibly 10,000 hours continuous. At 10mA - even longer. A lot longer. And with SB, you won't be needing a whole lot of brightness.
 

Audioguru again

Joined Oct 21, 2019
6,604
SuperBright LEDs are bright because their package focusses the light into a very narrow beam.
Maybe you need a wide light beam then it looks dimmer.
 

Tonyr1084

Joined Sep 24, 2015
7,766
Where did you get your SB LED's from?
Amazon.
SuperBright LEDs are bright because their package focusses the light into a very narrow beam.
Yes and no. Here's a video I did using Red, Green and Blue LED's, all SB's. The point in the video focuses on matching forward voltages and use of a single resistor to limit current to the stack of LED's. But you can see that their brightness doesn't specifically depend on being within a narrow beam path.
 

Audioguru again

Joined Oct 21, 2019
6,604
Of course the green LED is brighter. Our vision is much more sensitive to green and the red chemistry is very old and the new green chemistry is very new.
Here is a graph from the datasheet of the very bright (on axis) LED's directivity of only 15 degrees.
Some LEDs have a wide directivity of 120 degrees and are not as bright when on axis.
 

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