300 LED Rope Light Question

Each three-LED segment has a resistor to provide safe operation at 12V.

Thank you professor!

Mike

These are the strips I use all the time, as long as your supply is 12VDC (or less), you're fine.  That's measured voltage, not nameplate voltage.

I was going to post my own topic, then I see this one at the top of the electrical forum.

I'm taking the LED plunge. Largely because my Auto-Train is sucking up so much power with just 9 Superliners (and 12 autoracks), a single Williams Genesis won't even try to move it. When I got to 8 amps of current on the Z4000 and the loco only barely pulling out the coupler slack, I shut things down. I'll have to modify the reverse unit to start in neutral so I can double-head with a PS1 Genesis.

I have a reel of LED's on the way, and am aware of some other stuff to add. I am running this particular train conventionally, but at some point I will upgrade my Genesis diesels, a Proto-1 and a Williams, to Proto 2 (I provided some Amtrak horn recordings to MTH in hopes of getting those wimpy-sounding horns upgraded), so these cars will eventually run under DCS/TMCC.

I'm aware of the 20Ma current regulator, and a bridge rectifier. This train is being operated conventionally, but will eventually see its locomotives upgraded to DCS. For that reason, I understand should install a choke on each car to keep the lighting circuits from degrading the signal. I don't forsee much movement of this train under 12 volts owing to its weight (it may still need 2 locos to move it after all this) so the changes for conventional operation (to keep the lights on below 12 volts) may not be applicable. I guess I should put a capacitor in there as well.

So, in a nutshell, I'm looking for appropriate links from Mouser's site for the recommended hardware to start with (rectifier, regulator, choke and capacitor). I'll eventually upgrade the rest of my passenger cars, but for now these Superliners are the guinea pigs, so to speak.

One thing I haven't seen covered--since the Superliners have lights on both levels, can the strips on the two levels be hooked up in series off the one regulator, or should the two strips be in parallel?

---PCJ

The strips are wired in parallel.  When you look at the way they are connected end-to-end, there is a bus running down each side, so all of the three LED segments on the strip are in parallel when you use them as a single string.

I normally run the passenger car lights on less than the full rated current.  With just one strip down the center of my cars, 20MA on three groups was simply too bright!  You can fine-tune the current to get the illumination you desire.

I believe that I'll be running two strips in these cars. In the length of a Premier Superliner, I'm assuming 3/top 3/bottom or 4/top 2/bottom. 

So, if running 3 groups at 20Ma was too bright for the most part, then running six groups off the same regulator means I'll still have useful amounts of light if while running conventionally the track voltage dips below 12V (assuming I wire the cars for command operation)?

---PCJ

With that many lights, I assume you'll have plenty of light.   If you use a LM317L 100ma regulator in constant current configuration, you can select a single resistor to tune the current to any desired amount for the exact lighting you like.

Probably nothing too bright--prototype coach lighting isn't very visible in daylight. I ran across a discussion on prototype Superliners, and I can see now that I'll only need one or two tri-LED strips on the lower level.

I've never fooled around with variable regulators--I'm just getting around to understanding the function and use of the current-regulating component that was brought up in the last big LED thread (via the JCstudios article). You're saying I'd need the variable one in order to get an output under 20Ma? I skimmed the PDF on the regulator chip you mention, but I may have missed the part on how to connect it to resistors to get the desired current

---PCJ

Here you go, this is a "minimal" configuration for constant current from the LM317.  If you want to minimize flickering, you can stick a nice fat electrolytic in place of the .1uf cap.  If you happen to want more current, you can use the LM317T which handles over an amp.

Saved a copy and compiling a parts list. I'd still need a choke since these cars will eventually be run under DCS, correct?

---PCJ

There's also a 100mA version of the Supertex LED driver:

Mouser

although whether it is any more convenient than John's circuit I don't know.  The part is $1.62 at Mouser.

Yes, you'd want a choke (supposedly on the AC lines before the rectifier, though I'm not sure if that's well-established).  These are good for 560 mA and cost $0.24.

Yep, you'll need a choke, put it right at the input for the center rail to that rectifier.

