Last winter, as a frugal modeler, I did what a lot of others recommended and bought an after-Xmas string of 50 warm white LED lights on sale for $ 5.00. At 10 cents apiece, a bargain to be sure. I then used one, cut off the first 1/8th inch (where the focus cone is molded into the head) and gently sanded with 400 grit paper (to diffuse the glow), in a single circuit with a 470 ohm resistor, to light a 4"x4" small building. Worked out very well. So far, so good- but still a newbie at this LED stuff.
Now, as I complete a larger structure, I plan to use several LEDs to light two larger interior areas- a 12"x6" open first floor and a 5"x4"x3" upper structure (the Heljan #1752 HO railway station).
I was planning on using 2 lights in the upper area, divided by a black construction paper viewblock and 3 lights in the first floor, evenly spaced. (of course, I have been diligently painting to ensure no light leakage).
The LEDs Xmas string package box info states "average .02A" (amps) per LED w/ a 50-light string total of 4.8 Watts; the inside instruction sheet says, again, "4.8 Watts, but continues with .04Amps, and to confuse me more, it says "replace with only 3.4 volts, .068 watts LED type lamps provided with product (spares)".
Now the questions:
1. Please tell me what I am looking at per individual LED?
2. If my previous experience and HOn30Critter's previous illustrated helpful advice is followed, I should use a resistor with each LED in this larger circuit, correct?
3. Should the circuit be series or parallel?
3. Based on my above description of what I am doing, is my lighting plan sufficient or not enough? ( I plan on sanding the LEDs again to diffuse the glow)
The bargain price was great, the current confusion is greater!
Thanks for any advice on this.
Cedarwoodron
cedarwoodron2. If my previous experience and HOn30Critter's previous illustrated helpful advice is followed, I should use a resistor with each LED in this larger circuit, correct?
I agree with TZ.
.02amps = 20am. An "LED / resistor set" is one LED connected in series with one resistor.
Most LEDs are rated at 20ma. If you use 12VDC, you need a 470 to 1000 ohm resistor connected to each LED to get aprox 20ma. If you use a 4.5vdc supply, use a 100 to 150 ohm resistor connected to each LED to get aprox 20ma.
To rate a power supply for your LED needs, just add .02 until you get to the current rating of the supply and stop adding LEDs. Lets say that you have a supply that says "12vdc at 1 amp", then you can use up to 50 LEDs with 1000 ohm resistors, if the LED / resistor sets are connected in parallel. (.02amps times 50 = 1amp)
Elmer.
The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.
(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.
Not exactly.
an LED's current rating of 20ma is the maximum it cna handle withotu failing prematurely. The ACTUAL current flow is based on the resistor and supply voltage. The other thing to know is the LED's voltage drop. Since those specs aren't usualyl given with CHristmas light strings, we have to make a few assumptions, and play it safe.
Most white LEDs drop 3.2-3.5 volts. So we'll assume 3.2 volts for a worst case scenario. And most white LEDs have a maximum current rating of 20ma, so we'll use that as a maximim.
Now comes the power supply. Assuming a 12V DC regulated power supply, so the voltage under varying load is always approximately 12V. Compared to an unregulated power supply that when you have only 1 LED connected is actually somewhere around 16V, and if you connect a larger string drops to 10V. We don't want that.
So with a 12V power supply feeding through an LED that drops 3.2 volts, we have 8.8 volts left over. Feeding that through a 1K (1000 ohm) resistor means we have 8.8/1000 amps, or 8.8ma Round up to 9ma so it's easier.
THAT'S the number you add up to detemine how many LED/resistor sets can be connected in parallel ot the power supply. You don;t want to run the power supply constantly as max power, but 100 LEDs wired like this would draw 900ma, still under 1amp. A 1 amp supply would be close to fully loaded though, so you probably wouldn;t want to run a full 100 LEDs.75% load is a reasonable continuous load - that's about 80 LEDs.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
rrinker Not exactly.
True.
But that is an easy rule-of-thumb way to figure it without getting into the specifics which drive non electrical types crazy, and you have quite a bit of overhead so things don't heat up if you decide to tack on one or two more.
Yeah but using 20ma to calculate load then using 1K resistors is more than 50% overhead. I'm all for designing to worst case but if there are a lot of LEDs needed for structures and street lights and doing things like putting headlights and taillights in cars, you'll end up buying twice as many power supplies as you need. 2 instead of 1 is no big deal. 10 instead of 5 though - that starts getting significant.
Hi
A long time ago in a far, far land, (Air force) I worked as an E tech. If I remember correctly you have to account for the voltage drop across the resister. power = voltage X current. I may not be following this thread correctly, so forgive me if I'm wrong.
Lee
yankee flyer Hi A long time ago in a far, far land, (Air force) I worked as an E tech. If I remember correctly you have to account for the voltage drop across the resister. power = voltage X current. I may not be following this thread correctly, so forgive me if I'm wrong. Lee
That's exactly what I did in my calculations. 12V in, 3.2 across the LED, leaves 8.8 across the resistor.
Items in series, the sum of the voltage drops must add up to the supply voltage. Items in series, each gets the same current across it. So the LED and resistor get the same current, but different voltages across them
Items in parallel, each gets the same voltage as the supply voltage. Items in parallel, the currents add up. So if you hook LED/resistor pairs in parallel, each gets 12V, and the total draw on the power suppyl will be the sum of the current across each pair.
Guys,
For those of you that have installed LEDs in your structures, would you post the photos here, please.
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