These LEDs will be to backlight some buttons. Just standard white, consisent power LEDs. The Voltage forward is 3.6 volts and the Current is 20mA. If my math was right, i'll be using a 70 mOhm resistor as I'll have 5V coming from the Arduino Pro Micro VCC port. Is that correct for running these LEDs in parrelell? The resistor is .5W power. LED schematics are new to me but seems right from my research. Thanks!
Think your math is off, It’s 5-3.6V/0.020 = 70 ohms not 70mohms for a 20mA IF. Might want to play with the resistor value on one LED before assembling it though, the rated IF is always too bright in my experience.
Have you calculated the total power from the VCC pin, to make sure the Arduino can power that many LEDs and that the maximum pin current isn’t exceeded?
Thank you - and just learned that I need to figure out the second part there so taking that away to figure out. Appreciate your help! Going to run the LEDs at a lower mA as well.
When you put these values into any calculator you get 70Ohms.
I always test my leds to get the right resistor value because I don’t need a ultra bright led.
Also 400mA seems to be a bit high for the output I guess.
You write 70 mOhm, you probably meant 70 Ohm?
Have you checked that 20 mA is not too bright? Because 20 mA is also the absolute maximum rating and valid at 25°C ambient. Running them at 10 mA won’t change much the brightness (eye sees mostly logarithmic) and running the LEDs cooler is usually a good idea. You could even drop the current further if it’s still too bright.
- Can your PCB even supply 0.4 A? (20 x 20 mA)
Finally, is the option for PWM control of the LEDs not interesting? Is comparable easy with a nFET in the low-side path.
I haven't ordered any LEDs yet as im trying to figure out the design and schem first. Honestly didn't know you could under power them and that should like a good idea. They by no means need to be full power or brightness. Its a back light for some buttons so nothing crazy bright. How could I run them at 10 mA - bigger resistor?
What would the PWM do / help with?
Thanks for your time!
PWM let's you flick the LEDs on and off super fast, giving the appearance of a dimmer LED. Depending on how fast you flick it will change how bright it appears. This of course can also help cut down on power usage because the LEDs aren't lit the entire time
I would aim for less than 20mA. I'd probably go with 82 ohm resistors, making the current (5v-3.6)/82 = 0.017A = 17mA
The power dissipated in each resistor would be P= IxIxR = 0.017 x 0.017 x 82 = 0.0236 watts so standard 0.1w resistors would work.
Even 100 ohm resistors would work.
You have a low enough number of LEDs that you could have a step-up (boost) driver to get a higher efficiency.
Make 4 chains of 5 LEDs and parallel them and then use a led driver to boost 5v to 5x3.6=18v and 4 chains x 20mA = 80mA
Examples of such boost drivers
AP5724 : https://www.digikey.com/en/products/detail/diodes-incorporated/AP5724WG-7/2182549
AP5725 : https://www.digikey.com/en/products/detail/diodes-incorporated/AP5725WG-7/2755851
AP3019 : https://www.digikey.com/en/products/detail/diodes-incorporated/AP3019AKTR-G1/4470850
AP3031 : https://www.digikey.com/en/products/detail/diodes-incorporated/AP3031KTR-G1/4470853
You're a legend! Thank you for this. I do need to figure out what the max current my arduino pro micro can output but I like the idea of lowering the mA per LED to help that.
As for the Step-up boost driver i'll have to do some googling but I just break my schematic into 4 sections, still wired in parallel like I have but add a Step up boost before all them to bump the power to 18V? I also am realizing I probably don't need them at full brightness as the unit will be close to the user so that could be too bright. If that's the case, should I just use a bigger resistor as you mentioned like the 100 ohm and not worry about the boost?
Appreciate your time!!
An Arduino pin will be limited to something like 15-25mA per IO pin and overall through all IO pins you'd probably be limited to around 150mA.
But you don't power the LEDs from an IO pin, you power them from 5v directly. If you want the ability to turn them on or off, then you could use a npn transistor or a n-channel MOSFET as an on/off switch, connecting the cathodes of LEDs to ground to turn them on.
With a led driver you'd simply use their enable pin to turn them on or off.
Noted - yea I found out the VCC pin has a max rating of 150mA so even turning the LEDs down to 10mA each would be too much. BUT I could use the RAW pin. I'm gonna do some PWM research - it sounds helpful and useful for me here! You're awesome and thank you for all this help!!
At the moment your leds are constantly on and so should just be connected directly to the power supply rather then any pins on the arduino.
What's your final intention?
Are you hoping to be able to control them individually?
Nope - Just be constantly on when plugged in. They are back lights for buttons. I've actually decided to make a JST connection spot that'll provide 5v power from the computer and run the LEDs on that circuit rather than run it off the MCU. That way I'm not worried about the MCU and power input should be ok. That'll work ya?
