First, lets assume that I know nothing. Thats the easy part. I’ve got some run of the mill Christmas LEDs to light up some buildings. I’ve got a 12 volt 800 milliamp wall wart to power everything. From what I understand each LED is approx. 20 milliamps and 3.2 volts. Sound good so far? OK. Here comes the questions. If I wire several LEDs in SERIES what does that do to the amount of milliamps that I draw? i.e.: if I place two in SERIES do I draw twice as many milliamps- 40 milliamps, 3 in SERIES 60 milliamps, etc. or is there some electrical law that I’m missing. All information is greatly appreciated. Oh, and yes, I will place a resistor between the power supply and the LEDs. Thanks for your help.

-Kevin.

Kevin,

For LEDs in series, the current remains the same. 3 LEDs @ 3.2 volts each; 20 ma of current; you need a 110 ohm 1/4 watt resistor for 12 volts.

Bob

Sorry Kevin,

Make that a 120 ohm resistor. See my post at

http://largescalecentral.com/forums/topic/20586/making-receptacles-for-led-light

Bob

Love Bob Hymans idea on his link on his building very neat idea…

Here is a simple link to maybe just put in your Favorites for fast calks.
http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator

When you go to smaller LED’s like a Traix’s LED’s or larger Jumbos LED’s, just ck the package on the MA draw and you will be safe using this Calculator link. Hope this some added help.

If you use LED drivers, you don’t have to calculate resistor values. The driver is a current limiting device, and won’t let more than the necessary current pass. And they don’t seem to care about how many LEDs are in the circuit.

I wire mine in parallel, and have had up to 20 in a circuit, powered by a single LED driver power supply.

LEDS in parallel is not a good idea. You can not adjust the current properly, each draws what it wants and one could short and cause a fire or melted plastic which I would not want on any of my buildings or rolling stock.

Series is the only way to go. for series operation 20 ma means you need 50 ohms for every volt dropped, 10 ma is 100 ohms for every volt dropped, no special calculator needed.

So 3 3.2 leds gives 9.6 volts and 3 volts dropped from 12.6 volts (lead acid battery voltage) gives me 3 times 50 ohms for the resistor with 20ma bulbs.

Thanks for the info. I love this place! So let me get this straight. As long as I wire the LEDs in Series for each building the amount of milliamps I draw remains the same- 20 milliamps- no matter how many LEDs I use. So, each building can have 5 LEDs in it and the total draw from each individual building is still only 20 milliamps? Which would mean that my 800 milliamp wall wart should power an entire city? Sounds too easy…. I’m sure I’m missing something…… Your thoughts?

-Kevin.

Kevin,

Not quite. With a 12 volt supply, you are limited to three LEDs in the circuit. Like I explained above, three LEDs plus one 120 ohm resistor. The three LEDs take 3.2 volts each for a total of 9.6 volts. The 120 ohm resistor drops the remaining 2.4 volts. If you look again at my post about the LED receptacles, you will see that I have 15 LEDs. These are arranged in a series-parallel circuit, with five identical 3LED + 1 resistor legs. Each leg draws 20 mA, so the entire circuit draws 100 mA. With a 800 mA wall wart, you are theoretically limited to 40 legs with 3 LEDs and a resistor in each leg.

Bob

Bob- You are a patient man. And I’ve got a head full of rocks! Thanks for explaining this (again) to me. Now its making some sense. Electronic things were never one of my strong points. I hate to beat this subject to death but……

If I were to use more than 3 LEDs would they just be dimmer or would they not light at all?

If I used a larger, say 18 volt supply, I could then wire more LEDs per circuit. I would then need a larger resistor as well.

The beatings will continue………

-Kevin.

Kevin,

Not a problem with the questions … that’s how we all learn.

First answer - more than three LEDs on a 12 volt supply - if you put 4 LEDs in series and skipped the resistor, then each LED would drop 3 volts. They should illuminate at slightly less than full brightness. If you tried more than four, say five for example, then each LED would drop 2.4 volts, and that might not be enough to turn them on. My guess is they would illuminate, but be fairly dim.

Second answer - larger supply voltage - the answer is yes. If you used an 18 volt supply, you could put five LEDs in series with a 100 Ohm 1/4 Watt resistor. The circuit would draw 20 mA from the supply. The five LEDs would drop 16 volts and the resistor would drop the remaining 2 volts.

Here is the link to a neat wizard for calculating LED circuits. Just enter how many LEDs, the supply voltage, and the standard LED parameters (20 mA, 3.2 volts typical), and the wizard will design your circuit.

http://led.linear1.org/led.wiz

Bob

Bob- Thanks again!!! After asking the questions and re-reading the answers a couple of times, something in my rusted brain realized that no matter how many milliamps I draw, its the VOLTAGE thats important. LEDs run on VOLTAGE. Sometimes I need a good couple of whacks in the head with a tire tool to get the information to stick! Its all becoming quite clear now. Many thanks!

