Opinions needed for LED Lighting in the Roundhouse/Progress

LoS, what did you use on the flor between the rails. That floor looks great and your LED's are looking great too. Nice work on your roundhouse. I cant wait to get to mine

Because I cranked up the output voltage on that module until I got 3v at the resistor. The out put was showing like 12-13v before the resistor, needless to say it burned out the 2 LEDs after the resistor started smoking and burned the skin off my fingers. 

That's why I was asking if I should change the resistor to a different rating to get more than the 2.5v, as that is just too low. I want the LEDS to run at 3.2V, with a 5v power source, but I don't know how to calculate that..... so I just cranked up the voltage with what I had to see what would happen

I told ya, I'm an idiot when it comes to electrical stuff

Matt I used 2 layers of cabinet grade 1/4" plywood. They are actually 7/32" which is perfect when the 2 layers are sandwiched  together. It took 2 4x8 sheets to get what I needed because of the size at $25 a sheet. Between the outter rails and center rail was the same stuff, ripped on the table saw into strips with the outside edge beveled at 8 degrees to clear the angle of the rail. That was a tedious process to say the least. Then strips of scale lumber were pressed between the rail and the 1/4" plywood to cover the tie gaps. The good news is the scraps were used to cut the rear 3 portions of the roof (the sections with the smoke stacks).  

Here is an Ohm's Law Calculator.  You put the voltage drop desired, in this case 1.8 volts, and the current required, that would be the number of LED's times 20ma each, for 16 that would be 320ma.  When you press Calculate, you get a resistance of 5.625 ohms at 0.576 watts.  I'd probably pick a 5.6 ohm resistor at 2 watts for this use.

Thanks John. I have 2 questions based on that calculator...

1. Where did the 1.8V come from? 5v source - 3.2v LED drop = 1.8 from what I can guess. So if I had 6v then I would enter 2.8v.

2. The calculator shows .576 watts and you suggest 2 watts. What is the benefit of choosing 2 watt instead of a 1 watt resisitor? I'm guess it can handle a little more voltage before burning out?

You have a 5V supply.  I'm assuming that it's a regulated 5V supply, so I'm assuming that it's really 5V.  If it's not regulated, I suggest a regulated supply.  You specified that you have the proper current at 3.2 volts across the LED's.  If you haven't measured the current at that voltage, I'd do that so you can use the proper values in the calculator.

Assuming that 3.2V is the proper voltage to run the LED's at 20ma each, you need to drop 1.8 volts across the resistor, that's where that voltage comes from.  The reason I specify a 2 watt (or even larger) resistor is so it won't get as hot.  A 1W should work in free air, but it'll get fairly warm.

The value of having the series resistor is a very small mistake in the input voltage with unlimited current available can really cook LED's in a hurry.  If there's a resistor in the circuit, the effects of a small voltage difference is greatly reduced.  The LED forward current goes up very quickly after it reaches it's operating voltage, a couple of 10ths of a volt can really affect the MTBF of the LED.

The reason the two LED's went up in smoke is simple, you needed a much larger resistor for the greatly reduced current for two LED's vs. 16 LED's.  Two LED's would have needed 40ma of current, to drop 1.8 volts at 40ma of current, the resistor should have been a 45 ohm resistor.

You can determine that by plugging in the correct values to the calculator I posted above.

Looks great! One item you may want to explore is lining the interior of the shades with aluminum foil. This will redirect any "light" from the Led's back down to the floor. YEs it's tedious but the brighter light given maybe worth your effort. 

May I ask what size of Plastruct shades did you use?

There's something odd about that.  For the various LM2596 modules I've used, the adjustment screw changes the output voltage about 1 Volt per turn.  It is quite easy to set 3.0V as opposed to, say, 3.1V. 

Your module changes 5V in half a turn?  That would make it difficult to set the voltage with a resolution of, say, 0.1V.  Is this why you are now proposing to use fixed 5.0V voltage source (rather than one of the adjustable LM2596 modules)?

By all means stay the course with your current 5V method but here's some background that may fall in the TMI (too-much-information) category.

