http://www.ab-tools.com/en/software/resistorcalculator/
Colour codes and series or parallel setup values.
Uh oh. Now I’ve gone and done it. I think I just tore the fabric of the universe. I may have even screwed up that whole time and space continuum thing. Why, you ask?
Well, if you’ve been reading this thread so far, then you already know about my foray into LEDs. Here’s what happened today. I’m using my run of the mill 3.2 volt warm white Christmas LEDs (Martha Stewart brand, to be exact). I connected them AND a 470 ohm resistor in series to a 12 volt 500 milliamp wall wart. Now, we all know that I know nothing about any of this, but the theory is that I should only be able to connect 3 LED’s in series and have them light, right? Riddle me this, Batman……
http://freightsheds.largescalecentral.com/users/dieseldude/_forumfiles/IMG_4480.JPG
Yep. You counted right- 7 LEDs. No photoshopping this picture. No tricks up my sleeve. At no point during this experiment did my fingers leave my hand. So, why did this happen? Could the LEDs actually be less than 3.2 volts??? While they did dim a bit as I added the 7th LED, they were pretty bright until then. Heck, I would have tried for 8, but I ran out of alligator clips.
Just out of curiosity, how many milliamps am I now drawing with 7 LEDs in series?
Your thoughts?
-Kevin.
That’s weird…… My photo showed up as a link to my freight shed, instead of a photo in my post. Never happened before. Oh, well. Follow the link to the photo. There really are 7 lit LEDs. I swear.
-Kevin.
LEDS will light up on less than the rated voltage.
Rated voltage is the max allowed for getting the rated life spec.
lower voltages will increase the lifetime but reduce the amount of light output.
So, 7 in series on 12 volts would greatly lower the current. These will light up, but not at full brightness.
Whew…. Thats a relief. I thought I’d need a new flux capacitor or something. Luckily there’s a logical reason, instead. Thanks, Dan.
-Kevin.
dieseldude . said:
Whew…. Thats a relief. I thought I’d need a new flux capacitor or something. Luckily there’s a logical reason, instead. Thanks, Dan.
-Kevin.
Kevin
Save yourself the aggravation and pickup some L.E.D. Drivers at 32 cents a piece and then you dont need to figure out resistors cause you dont need them.
Nick
I know, Nicky. There’s probably an easier way. But, other than reading about my electronic adventures, where are you gonna get this quality of entertainment in this price range? Besides, I’ve gotta learn somehow and this site has so many great teachers. Who knows, maybe I’ll save some other large scaler some frustrations by posting my trials and tribulations. On top of all that, you’ve gotta admit, I’m a lot of fun to watch.
-Kevin.
dieseldude . said:
I know, Nicky. There’s probably an easier way. But, other than reading about my electronic adventures, where are you gonna get this quality of entertainment in this price range? Besides, I’ve gotta learn somehow and this site has so many great teachers. Who knows, maybe I’ll save some other large scaler some frustrations by posting my trials and tribulations. On top of all that, you’ve gotta admit, I’m a lot of fun to watch.
-Kevin.
Gotcha… 
So, Dude, my friend… I believe if you were to get out your trusty multi meter [Don’t have one? Rush over to Harbor Freight and spend $7) and measure the voltage on both ends of that string where the wall wart connects I b’leve you will find your voltage hasn’t dropped that much. Some to be sure, but your wart should still be pumping out plenty of them volts.
Reason: Your LEDs consume AMPS, not VOLTS. You might be nearing the capacity of the wart to produce current (Amps), but until you exceed that capacity, the Volts should not drop much.
Easiest way to understand Current Vs. Voltage is to compare to water where Volts is similar to Pressure (PSI) and Amps are similar to Volume (Gals per Min). You can have lots of water pressure, say in a pressure washer, but not produce much volume because of the size of the opening (Hey; that’s Resistance or technically speaking; Friction Loss) - However, If you have a large opening, like the spout on a water tower; you need hardly any pressure to get a huge amount of volume. Try filling a steam engine tender with a pressure washer - you’ll be there all day and not even notice the level change.
Clear as mud, right?
Oh yeah. Making all kinds of sense now, Jon. I should have stayed in school. Got my MBA. Became a successful stock broker. Made millions with some crazy Ponzi scheme and just bought everything that I wanted.
But, I digress. So, what you’re basically saying is, that there is nothing wrong with connecting all this junk together with some good old fashioned duct tape, crossing my fingers, and hoping that it all lights? No?
