RMweb Premium kevinlms Posted March 25, 2020 RMweb Premium Share Posted March 25, 2020 30 minutes ago, DavidCBroad said: You need two, or one for each LED. Exactly, but my reply was to Quarryscapes who advocated a voltage regulator to eliminate resistors. Don't know why anyone would go to the trouble of eliminating resistors, when they are the cheapest product, in the electronics industry. 1 Link to post Share on other sites More sharing options...
AndyID Posted March 25, 2020 Share Posted March 25, 2020 46 minutes ago, DavidCBroad said: You need two, or one for each LED. Looking up the resistor calculator I find that a typical white LED needs a 390 ohm resistor for 12 volts the forward current is 20ma and the resistor power dissipation is 144mw, well within a 1/4 watt resistor. Increase the supply voltage by 50% to 18 volts, not unusual, and keep the 390 ohm resistor the figures are forward current 40ma, and resistor dissipation 467mw. That is a 100% and 300% + increase respectively from a 50% increase in voltage. Its enough to severely shorten the life of the LED and make a 1/4 watt resistor very hot indeed. My latest LED set up has a 56 ohm resistor, 5 volt supply, forward current 20ma, and resistor power dissipation 56mw. Something is slightly off. 20mA through 390 ohms dissipates 156mW. Assuming it really is 20mA the correct resistor for 20mA from an 18 volt supply is 690 ohms. Obviously it's a good idea to know what the actual power supply voltage is. It doesn't need to be regulated. All you need to do is measure it with a multi-meter and there really is no excuse for not owning one considering how inexpensive they are. 1 Link to post Share on other sites More sharing options...
RAF96 Posted March 25, 2020 Share Posted March 25, 2020 There is no reason to run an led at its max spec values. Most will run at half that value. I use a standard 1k resistor on all my leds colour legs with an extra one in the main common power feed if they are a bit too bright. I normally use common anode 3-legged bi-colour leds for DCC purposes, hence the extra dumb down resistor where I have them fed from track voltage. Link to post Share on other sites More sharing options...
Junctionmad Posted March 25, 2020 Share Posted March 25, 2020 (edited) Unless you are looking at high power white leds , most leds these days are happy at 2-3 mA , for standard 3mm red , green , yellow I use 1K8 at 5V Edited March 25, 2020 by Junctionmad 2 Link to post Share on other sites More sharing options...
Crosland Posted March 25, 2020 Share Posted March 25, 2020 On 24/03/2020 at 08:47, Quarryscapes said: You could look into using an LM317 regulator IC to provide a dedicated lighting feed and remove the need for resistors altogether (except the voltage setting ones for the IC of course) Only if you are using the LM317 as a constant current source. Otherwise, you should ALWAYS be using resistors. 2 Link to post Share on other sites More sharing options...
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