Model Railway Forum

Hi, I will be fitting quite a few red/green bi-led’s & wondered the best resistor to use.

Green led = sv 20v / fv 3v / FC 25ma / = 566 ohm resistor
Red led = sv 20v / fv 2v / fc 30ma / = 720 ohm resistor

750 ohm 1/8W Carbon Film Resistors is the nearest i can seem to get. Will this be on the safe side & not dim the green ones to much ??
Thanks

Hi
Use this calculator and it gives the minimum resistance needed. Increasing the resistance a little will not cause any noticeable losing in LED brightness and will reduce current consumed. http://www.hebeiltd.com.cn/calculator/v5/led.php Try to reduce the forward current you don't want to run them at their rated maximum
eBay is always a good source for resistors.

Brian wrote: ↑Thu Dec 09, 2021 7:29 am Hi
Use this calculator and it gives the minimum resistance needed. Increasing the resistance a little will not cause any noticeable losing in LED brightness and will reduce current consumed. http://www.hebeiltd.com.cn/calculator/v5/led.php Try to reduce the forward current you don't want to run them at their rated maximum
eBay is always a good source for resistors.

Thanks Brian. What a handy calculator, gives that bit extra info.

I would be looking at using 10millamp as the highest current.
So with a 20 volt DC supply. 3v or 2v forward and 10ma forward, the minimum value of resistor is 1K8 (1800 Ohm) at 1/2 (0.5) watt rating. These are the minimum ratings, you can go higher ohms.

Examples...
Typical 1K8 0.5watt - 20 for £1.60 (£0.8p each) ... https://www.ebay.co.uk/itm/143346195479 ... SwqLFdRaAY

Or

2K2 0.5Watt - 25 for £1.99 (just on £0.8p each) https://www.ebay.co.uk/itm/261964679700 ... SwiHFdVTTB

Thanks Brian, I think i get confused with some calculators giving different read outs with the same input. Got a feeling some might put in a % safety margin. Will try your recommend resistor's.