LEDs and resistors; a simple query.

Thread Starter

LesB

Joined Oct 19, 2022
8
I'll start with an apology. I sneaked into this forum as I find it interesting. (I'm a mechanical design guy and electronics has always eluded me lol)
I'm sorting a little project where I'm making a water depth indicator with a float, magnet and linear array of reed switches. The indicator panel uses a number of individually addressed 5mm leds. It's vehicle based so 12v. The leds don't need to be madly bright or anything but I have no idea what value resistor to put in. Any suggestions would be much appreciated.
Thanks,
Les
 

panic mode

Joined Oct 10, 2011
4,866
typical indicator LED will have max allowable current of 20mA and Vf of about 2.5V
you do not want to use max current. it is much more reasonable to stick with 5-10mA.
lets say you chose 8mA

then series resistor will need to drop 12V-2.5V=9.5V
and since current is 8mA that means R=V/I = 9.5V/0.008A = 1107.5 Ohm
you can round that up or down to a nearest common values like 1k or 1,2k

note that resistor power also need to be considered. voltage in vehicle fluctuate and it may be up 14.5V

then use P=V^2/R

and as always there need to be some safety margin. if we consider Vf of the LED to be the safety margin and R=1k you get power dissipation of 0.21W so resistor of 1k, 1/4W should be fine. you may use larger power rating too (0.33W or 0.5W or even higher if you like)
 

Thread Starter

LesB

Joined Oct 19, 2022
8
typical indicator LED will have max allowable current of 20mA and Vf of about 2.5V
you do not want to use max current. it is much more reasonable to stick with 5-10mA.
lets say you chose 8mA

then series resistor will need to drop 12V-2.5V=9.5V
and since current is 8mA that means R=V/I = 9.5V/0.008A = 1107.5 Ohm
you can round that up or down to a nearest common values like 1k or 1,2k

note that resistor power also need to be considered. voltage in vehicle fluctuate and it may be up 14.5V

then use P=V^2/R

and as always there need to be some safety margin. if we consider Vf of the LED to be the safety margin and R=1k you get power dissipation of 0.21W so resistor of 1k, 1/4W should be fine. you may use larger power rating too (0.33W or 0.5W or even higher if you like)
Hi,

Many thanks for taking the trouble to reply. I'll get hold of some 1k resistors and see how it looks. I'm sure it'll do what I need. As said, they don't need to be massivly bright as long as I can see them. They are part of a water gauge for our motorhome. My intention is that it only works when I press a button on the panel so they're unlikley to be on for more than a few seconds at a time.

Thanks again, Les.
 

dl324

Joined Mar 30, 2015
18,220
The leds don't need to be madly bright or anything but I have no idea what value resistor to put in.
Get some ultrabrights and you can operate them at very low currents.

From Jameco's 2017 catalog (they don't print them anymore...):
jameco-ultrabrightLeds.jpg
 
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