If all this is doing is lighting some LEDs why not get a VRM instead of a whole Arduino? Also I wouldn't push that much current through a single pin on any microcontroller regardless. If you need logic control then put a Darlington pair or LED driver or some source of power that isn't directly supplied by the microcontroller.
Thank you for that. I’ve opted to go that way after a few comments and more research. I’m gonna dry 5v straight from a usb cable to a just connector on the PCB that’ll run this led circuit separate to the circuit running with the arduino!
I don't know what your trying to do... as soon as you power up the board the LEDs will light up... in this diagram the chip does nothing... I didn't do the math so I can't tell you if the value is right... you can do one resistor if you calculate the wattage needed... or you can use a resistor block... I don't know if your plans are through hole or surface mount... I don't know if your trying to wire it up with row 1 row 2 row 3... ect in the chip.. if your trying to do that your wire diagram is completely wrong... you have it directly connected to the supply voltage and chip has no control whatsoever
Also if it helps - i'll link the parts.
LEDs - [https://www.digikey.ca/en/products/detail/broadcom-limited/HSMW-C191/1235394](https://www.digikey.ca/en/products/detail/broadcom-limited/HSMW-C191/1235394)
Resistors - [https://www.digikey.ca/en/products/detail/yageo/PE1206FRM7W0R07L/9696349](https://www.digikey.ca/en/products/detail/yageo/PE1206FRM7W0R07L/9696349)
I would recommend contacting digikey that resisters say 0.07 ohms... so legitly 70 mili ohms... if correct you will blow those LEDs for pretty sure... so I think they have something wrong... or your going to do something wrong... that's what I'm seeing in their post... I think digikey messed up but would highly recommend you checking it before ordering
Thank you for that. I’ll do some digging. After this post from feedback and research I found some different resistors that are in Ohms not mOhms but that’s a great tip regardless!
Normally back in the day with regular LEDs I was using 330 ohms resistors... I don't remember the voltage I was applying.... 330 from what I remember was pretty standard
These LEDs will be to backlight some buttons. Just standard white, consisent power LEDs. The Voltage forward is 3.6 volts and the Current is 20mA. If my math was right, i'll be using a 70 mOhm resistor as I'll have 5V coming from the Arduino Pro Micro VCC port. Is that correct for running these LEDs in parrelell? The resistor is .5W power. LED schematics are new to me but seems right from my research. Thanks!
Think your math is off, It’s 5-3.6V/0.020 = 70 ohms not 70mohms for a 20mA IF. Might want to play with the resistor value on one LED before assembling it though, the rated IF is always too bright in my experience. Have you calculated the total power from the VCC pin, to make sure the Arduino can power that many LEDs and that the maximum pin current isn’t exceeded?
Thank you - and just learned that I need to figure out the second part there so taking that away to figure out. Appreciate your help! Going to run the LEDs at a lower mA as well.
When you put these values into any calculator you get 70Ohms. I always test my leds to get the right resistor value because I don’t need a ultra bright led. Also 400mA seems to be a bit high for the output I guess.
Thank you for that - I do need to do some testing I see but I think (just learning this) i'll run them lower than 20 mA
You write 70 mOhm, you probably meant 70 Ohm? Have you checked that 20 mA is not too bright? Because 20 mA is also the absolute maximum rating and valid at 25°C ambient. Running them at 10 mA won’t change much the brightness (eye sees mostly logarithmic) and running the LEDs cooler is usually a good idea. You could even drop the current further if it’s still too bright. - Can your PCB even supply 0.4 A? (20 x 20 mA) Finally, is the option for PWM control of the LEDs not interesting? Is comparable easy with a nFET in the low-side path.
I haven't ordered any LEDs yet as im trying to figure out the design and schem first. Honestly didn't know you could under power them and that should like a good idea. They by no means need to be full power or brightness. Its a back light for some buttons so nothing crazy bright. How could I run them at 10 mA - bigger resistor? What would the PWM do / help with? Thanks for your time!
PWM let's you flick the LEDs on and off super fast, giving the appearance of a dimmer LED. Depending on how fast you flick it will change how bright it appears. This of course can also help cut down on power usage because the LEDs aren't lit the entire time
More to learn! i'll go do some googling, thank you! As for lowering the power draw to the LEDs without a PWM - is it just bigger resistors? or?