But seriously…. When I connect more than 3 LEDs to my 12v wall wart, just for fun let’s say 7, as my ridiculous example above- how do I determine the amount of power I’m drawing (volts, milliamps)?
Is connecting many LEDs in series, such as my example, a bad idea?
-Kevin.
Again, as you add lights in series, you lower the current flow thus dimming the light.
This can be good if you do not want full brightness of the leds.
For a head light on a train, you would want the current to be at full spec (usually 10 or 20 ma) to get max brightness.
For passenger cars, this may be too bright, so less current would be better for less light!!
Nick, what are these $.32 led drivers you speak of and where can I find them?
What Dan said. To determine your load you would need to know the specs of your LEDs - but since you are scavenging them you probably don’t have the specs. You could always use that $7 meter in series set to the MA scale to determine the current draw of each individual, or the entire string. Well, maybe not each individual with a $7 meter, but for sure with a $70 meter!
An LED has a minimum voltage. Below that it just won’t turn on. Above that, it has a voltage it likes and it will draw as much current as it needs to in order to get that voltage, so if you increase the volts, your leds will draw more current and the volts across the resistor will increase, but the leds will stay about the same.
Your “12v 500mA wall wart,” might be a transformer and rectifier. In this case, it provides “humps” of voltage that average out to about 12.6 volts, but the peek voltage is 18 and that could be why your 7 leds are still alight. They’ll be quite happy running this way.
So Jon, former Nickel City electronic enthusiast friend of mine… I actually posted my LED specs during this electronic extravaganza (at least I believe these are the specs you speak of)—
3.2 Volt
20 milliamp
(As well as my 12 volt 500 milliamp wall wart)
I guess I was thinking that I could take these specs along with the number of LEDs I wanted to solder together (more than 3), add a resistor if necessary and plug them into a formula to find out how much juice I’m drawing. But, as you know, I’m flying by the seat of my pants here. I did try using the LED Wizard that Bob mentioned earlier in my quest, but it only allows me to place 3- 3.2 volt, 20 milliamp LEDs in series given my 12 volt supply. I’m certain that the LED Wizard is trying for maximum brightness (not a priority for my particular project).
I’d like to know how much power each building in my town draws so that I don’t create a G gauge blackout when I wire them all to the wall wart. Which of course would lead to a lot of G gauge looting and other assorted violence. My current G scale police force is not currently equipped for that type of catastrophe. Not to mention that they don’t even have a police station yet (rest assured when I do build a police station it will be lit with some 3.2 volt, 20 milliamp LEDs. Probably Martha Stewart brand).
I was seriously thinking that there would be a simple formula that electronic type folks use instead of wiring up everything and then using a meter to get the answer. But hey, what do I know?
So, if I have more than 3 LEDs wired in series, etc, etc, etc. I need to connect everything up and use a meter? No formula? No math? Huh?
As I said… I know nothing. Maybe less.
-Kevin.
Terry Burr said:
Nick, what are these $.32 led drivers you speak of and where can I find them?
An LED driver is a device that looks like a small resistor or transistor. It limits the current passing through itself to 20 ma, regardless of the voltage applied (as long as it’s DC and doesn’t exceed the design specs).
They deliver the current to one LED or dozens, and you don’t have to calculate the resistor value. If you’ve got a city block, you can wire up one or more buildings to the driver.
If you power your passenger car lights from track power (as I do) the circuit is a little more complicated, but not much. You need to make sure the voltage is always in the same direction (a rectifier), and it’s good practice to include a capacitor to store enough electrons so the lights don’t flicker over dirt on the track.
This is a passenger coach powered by the driver from the track: http://freightsheds.largescalecentral.com/users/dick44/led_driver/lights_on.jpg
In my Freight shed is a folder called led_driver with more pics and a pdf of the circuit.
Hope this helps!
There is detailed information on these LED drivers on my web page here:
http://www.trainelectronics.com/LED_Articles_2007/LED_104/index.htm
dave
Terry Burr said:
Nick, what are these $.32 led drivers you speak of and where can I find them?
Terry, They basically make LED installs easy I use them as is with a couple of LEDs each
for Buildings and when I use them for passenger cars I add a rectifier like in the picture
and since have been adding capacitors for energy storage “anti flicker so to speak” They work well and simple to use and cheap. Hers an old picture of when I 1st started playing around with them
a couple of years ago. Al Electronics is were I think I bought them from.
Not to derail a thread but Kent posted a pic with some lead weight holding wires down… I had never seen ones other than mine. Got mine years ago thrown in the dump pile behind the town news paper office. Are yours the same? Later RJD