I am not good with electronics so I hope to get a answer here . What resistor do I need to drop a 12 volt power supply to 3 volts to power some LED’s ? Thanks in advance .
Mike… “MY PERSONAL PREFERENCE”… when powering LED’s is a 480ohm resistor…which reminds me I’m out of them and Ric Golding owes me a batch !
Bingo !!! 480 ohm it is . Thanks Mr. Rooster
NO NO NO, the resistor is dependent on the 9 volt drop AND the current. for 20 ma leds 480 is ok for 1 20ma LED. BUT for 10ma leds the value is double that!!!
When someone needs resistor values, led currents and dropping voltages are needed for accurate information.
A better way to figure out a resistor value is:
For a 10ma led, multiply the droping voltage times 100 ohms, in this case 9 times 100 (900) and the closest resistor is 910 ohms.
For a 20 ma led multiply the dropping voltage times 50 ohms which gives 450 and closest resistor is 480 ohms. Always size the resistor at the next highest value.
These numbers are for single leds, or for leds in series. Do not assume that only one 20ma led is being used without getting further info from the original post.
Identical leds in parallel can be done, and resistor sizes wil be smaller, however this is not a good thing to do.
I have placed a pair of 10ma leds in parallel for dual headlights and used a CL2N2 for the 20 ma current with great success, no resistor needed.
When I installed LED lighting in my 6 Bachmann J&S coaches I used 5 LEDs per car in parallel and 1 480 ohm resistor. They have been working just fine since 2012 with analog DC of o-24 volts!
You could use a led resistor calculator. There are lots of them that will show up with a search. Here is one. https://ledcalculator.net/ You can always add more resistance if the led is too bright. Be very cautious about about using less, it could cause smoke.
When I first started using LED’S I to wondered about what resistor to use, so I talked with a friend who had been using them for years and his answer was put a 100 ohm resistor on everything and 99% of the time it works just fine. Now I know you electronic minded people out there will say, you got to figure it out by the electronic bible, I say here is my bible, 100 ohm resistors. I buy my LED’s on eBay in packs of 20 plus, I mainly buy 12v, but the smaller LED’S like 402 and 603 are usually in the 3-5v range, I buy them with the wires and resistors installed, I try them on 14.8V track power and sometimes I add another 100 resistor just because the brightness is still too bright. For a test I burn the LED for 24 hours on 14.8v just as a test, I don’t want to have to remove a burned out LED after it’s been installed, not that much fun. I use the KISS system on LED’s, I find it work, you do what you think needs to be done, but the 100 ohm works out for me. Joe, I see you use the 480 ohm resistor and all is working for you, as soon as I use up the 10,000 100 ohm LED’s I purchased on eBay I will try the 480’s, just kidding about 10,000, actually I purchase assortment of resistors on eBay of all different ohm’s, I think is was a 750 pack.
trainman
Another note on LEDs and resistors: It doesn’t matter electrically which leg on the LED the resistor is connected to. It will still control the current through the LED and that’s all that matters. (https://largescalecentral.com/externals/tinymce/plugins/emoticons/img/smiley-wink.gif)
this is what i love/hate about things like this. you get one answer that says they have used “X” for years , and another that insists you need to use “Y” or “W” and if you use the other you will blow up the whole railroad, burn out a LED or maybe not have a problem at all and they both work just fine. I am not saying anything bad about either person or their method of doing things, Its just a little confusing to someone like me who has only a very basic understanding od electrical things and how to not release the magic smoke. I want to swap my locomotives to LED’s and my cabooses too, but one day will try maybe both methods to see which works better. anyway this one will be saves for future reference
double post, not for post count, for thinking I didn’t hit send
mike dorsch said:
Bingo !!! 480 ohm it is . Thanks Mr. Rooster
You are welcome Mr. Mike
(https://www.largescalecentral.com/externals/tinymce/plugins/emoticons/img/smiley-wink.gif)
Pete,
BOTH methods work.(https://largescalecentral.com/externals/tinymce/plugins/emoticons/img/smiley-wink.gif)
Thanks everyone for your input . I knew I could get the answer here . I’m going to get some 480 ohm resistors . I have plenty of bright white LED’s to experiment with. I’ve been using some LED’s outside for years but I used the last resistor up this week . I got them way back from Radio Shack and lost the little sleeve they came with. My last little project I did was to put some LED’s in a 1/24th scale car and hard wired it into my railroad 12 volt power supply. Looks really great at night and the headlites face the tracks so when a train goes by in the dark the yellow reflective tapes on the side of the cars stand out . Thanks again !!!
OOPs! Accidental post here .
It is very interesting that John has all those 100 ohm leds. Did he measure those with a ruler??? he he he.
If the LED is too bright you can increase the resistor value, this will decrease the current which will dim the LED.
You can try values between 1,000 ohms to 10,000 ohms. To see any significant dimming try the 4700 ohm to 8200 ohm range.
Standard values to use: 1000, 1200, 1500, 1800, 2200, 2700, 3300, 4700, 5600, 6800, 8200, 10,000 Ohms.
Dennis
Dennis,
I’m glad to see your post. I have played with the resistor value above the minimum required many times to get the light effect that looks most realistic. Especially in steam locos where you want that incandescent somewhat soft look. Older first generation diesels will also benefit from a little higher resistance. For those that want to increase the value but don’t have the value you want you can add a second, or more in SERIES with the first one. The total resistance will be what the individual values add up to. DO NOT put them in parallel. That will reduce the total resistance.
I have used the LED Calculators on the Ngineering web site for years.
Good information on the placement of dropping resistors in different wiring circuits as well.
For single LEDs Power supply voltage minus TYPICAL voltage of LED divided by the mA of the led.
Thats how I do it. For a string then I use a LED calculator…lots around on internet.