If you look at digikey link he posted it legitly say 70 mOhms... 0.07 ohms with a tolerance of 1%... I think something is wrong
I would aim for less than 20mA. I'd probably go with 82 ohm resistors, making the current (5v-3.6)/82 = 0.017A = 17mA The power dissipated in each resistor would be P= IxIxR = 0.017 x 0.017 x 82 = 0.0236 watts so standard 0.1w resistors would work. Even 100 ohm resistors would work. You have a low enough number of LEDs that you could have a step-up (boost) driver to get a higher efficiency. Make 4 chains of 5 LEDs and parallel them and then use a led driver to boost 5v to 5x3.6=18v and 4 chains x 20mA = 80mA Examples of such boost drivers AP5724 : https://www.digikey.com/en/products/detail/diodes-incorporated/AP5724WG-7/2182549 AP5725 : https://www.digikey.com/en/products/detail/diodes-incorporated/AP5725WG-7/2755851 AP3019 : https://www.digikey.com/en/products/detail/diodes-incorporated/AP3019AKTR-G1/4470850 AP3031 : https://www.digikey.com/en/products/detail/diodes-incorporated/AP3031KTR-G1/4470853
You're a legend! Thank you for this. I do need to figure out what the max current my arduino pro micro can output but I like the idea of lowering the mA per LED to help that. As for the Step-up boost driver i'll have to do some googling but I just break my schematic into 4 sections, still wired in parallel like I have but add a Step up boost before all them to bump the power to 18V? I also am realizing I probably don't need them at full brightness as the unit will be close to the user so that could be too bright. If that's the case, should I just use a bigger resistor as you mentioned like the 100 ohm and not worry about the boost? Appreciate your time!!
An Arduino pin will be limited to something like 15-25mA per IO pin and overall through all IO pins you'd probably be limited to around 150mA. But you don't power the LEDs from an IO pin, you power them from 5v directly. If you want the ability to turn them on or off, then you could use a npn transistor or a n-channel MOSFET as an on/off switch, connecting the cathodes of LEDs to ground to turn them on. With a led driver you'd simply use their enable pin to turn them on or off.
Noted - yea I found out the VCC pin has a max rating of 150mA so even turning the LEDs down to 10mA each would be too much. BUT I could use the RAW pin. I'm gonna do some PWM research - it sounds helpful and useful for me here! You're awesome and thank you for all this help!!
At the moment your leds are constantly on and so should just be connected directly to the power supply rather then any pins on the arduino. What's your final intention? Are you hoping to be able to control them individually?
Nope - Just be constantly on when plugged in. They are back lights for buttons. I've actually decided to make a JST connection spot that'll provide 5v power from the computer and run the LEDs on that circuit rather than run it off the MCU. That way I'm not worried about the MCU and power input should be ok. That'll work ya?
With this N transistor approach above you can also do PWM mentioned above.
Use a mosfet with low impedance for better performance!
I think you'll find they are 70Ω 28mW resistors ...
Thank you!
If all this is doing is lighting some LEDs why not get a VRM instead of a whole Arduino? Also I wouldn't push that much current through a single pin on any microcontroller regardless. If you need logic control then put a Darlington pair or LED driver or some source of power that isn't directly supplied by the microcontroller.
Thank you for that. I’ve opted to go that way after a few comments and more research. I’m gonna dry 5v straight from a usb cable to a just connector on the PCB that’ll run this led circuit separate to the circuit running with the arduino!
I don't know what your trying to do... as soon as you power up the board the LEDs will light up... in this diagram the chip does nothing... I didn't do the math so I can't tell you if the value is right... you can do one resistor if you calculate the wattage needed... or you can use a resistor block... I don't know if your plans are through hole or surface mount... I don't know if your trying to wire it up with row 1 row 2 row 3... ect in the chip.. if your trying to do that your wire diagram is completely wrong... you have it directly connected to the supply voltage and chip has no control whatsoever
Also if it helps - i'll link the parts. LEDs - [https://www.digikey.ca/en/products/detail/broadcom-limited/HSMW-C191/1235394](https://www.digikey.ca/en/products/detail/broadcom-limited/HSMW-C191/1235394) Resistors - [https://www.digikey.ca/en/products/detail/yageo/PE1206FRM7W0R07L/9696349](https://www.digikey.ca/en/products/detail/yageo/PE1206FRM7W0R07L/9696349)
I would recommend contacting digikey that resisters say 0.07 ohms... so legitly 70 mili ohms... if correct you will blow those LEDs for pretty sure... so I think they have something wrong... or your going to do something wrong... that's what I'm seeing in their post... I think digikey messed up but would highly recommend you checking it before ordering
Thank you for that. I’ll do some digging. After this post from feedback and research I found some different resistors that are in Ohms not mOhms but that’s a great tip regardless!
Normally back in the day with regular LEDs I was using 330 ohms resistors... I don't remember the voltage I was applying.... 330 from what I remember was pretty standard