Comments on: Drive LED or Opto-isolators with AVR http://www.scienceprog.com/drive-led-or-opto-isolators-with-avr/ ScienceProg serves scientific, embedded, biomedical engineering, physics based contents. If you want your article or scientific project to be on this site, do not hesitate to contact. Wed, 08 Sep 2010 00:13:56 +0000 hourly 1 http://wordpress.org/?v=3.0.1 By: scienceprog http://www.scienceprog.com/drive-led-or-opto-isolators-with-avr/comment-page-1/#comment-28538 scienceprog Thu, 21 Jan 2010 10:32:55 +0000 http://www.scienceprog.com/drive-led-or-opto-isolators-with-avr/#comment-28538 Yep you are right. For smaller supply voltages is more visible, but for higher this can be neglected to simplify calculations and do it faster. But if keep in mind that it is better to chose at least 50% higher resistor values to preserve LEDs lifetime, such calculation can be accepted. Any way thanks for pointing that out. Yep you are right. For smaller supply voltages is more visible, but for higher this can be neglected to simplify calculations and do it faster. But if keep in mind that it is better to chose at least 50% higher resistor values to preserve LEDs lifetime, such calculation can be accepted. Any way thanks for pointing that out.

]]> By: bootstrap http://www.scienceprog.com/drive-led-or-opto-isolators-with-avr/comment-page-1/#comment-28535 bootstrap Thu, 21 Jan 2010 03:34:48 +0000 http://www.scienceprog.com/drive-led-or-opto-isolators-with-avr/#comment-28535 Your calculation of the resistance does not take into account the voltage drop of the LED. The voltage you divide by the current is the supply LESS the LED voltage drop. eg Blue LED drop is ~3V so 5V-3V = 2V. 2V/20ma = 100 ohm. Red LED drop is ~1.8V so 3.2V/20ma = 160 ohm. Green ~2.2V drop (140 ohm). These are the MINIMUM resistance values since - as you correctly stated - the maximum sink current of an AVR is 20 ma per pin. Also, if you are advising people to interface silicon components to relays, insist they use a freewheeling diode across the coil to prevent back EMF spikes damaging components. Idealy this should be a 'fast' diode, but that is getting picky. Otherwise good stuff dude. Your calculation of the resistance does not take into account the voltage drop of the LED. The voltage you divide by the current is the supply LESS the LED voltage drop. eg Blue LED drop is ~3V so 5V-3V = 2V. 2V/20ma = 100 ohm. Red LED drop is ~1.8V so 3.2V/20ma = 160 ohm. Green ~2.2V drop (140 ohm). These are the MINIMUM resistance values since – as you correctly stated – the maximum sink current of an AVR is 20 ma per pin.

Also, if you are advising people to interface silicon components to relays, insist they use a freewheeling diode across the coil to prevent back EMF spikes damaging components. Idealy this should be a ‘fast’ diode, but that is getting picky.

Otherwise good stuff dude.

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