-Kevin.

Kevin,

I didn’t mean to give you the wrong impression that VOLTAGE is the determining factor for LED circuits, although it does determine how many LEDs can be in each leg of the circuit. It is more useful to think of driving a LED with a specific current, instead of voltage. It is always best to select the appropriate current-limiting resistor to achieve a target 20 mA current in the LED. Specs and performance curves vary for LEDs, but 20 mA and 3.2 Volts are good defaults. That is why the wizard link is so useful; once you enter the specs and the voltage, it does all the calculations and design for you.

Bob

I buy my LED drivers from LSC member Dave Bodnar.

I simply tell him how many LED’s I’m using for the project and its done.

My eyes glaze over when I try to configure resistors, diodes, etc. I let the Pro do the figgerin’.

No problem, Bob. Some of this is sinking in, slowly but surely. I’ll probably make some rookie mistakes, but I’ve gotta start somewhere and this site is full of helpful people, like you, to keep me from doing anything real dumb. I’ve been researching this whole LED wiring mystery for a while and it does get a bit confusing from time to time. I’ve been reading another topic in this forum-

LED light help!

Where they were discussing some of the strip LED lights (not the same design as my Christmas string but same principal, I’m guessing). At first it was confusing me because one poster said he lit the entire strip with a single 9 volt battery (to test the LEDs). And yet, the strips could be cut into 3 light sections and still work on the same 9 volt battery. Something made me think that the voltage then, was not that important (I know better, now). Naturally, I thought I just needed enough milliamps and I could string together as many LEDs as I needed (boot to the head). Not so. After reading farther, I realized that each 3 light set was wired in series with a resistor and then each of those sets were wired together in parallel to form 1 long strip. So, each 3 light/resistor set receives the same voltage. We both know that I don’t do math, but wiring each 3 light/resistor set in parallel seems like that would solve the dilemma. All sets get equal voltage and each set draws a fraction of the total milliamps from the battery. Sound pretty close?

Hopefully, some of my rookie questions/mindless bantering and your expert answers will help out other electronically challenged large scalers, as well. If nothing else, I’ll demonstrate what not to do.

OK. I’m ready to teach rocket science at the collegiate level, now.

-Kevin.

Kevin -there are a number of articles on my TrainElectronics.com web site that deal with LEDs.

Have a look at

LED Lights for Coaches

and

LED Related Articles:

• A Simple Constant Brightness LED
• LEDs 101 - Introduction & Overview
• LEDs 102 - Using them Onboard Trains
• LEDs 103 - Turning them On and Off
• LEDs 104 - A Better Constant Brightness Circuit for LEDs

Note that the first article has up to 25 LEDs wired in parallel - I have had good luck doing this so long as the LEDs all come from the same batch. If you mix LEDs it is not a good idea to wire them in parallel unless you add a current limiting resistor to each one.

Note that LEDs 101 talks about wiring LEDs in series and parallel.

Please let me know if you have any questions.

dave

Usually, I wire my LEDs in parallel, so that when one goes out, they don’t all go out. I do use an LED driver for most applications, as it controls current, which, I think, is the most important part of LED circuitry.

As I understand them, LEDs will draw as much voltage as they need to burn out, and they don’t have much range. But the LED driver limits current draw, and you can have lots of LEDs wired in parallel.

In my passenger coaches, I’ve got four or five LEDs in each coach (wired in parallel). I’ve got five coaches and a diner, so there’s 24 - 30 LEDs in a parallel string. My LED driver circuit lights them all from track power.

With the help of Dave Bodnar, his articles, and some other people who chimed in from this board, I got some printed circuit boards and have built perhaps half a dozen of these circuits. They work fine. I’ve had one “testing” 24 hours a day, seven days a week for almost a year, with six LEDs. No problems yet.

If you’d like to see the circuit (or the board), reply off line. Don’t want to hijack this thread!

Dave- Thanks. I’ll check out your site for more help. Sounds like you’ve got quite a bit of info there.

Dick- Thanks for the info. Much appreciated.

-Kevin.

Be careful with the 3.2 bolts mentioned for LEDS as this is not always true.

There are many leds that are less than 3.2 volts to light up and you will find colored leds have different voltages for each color.

Always check the ratings and I test the leds for voltage drop if unknown.

Get your hands on a meter. Test the output voltage of your walwart first!
Some unregulated ones put out different voltages than they claim. I’m not sure what to tell you except that you should test for voltage.
I have a couple wall warts that vary by more than 5 volts each. They probably change with a bigger load too. I don’t think a couple of LEDs is a big load???

I updated the led page at http://barefootelectronics.com/led001.aspx to include a section on using multiple LEDs.