0
\$\begingroup\$

I'm tinkering with an Arduino with more knowledge in software than hardware & seeking clarification that I understand specifying a resistor on my project correctly?

I am using an Adafruit Feather Huzzah ESP8266 (https://cdn-learn.adafruit.com/downloads/pdf/adafruit-feather-huzzah-esp8266.pdf) microcontroller. This reportedly provides a peak output of 500mA at 3.3v. The manufacturer recommends that I keep the draw less than 250mA.

I plan on connecting an LED (multi colour) http://uk.rs-online.com/web/p/visible-leds/6194943/ to the pins, which will then have a resistor connecting the LED to earth.

This LED has a forward voltage of 2-2.5v, a DC forward current of 25-30mA & a peak forward current of 140-160mA.

On this basis, I've ended up at this site, specifying the right resistor to use: http://ledcalculator.net/

I've inserted 3.3v as the Power Supply Voltage, 2.5v as the LED Voltage Drop & 25mA as my LED Current Rating. This suggests a 33Ω resistor.

The item that I'm looking to clarify is, what would you consider to be the correct forward current to specify? Would you specify the lowest amp draw specified or the draw at spikes (i.e: 160mA)?

\$\endgroup\$
3
\$\begingroup\$

The datasheet gives the Absolute Maximum LED current of 30 mA for the red LED, and 25 mA for green. However, the "Electrical Characteristics" table shows test conditions for 20 mA. I'd plan for 20 mA or less for each LED, which would require a 40 Ohm resistor. The next standard resistor value is 43 Ohms, so I'd use that.

LEDs are not at all fussy about current, as long as it doesn't exceed the Absolute Maximum value - you might find that these LEDs are bright enough for your use with only 10 mA or less...

\$\endgroup\$
  • 1
    \$\begingroup\$ Thankyou for your answer. So of I've interpreted correctly - having a bigger resistor will give you a bit of safety? Also, the bigger resistor (will lower the brightness, but 43ohm won't in reality make much of a difference (whereas a larger say 1k resistor would)? \$\endgroup\$ – Mr T Aug 20 '17 at 1:53
  • 1
    \$\begingroup\$ @MrT: With a 1K resistor, the LEDs may be barely visible in a dark room. However, I've got some green LEDs that I had to reduce the current to less than 1 mA to get them suitably dim. \$\endgroup\$ – Peter Bennett Aug 20 '17 at 2:06
  • 1
    \$\begingroup\$ @PeterBennett -- probably GaN greens -- GaN LEDs are noted for being able to get a lot of light out of not a lot of If \$\endgroup\$ – ThreePhaseEel Aug 20 '17 at 2:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.