I found this website that draws out a schematic for led lighting. I don't know what values to enter in the boxes as the lights have a range. Do you enter the minimums or maximums of the 2ma- 20ma current rating and what do you enter for the 1.8v-3.3v voltage forward rating?
http://led.linear1.org/led.wiz
Leds are 0603 (1608)
I plan to use an old tansformer for power (I have a few deadbeats hanging around that must start earning their keep)
I plan to put 36 to 42 leds in the roundhouse.
I am assuming that the resister they suggest offers the maximum brightness, can the size of the resister be changed to offer a more dreary look inside the RH?
Do I use A/C or D/C for the power?
As always thanks.
Brent
"All of the world's problems are the result of the difference between how we think and how the world works."
Don - Specializing in layout DC->DCC conversions
Modeling C&O transition era and steel industries There's Nothing Like Big Steam!
The calculator wants the LED forward voltage, and the LED current, and the supply voltage - which needs to be DC. The calculator then takes into account the drop and current and determines the resistor.
Never design to the meximum limit - since LEDs are rather sensititve to overcurrent, a slight rise in voltage would put you over the limit if you built it to run right at 20ma or whatever the LED specs say. Pus resistors have tolerances - unless you used an expeisivne ultra precision resistor. Standard resistors are 5% tolerance, so it ould be as much as 5% less than the nominal value - again putting the LED current over the limit even if the voltage remains constant. Middle ground works pretty well - note if you plug in typical values used for DCC decoders and LEDs, you get about a 1K resistor, or if you plug in a 1K resistor, you get about 9ma.
You need to plug in values form the datasheet for your specific LED. There will be a Vf(typ) parameter which is the forward voltage, and it may specify the current at that Vf, or it may just give a max current. Pick no more than 75% of that current limit as the value to use. You'll probably want to test this out before installign it on the layout, beause mostof the small SMD LED are VERY bright, so you may want to use a MUCH lower current number (larger resistor) to keep them looking like bulbs and not miniature fusion reactors. 2K-4.7K is not out of the question, that is less than 4.5ma at 12V with a typical white LED.
The calculator is a good starting point, but this will probably require some experimentation. Just don't go with any smaller resistor than the calculator shows, or raise the voltage without recalculating. Bigger resistor/lower voltage won;t hurt anything, you'll just eventually reach a point where the voltage is below the minimum forward voltage and the LED won't light, or the current will be below the minimum current and again it won't light up. But it won't harm anything.
There was a picture posted in an electronics forum I'm on, the guy had a 4KV (4000V!) lab power supply which also has current limiting. He had the current limit set to 10ma, and an LED connected right to the 4KV output - nicely lit and NOT exploding. Probably the best proof that LEDs are current devices, not voltage devices.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Slightly off topic, but I found this book very useful for my hobby needs -
Practical Electronics for Inventors
http://www.amazon.com/gp/product/0071771336
There is new edition coming in a month.
I usually go by spec, if you have a spec for your LEDs then there should be forward current If and forward voltage Vf, given your supply voldate Vsup the resistor value is
R=(Vsup - Vf) / If
Increacing forward current may reduce life of your LED, increasing it above absolute current rating will blow it off, reducing forward current will result in less light. Also keep in mind resistors max power rating, for your scenario it would be
P=(Vsup - Vf) * If
If you want to drop 3v at 20mA then that is 60mW, so pretty much any resistor can do, if your Vsup is 12v and you want to drop 10v at 20mA then resistor needs to handle 200mW, so you'd need to use at least 1/4 W resistors.
Slava
Visit my blog to see progress on my UDTR Model Railroad
http://slavak815.wordpress.com
Hi Brent.
I hate doing the math for these situations! I admire those who can make sense of it.
I would suggest a somewhat simpler method for getting the right amount of light out of your LEDs.
First, set your transformer to 12 volts.
Next, get yourself a small assortment of resistors to experiment with. I would suggest 2.2K, 3.3K, 4.7K, 6.8K, 10K, 15K, 22K and 30K (or there abouts as long as the numbers are close).
Assuming that you are going to have a resistor for each LED, the next step is to simply test the brighness starting with the 2.2K and working up. Once you get close to the brightness you want you can fine tune the setting on the power pack if need be. Its safer to turn the power down rather than up.
Once you have the right setting, lock the throttle knob in place with a small wedge and some epoxy (you might want to wait until the roundouse is installed before doing that).
These 0603 LEDs are hooked up to 30.1K resistors:
Here they are installed:
They are actually not as bright as the camera suggests.
The idea of using brass bus bars (wires) is good. This is a rather crude installation which was done very early in my LED and scratch building career. The lighting is getting re-done when I detail the interior structure. The principle will be the same but I will attach the bus wires to the beams and use 0603s instead of 3mm LEDs, and SMD resistors:
The lights are a bit too bright for your purposes:
Dave
I'm just a dude with a bad back having a lot of fun with model trains, and finally building a layout!
Don, Randy and Dave.
You guys have made it all crystal clear, thankyou.
I was going to ask about getting a selection of resisters and experimenting, as well as using the throttle to adjust the light intensity. Dave you are a mind reader answering before I ask the question.
