My past layouts used building lights as an after-thought. This time I've added a buss throughout to connect building lights to. But I haven't figured out how to power it yet. My last layout, which was destroyed in the 2011 floods, used a similar #14 wire buss with a 5amp A/C power supply. Then at spots near building clusters, I added GRS Litepac's to power 1.5v GOW bulbs. I was not happy with the result and would prefer to use LED's this time. However GRS does not make a LED version. Anybody have any suggestions? My lighting buss is a 32' run of #14 wire. Can I just power it with a 3v DC power supply? And if so, how many LED's can I expect to power per amp? Thanks - Bill
It may be a slightly more work but you may want to just put in a 5 or 12 volt bus and use current limiting resistors to drop down to what you need. 5 volt and 12 power supplies are pretty common. This will also allow you to power other accessories in the future.
LED's generally use at a maximum 20-30 milliamps of current. LED's are current driven not Voltage driven (although they do have a minimum voltage limit somewhere between 1.5 and 3 volts, depending on color). This means you can run them off of 12 volts but you will need a resistor.
Ohms law can be used to give you the right resistors for your LEDs. It States that V=IR where V is your buss voltage (5 or 12 volts), I is the amount of current that is drawn (I would say 15 milliamps (0.015 amps) is good number to use for LEDs') and R is the amount of resistance in Ohms.
Lets do an example.
for a 12 volt power bus:
V= 12 volts
I = 0.015 amps (15 milliamps)
solve for R
R= V/I --> R= 12 volts/ 0.015 amps = 800 ohms. I would go with at least a 1kohm (1000 ohm) resistor.
1kohm is my standard resistor if I am using a 12-14 volt power supply.
Another example:
for a 5 volt power bus:
V= 5 volts
R= V/I --> R= 5 volts/ 0.015 amps = 330 ohms, I would use something around 480 or there abouts.
I hope this information helps
Colorado Front Range Railroad: http://www.coloradofrontrangerr.com/
Wow - sounds complicated. I was hoping to find a power supply just for LEDs. I find soldering a resistor to each LED to be a challenge. Why can't the power be "resisted" at the source? Better to have one resistor than 100? But obviously, I don't really understand the technicalities. Thanks for the reply - Bill
The voltage drop through a resistor varies with the load. More LEDs, more load. Fewer LEDs, less load. That's the main reason you can't just 'resist' at the source.
You CAN wire LEDs in series, and then use one resistor for the group. But not too many. You need to know the voltage of the LEDs, but most white ones tend to be around 3.5 volts. If you wire 3 of them in series, that's 10.5 volts total, which you could connect to a 12V DC power supply with one resistor of about 150 ohms.
One resistor per LED is still the best though, because there's less chance to goof it up. If you're using lots of LEDs and buying from one of the bulk eBay suppliers, they also have resistors in bulk, which winds up costing maybe a penny each. Work assebly-line fashion, twist a bunch of resistor leads to LEDs leads, then in a couple so seconds solder them all, build up a supply ahead of time. If you keep your soldering iron clean, this doesn't require all the extra steps like fluxing and stuff. Touch iron to leads, quick tough of solder to leads, done. In less time that it takes to read that sentence.
You CAN wire up a simple adjustable regulator as a constant current source to run LEDs from with no resistors, but you will need multiples of this circuit because one doesn't handle 100's of LEDs, so it ends up being just as complicated as just connecting a resistor to each LED.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Regardless of how you end up wiring things, make sure your power supply is protected. Most do not have breakers built in. To keep them from burning down your house, many manufacturers build in one-time fuses. But, they aren't "field replaceable," so once they go you have an expensive and very unattractive paperweight.
I've installed cheap fuse holders and fuses from Radio Shack on my lighting bus lines. As much as I hate blowing a fuse, for some reason I smile every time I have to replace one.
It takes an iron man to play with a toy iron horse.
Thanks Randy - Guess I understand it now. Does it matter which LED lead gets the resistor? Also I was exaggerating a bit with the 100 number. It's not likely I'll have more than 25 or 30 at the most. So it sounds like a 12v 1amp supply should work fine. Regards - Bill
Thanks MisterBeasley - Actually my entire layout is powered by a fused power strip that is turned on only when I'm running the layout. So it should be pretty safe. BTW I had dreams of building a NYC subway layout but got disappointed when Walthers didn't expand the P1k models. They seem to be available on eBay but not at prices I can afford. I find that retired folks need to have smaller dreams. - Bill
You need to protect the output of the power supply - most little wall plug transformers have protection inside, but it's a one-time fusible link., Short the output of the power supply and - go get a new one, because you can't fix it. An external fuse of lower rating then the power supply, will protect the power supply and blow before the internal non-replaceable one will.
20-30 LEDs, you wouldn;t even need a 1 amp power supply, the total there, depending on resistor used, would be under half an amp. For a typical white LED with 12V power supply, use a 1K resistor, that result in about 9 mA per LED, 30 of them would be 270mA, or 0.270 amp.
The resistor can go on either lead of the LED. But if you make them up ahead of time, it would be best to pick one and be consistent. LEDs have their polarity marked in various ways, often one lead is longer than the other (until you cut them to fit....) and/or they may have a flat in one side of the base. If you are consistent in hooking the resistor to the same side of every LED, then you can more easily remember which side goes to the + power and which goes to the -. LEDs do matter. I'd make them all up so that the resistor is ALWAYS + or ALWAYS -, not hald and half, which will only confuse you later. Good thing is, hook an LED with resistor up backwards, no harm, it just won't light up. Flip it around and it should work.
Randy is right. The power strip will protect your house, but you need to fuse the outputs of the power supplies individually. Get a fuse rated just a bit below the output of the power supply - like a 4-amp fuse on a 5-amp supply. Don't worry about the voltage rating on the supply. That's a maximum voltage, so a 125 volt fuse will be just fine on a 12 volt supply.
Keep looking for those subway cars. I see them every now and then, new in the box, at train shows or eBay. Walthers will probably release new versions, typically with nothing "new" but the road numbers, every couple of years. At that point, go to www.trainworldonline.com and buy them there for a much better price.