Setting LED voltage/resistor value? - Page 1
Forum section: electronics / projects

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: August 23, 2019, 05:00:08 pm
content: So I'd like to add a small LED to indicate that the power switch is on in a very basic circuit, however I've never known how to figure out what voltage to set them at (in this case resistor value to drop 12V DC).Do I need to use my DMM in its diode test function? Will that give me the minimum voltage but not the max or ideal voltage?I know different LED colors will have different required voltages. Surely I would then need to use Ohms law to figure out what resistor value I need to drop the VCC down to.

author: ledtester
author_rank: Super Contributor
timestamp: August 23, 2019, 05:19:36 pm
content: All LEDs have voltage drops of 2 to 3 volts and will be adequately illuminated with 10 ma, so just base your initial resistor calculations on that and adjust as necessary.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 23, 2019, 05:25:08 pm
content: Some leds will light at 1.25v ... and have 20 ma current consumtion ,,,

author: MarkF
author_rank: Super Contributor
timestamp: August 23, 2019, 05:28:50 pm
content: You do not set the voltage for LEDs. You set their current.Most individual LEDs have a max current of 20mA and a forward voltage drop between 1.5V to 3V.If you want a quick and dirty calculation, just ignore the LED voltage drop and calculate a resistor based on the supply voltage and a 15mA to 20mA current. Realizing that the current will be slightly smaller due to the voltage drop across the LED.Otherwise, the resistor value = Rmin= (Vsupply- Vled) / Iled

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: August 23, 2019, 05:36:04 pm
content: Good to know thanks!What about the safe max current for LED's? Surely they must exhibit some strange behavior that I could capture on my graphical sampling DMM before they die

author: DimitriP
author_rank: Super Contributor
timestamp: August 23, 2019, 05:37:46 pm
content: Another LED Series Resistor Calculator lives here (and in a few other places):https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-led-series-resistor

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: August 23, 2019, 05:39:08 pm
content: Quote from: DimitriP on August 23, 2019, 05:37:46 pmNice to see another forum member that share the same name

author: ledtester
author_rank: Super Contributor
timestamp: August 23, 2019, 05:41:43 pm
content: 20 mA is what I limit continuous current to, but most LEDs can be pulsed at much higher currents (on the order of 100 mA for ordinary LEDs, much higher for specialty LEDs - check each device's datasheet.)An LED without a series resistor doesn't last long:

author: MarkF
author_rank: Super Contributor
timestamp: August 23, 2019, 05:43:24 pm
content: Quote from: drummerdimitri on August 23, 2019, 05:36:04 pmYou would need to check the datasheet for the particular LED. I don't know of any way to measure max current.As a rule of thumb, the max LED current is 20mA. Going over their max current will shorten their life. That is why I suggested using 15mA instead of 20mA. It's a compromise between brightness and LED life.