I haven't decided on how to proceed as far as mounting the lights as I am waiting for all the bits to arrive in the mail. Copper tape is another option. One guy used the magnet wire to string his lights in an engine shed and that had a very early 1900s look to it. I am currently thinking that using magnet wire suspending the Ngineering shades hanging down from the resister chip with painted copper tape as a buss may be the way to go. We will see.
One more question, can you use two (or more) resisters in a series to double up the resistance? Say 2 x 1K to make 2K resistance? Or a 1K + 3K to make a 4K?
Thanks.
Yes, resistors wired in series just add the total resistence. Good for testing, but if you put 5 or 6 1K resistors in series and it's what you want, in the real circuit replace it with the nearest actual value. For standard 10% resistors that would be a 4.7K or a 5.6K.
Resistors in parallel is a bit more complex. This site has lots of cool calculators: http://www.daycounter.com/Calculators/ Two of note are the parallel reistor calculator and the standard resistor calculator - for the alatter, you plug in the resistence value you want, and select 10%, 5%, or 1% tolerance. It then tells you the nearest standard resistor.
I really like the idea of having the lights suspended by magnet wire. Very prototypical. You may want to consider using two or three LEDs in parallel to limit the number of wire spans. You can use a single resistor for two or three LEDs. The only change will be that the value of the resistors will be lower to get the same light levels as you will from a single LED.
Something to think about is how you are going to solder the magnet wires to the copper tape. If the beams are plastic and the tape is already stuck to them you might damage the beams with the heat.
I would suggest two options:
- One is to use wooden beams because they won't melt and any slight scorching will be covered by paint.
- The other is to assemble the lighting first and then put the whole thing in place. Getting all the measurements correct will be fussy, as will handling the system once it is assembled.
I would suggest doing a mock up first to see what works best and easiest. Others may have better suggestions on how to assemble the lighting.
Something else to consider is how you will get access to the stalls with the lighting in place. You could leave the beams where the lighting is attached loose so the beams and the lights can all be moved to one side. Micro connectors would make the whole thing removable. You could use pins on top of the posts to hold the beams in place or you could make the whole post and beam system removable. It could even be done in sections to make handling easier.
Alternately, you could make the whole roundhouse removable from its floor.
You can make your own micro connectors by cutting these into pairs:
http://www.ebay.ca/itm/111802157530?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
DigitalGriffin...You shouldn't hook two LEDs together in a straight path as that will make the above equailtion invalid...
No it doesn't, you just have to add the forward voltage of the LED's in series for the Vforward value. In fact, wiring LED's in series is a great way to reduce the current consumption. For example, if he has 42 LED's all in parallel operating at 10ma, they will consume 420ma. If he has a 12 volt supply and they are 3.5 volt white LED's, then he could wire them in groups of three in series(adjusting the resistor accordingly) and they would only draw 140 ma total.
CSX Robert DigitalGriffin ...You shouldn't hook two LEDs together in a straight path as that will make the above equailtion invalid... No it doesn't, you just have to add the forward voltage of the LED's in series for the Vforward value. In fact, wiring LED's in series is a great way to reduce the current consumption. For example, if he has 42 LED's all in parallel operating at 10ma, they will consume 420ma. If he has a 12 volt supply and they are 3.5 volt white LED's, then he could wire them in groups of three in series(adjusting the resistor accordingly) and they would only draw 140 ma total.
DigitalGriffin ...You shouldn't hook two LEDs together in a straight path as that will make the above equailtion invalid...
Yes I know. But the equation isn't as clear as that. A layman may just assume the 1 Vforward may be all that is needed. And I didn't want to complicate the equation by writing Vsource - (Vforward1 + Vforward2 +... Vforwardn)= I R and explaining it.
I think however I do the lighting it will have to be a prefab. I have had a lot of luck building things for people at my house and upon delivery having them fit perfectly into place. I have visited a lot of websites on lighting models and have found a host of different ways to do things and Google Images is amazing for getting ideas.
I was sitting here yesterday reading the forum and noticed the 3 hole punch on the desk and thought the holes were about the same size as the shades from Ngineering. After picking up the ruler that was also sitting on the desk (good thing those kids never put anything away) I thought OH OH! what am I getting myself into, these are awfully small. So I punched out a bunch of "lampshades" out of paper, stuck pins through them to simulate wire and it was off to the roundhouse.
I think a row of hanging lamps down either side of each stall will look really good as the guy that used magnet wire to hang his lights did something along those lines. It needs to be dull in there though. I came across some early days roundhouse pic's and they came across as dim, dismal places to work and the workers were walking around with lamps in their hands. So puting in guys holding lanterns might also add to the scene.
I was/am a little concerned about my grimy roundhouse floor, so I stuck it, my head and a dim light under a heavy blanket to see how it looked through a small opening. My wife came along and asked if all train guys are as weird as me. Anyway what I've done so far looks good under a blanket at ground level. I still have add more details like drains in the pits and stuff. Fixing it is only a little more paint away if I don't like it.
Thanks for the additional answers guys.