So while the parallel-bus method greatly simplifies assembly, it can be problematic if you're trying to push the LEDs to their limit.  I took a sample of 3 warm-white flat-top 3mm LEDs out of a bag and here's a graph of the current vs. voltage.  Note that in the bus method all LEDs see the same voltage.  It doesn't matter if you have that single resistor "protecting" the bus.  After the resistor does its thing to drop the voltage, all the LEDs see the same voltage. 

For these 3 LEDs (note: these may NOT be the same type as what you have) if you apply 3.1V to the LED the current flowing in each LED varies from about 19 to 23 mA.  If I had sampled more LEDs the spread would increase but this is representative.  As GRJ points out note how the current takes off with small changes in voltage when you start pushing the LEDs.

So in your first example of 16 LEDs you turned up the module to 5.0V and measured 2.5V across the 10 ohm resistor.  This means 2.5V / 10 ohms = 250 mA is flowing into 16 LEDs or about 16 mA per LED on average.  Some LEDs were pulling more, some LEDs were pulling less.  You just don't know in the parallel bus method. Again, my curves may not represent your LEDs.  That is, in your example, starting from 5V, if the resistor dropped 2.5V, that's 2.5V at the LED parallel bus.  If the LEDs in my graph were presented with 2.5V, they would draw a negligible amount of current (off the chart to the left).  I'm surprised at your numbers but I'll take them at face value.

Next. If you plan to push the LEDs to the limit, one limit is power dissipation in the LED itself.  In the same way that the resistor is rated at 1 Watt or whatever, if you're lucky you might find a published specification for the LED.  Here's a chart for a 3mm warm-white flat-top LED on eBay.  It took me several listings to even find one that gave this level of detail.  Now we can exchange stories about the credibility of specifications from these low-cost eBay sources but that's a different discussion. 

So here they say a maximum power dissipation of 80 mW.  Power is calculated by Voltage x Current.  So in my chart, at 3.1V, the 19 mA LED is dissipating 3.1V x 19 mA = 59 mW.  The 23 mA LED is dissipating 71 mW.  So even if the average current to each LED seems acceptable, a "proper" design must account for the spread or component-to-component variations.  Just looking at the curves you can see that a small increase in voltage would push the upper curve above 80 mW for that LED.  Again, this goes to the notion of pushing the LEDs to their limit.  A rising tide lifts all boats...

Stan, you have perfectly illustrated why I prefer higher voltage and series wiring for the LED's.   Let's say you go with 24VDC for the supply.  You can run six or seven LED's in series with a small dropping resistor.  Since the total circuit current is only 20ma, you can use 1/4W resistors with no issues.  Also, a small variation in voltage is of minimal risk of significant over-current issues.

If you want maximum light from the LED's, I'd use six in series and a 50 cent CL2 constant current regulator for each string using that same 24VDC supply.  This gives you maximum light output withing the LED's ratings, and no risk at all of cooking any LED's.

You can wire LED's in parallel, but you do multiply the issue of precisely regulating high current.

John I'm use the AC/DC converter module with adjustable output voltage. So I can adjust the output voltage to whatever I want. I have been using 5v and 3.2v, except for when I cranked it up and burnt out the resistor and LEDs.

Stan apparently that first module I was playing with was bad, because the adjustment was really sporadic and then it burnt out... the bridge rectifier went up in smoke. The others have been fine, and have a very sensitive adjustment, so .1 voltage adjustments is easy. I was using 3.2v for the pictures I posted above with the lights on.

So how bout I wire them like this..... 5v power source and a 100 ohm 1/4 watt resistor on each LED as pictured in this calculator? 

l 

I would assume this would give me a safe and consistent string of lights?

Is that still parallel wiring?

It's probably easier to use higher voltage and wire some of them in series with one resistor for each series string.  Less wiring and fewer components.

That's exactly what I'm using for my regulator, with a PW ZW as the power source.

You could easily get 18-20 volts out of that combo, which would allow you to string a few in series...

Because I am doing them in banks with on/off toggles, I really need 2 LEDS per roof beam in each section, that's where I came up with 16. I have the 100 ohm and 150 ohm 1/4 watt resistors, so wiring them all individually is no big deal to me, if that's what I have to do.

Since I need 2 LED's per stall/beam section, how bout this John?

I don't know what options the LED calculator gives you but since you have 100 ohm resistors, I suggest you insert different source voltages until it tells you the resistor should be 100 ohms.  That is, you have a variable voltage source so take advantage of it.  (Spoiler alert: the answer will 2 * 3.2V + 20 mA * 100 ohms =8.4 Volts).

If you are happy with the 2-series LEDs plus 100 ohm resistor wiring, wire or a few sets up.  Then apply about 8.4V and measure the voltage across the resistor.  You are looking for 2.0 Volts as that means 20 mA is flowing into the string of 2 LEDs.  It might be something other than 2.0 Volts since you don't really know if your specific LEDs draw 20 mA exactly when 3.2 Volts is put on them.

The bigger issue is whether you can, with a few sections of LEDs, estimate whether even 20 mA per section will be bright enough.

Well, since the LED's are rated at 20ma as a rule, if they're not bright enough that's a different issue.  You can get higher current LED's, or add more LED's to solve the issue.

These are the LEDs I ended up using. I just went with the middle of the road at 3.2 forward voltage.

I have 1/4 watt resistors of 100, 120, 130, and 150 ohms.

I have 1/2 watt resistors of 120 and 150 ohms.

These LEDs will be plenty bright once I get them all in. The front section looks good I think, I'm just trying to protect my time so I don't burn anything out and have to redo it.

Based on that LED calculator, I can go with 8.6 volts and use my 120 ohm 1/2 watt resistors, or 9 volts and use my 1/2 watt 150 ohm resistors. The 1/2 watt would give me a little bit more security in voltage variance if I'm understanding what you guys are saying. Either way, it's past time for a drink!

I really appreciate all the time that John and Stan are giving me with opinions and explanations. Without this forum and the guys here, I would of just stuck a GE 100 watt light bulb on the floor of the roundhouse and called it a day!

I think you have it, sounds like a plan.

If you are happy with the 2 LED configuration at 20 mA, then the 1/4W 100 ohm is fine.  The calculator should tell you the resistor power is 0.04 Watts or 1/25 Watt.  A 1/4 Watt resistor can "handle" 6x that.

Hook up one series pair of LEDs with the 100 resistor, and measure the voltage across the 100 ohm resistor.  You should read 2.0V if 20 mA going through the resistor.  That is current into the LEDs will be the resistor voltage divided by the resistor value.  2.0V / 100 ohms = 0.02 Amps or 20mA.  If you read 2.5V then its 25 mA, etc.  Adjust the source voltage accordingly until you read 2.0V across the resistor.

Well I wired up a pair in series, with a 1/4 watt, 100 ohm resistor, running 8.4 volts. The meter shows 1.9 volts through the resistor (I assume that is 19 milliamps), which is close enough for me. I let them set there on the bench for a few hours and all was well. Still bright, and no heat in the resistor.

For the next section in the RH, this is how they will be wired.

Thanks again John and Stan for all the advice. Stay tuned to see what I burn up next.  

Sounds like you have a launch window.   I'm guessing it'll be pretty bright in there when you light up all the LED's!

The RH is big, 36" deep in every stall, and with the 2nd story through the center section, it needs a ton of light. With the adjustable modules, I'll just turn the voltage down a bit if it's too bright. After experimenting based on Stan's advice and showing that hangar, I would rather have too much light, and dial it down a bit, than not have enough and be stuck with it.

Now that overhead lighting has been beaten into submission...

I assume the guys in the RH didn't just sit around drinking coffee all day so how about a couple guys welding a wheel or side-rod?  A flickering blue/white LED that occasionally pops on might make a nice effect.  Your thread title doesn't specify only overhead RH LED lighting!

I'd also think there'd be task-lighting at ground level rather than just depending on overhead lights?  I dug up this photo of 1/48 figure holding a flashlight. An LED illuminates a flexible fiber strand that runs through the figure. That thread topic was how to model an operating flashlight for a camping diorama but I'd think RH workers might have used flashlights for inspection?

So to the extent you may later want to add ground-level LED lighting, now might be the time to provision for this with an auxiliary DC power bus on/under the RH floor.

And I still can't let go of the swinging door of the RH.  You previously suggested this was a roll-up door (which I agree would be challenging) but I was reading the RH instructions linked earlier.  It implies the doors are the swinging type and can be assembled opened or closed providing a glimmer of hope that they can be made to operate.  Two words ..."wow factor"

Stan I already planned on a welder or 2 inside the RH. Nice effect and I've seen it done by others. I think Patrick H had a welder in his. Maybe even a shop light over a work bench or above some storage shelves.

I will have an AC bus running over there, as that's what powers the LEDs for the RH. It has a terminal strip so I can just add more converter modules where needed.

Now the swinging doors..... stop it already!  Although Dennis Brennan (my RH is not a Korber kit) gave the option for swinging doors, or roll ups, I went with roll ups. No changing that now. The roll up housings are in and trust me, they ain't coming out without complete destruction of the front stall openings. We'll just have to figure out something else for a "WOW" factor.   

Of course, the roll-up doors are motorized, right?

In my head....YES they are! and they go up and down reeeal slow.

Is that "in your head", or are they really motorized?

What kind of question is that, John?  What's the difference?

Seriously, this thread has been extremely useful.  Let me echo the thanks for John, Stan, Bob and the others who weighed in with great ideas.

George

Not at all, the resistor can be anywhere in the circuit, it has no polarity or position sensitivity.

In other words any of these will work...

Once again Stan goes that extra mile with great graphics!

Well the front section and center section has been lit. I also added lights leds above the front doors.

A set of 2 LEDs wired in series for the center section.

The sets of 2 installed and connected to the feeders. Then it was all painted flat black.

Piece of conduit (plastic tubing) on the right, that runs through the floor to guide the feeder wires into the RH.

Lights above the doors.

You can see the tubing coming through the bottom of the layout, and the AC/DC converter modules mounted to the bench work below the RH. Eventually they will be connected to an On/Off toggle so each section of the RH can be lit separately.

50 in, 34 more to go!

That's some very impressive work!

 I was just worrying about how bright and what color my lights would be. I grouped them in sets of five and wired the three sets together. I'm happy with the color and brightness. I mad I just threw them in.

 The first thing a train guy did when visiting was to stick his head in there to look inside the building to see how it was detailed. You can see my light's wires hanging down and not very neat. I used old Christmas tree light wires and sockets and they still have that twist in them. It looks sloppy up there.

 Looking now at how you guys did yours, makes mine look even worse.

I couldn't comment here as I've seen too many types of LED lights to know, just how many or where they should be placed would be guessing.

 So I smoke up the building and don't look up there!

 Yeah, OK. But don't look up in there!!!

Thanks. I usually do "good enough".

I appreciate the work you're doing. Really first rate.

After a few years, I still haven't glued on the gutters and facia I made!

Answer: More than you can shake a stick at.

Each 3mm LED requires about 3.1V @ 20mA.  Power = Voltage x Current so Power per LED is 3.1V x 0.02A = 0.062W or 62 mW.  So if you were able to convert the AC source with 100% efficiency, you could power over 4000 of your LEDs. 

But in your configuration, your loads are 2 LEDs plus a 100 ohm resistor which per your measurements requires 8.4V @ 20mA.  Power = 168 mW (for 2 LEDs), or 84 mW per LED.  So again with 100% conversion efficiency, you can still power over 3000 LEDs.

But your AC-DC converter module efficiency will probably run, say, 75% at your power levels.  So 275 Watts becomes, say, 200 Watts available.  That's still over 2000 LEDs.

Of course you can't cram 200 Watts through one eBay converter module.  So the task is to spread the power (heat) around so that each module only drives maybe 5-10 Watts or, say, 100 LEDs max.

Bottom line, you have PLENTY of power to drive LEDs with plenty leftover to power motorized roll-up doors (real or imaginary).