John;

Thanks for posting that handy LM317 circuit.

Rod

No problem Rod, I had posted that in another forum some time back, so I saved it in case it was of use.

It's even "tested", I used this in a set of passenger cars.

I agree with the previous post, use one LM317 for all the lights.  If you end up with a total of less than 40ma, the LM317L in the TO-92 package will do the job.  Otherwise, I'd also go with the LM317T, it'll handle several hundred MA without a heatsink of any kind.

On one of my conversions, I removed the resistors for each three LED group to have a lower voltage capability.  I also suggest considering a voltage doubler for conventional running to replace the diode bridge.

          Mike,

   Where did you purchase the reel of leds?

          Thanks Bob K.~

Here you go: 5 Meter Warm White LED Strips

stan2004:

I was assuming one regulator per car. Currently I'm waiting on my reel of LED's to arrive so I can determine how many 3-unit strips I'll need to evenly light up a Premier Superliner. I'm not too concerned about the lights dimming at low speeds since these cars are dedicated to a heavy consist (Auto-Train) and won't do much running at low voltages. I'd just like to be able to run them conventionally at the outset, but be compatible with DCS operation once the locos assigned to them get upgraded.

The other details are hashing out a 'shopping list' of what other components I'll need so I can place one order with Mouser for enough parts to upgrade all 13 of my MTH Superliners (and possibly four K-Line ones--but those won't be part of the train I'm assembling). If this works out, I'll use the same arrangement on the rest of my passenger/commuter/subway fleets.  

So far as the initial Superliner project goes, each car may actually have less than 18 LED's since the lower level only has "passenger space" between the trucks, so there will be fewer lights on the lower level vs the upper. I expect to have the lower level strip jumpered to the end of the upper-level one, or vice versa.

Is the electrolytic capacitor in your photo of the "virtually flicker-proof" size?

---PCJ

If you're going with a bunch of LED's and higher current than 20ma, you'll need more capacitance.  I'd seriously consider a pair of 4700uf 35V caps for your flicker control.  I recommend dual caps because the single 10,000uf will be pretty large in diameter and may not be so easy to hide.

The cap in photo 470uF/35V.

This is a Your Mileage May Vary situation. It depends on how many LEDs and at what current, track voltage, how clean your pickups/wheels/tracks are, your personal tolerance for flicker especially over switches, and so on. 

That said, as John suggests, you need to be in the 1000's of uF (35V) for what you're doing (or what I think you're doing!).  Determine where you have space in your car and you'll probably need to use multiple (in parallel) caps. I'd plan to experiment with more/less capacitance before settling on a final answer and placing the "big" Mouser order.

As mentioned previously, if you can access/alter that 150 ohms resistor, you can get back a few Volts of headroom.  This means the same capacitor can power the LEDs for longer during a power loss. There's some arcane math we ought not to get in to, but a few Volts of additional headroom can translate to, say, 1000 uF of bonus capacitance so it's something to consider.  I don't know what strip you have but the resistor is probably a "1206" (0.12" x 0.06") surface-mount package which are a few pennies each at Mouser.

Well, if brightness is related to current, I probably want to run them at less than full voltage, and going by your experience, maybe a bit less than 20Ma. The prototypes didn't run theirs blindingly bright (that's for Horizon cars ), so neither will I. It's far from a done deal that there'll actually be 18 lights in each car. It's quite possible that I'll have fewer than that. I'll know once the LED's arrive, probably 3/30.

---PCJ

For the record, an LM317 in current mode eats up 1.2V of additional headroom.  That is the amount of voltage drop across the current sensing resistor in series with the load.

A 10,000 uF capacitor will drop only 1 volt in 1/2 second at 20 mA.

I can't believe anyone can run those LED strips at 20 mA per section!  I am using 20 mA total to drive 9 or 10 of the 3-LED groups.

I was being generous with the capacity, no disappointment that way.

Jumpering out the resistor on the strip segments basically lowers the voltage required for each three LED segment.  Of course, I discovered since I didn't want to run at full brightness anyway, that was a redundant step after the first set of passenger cars.

I agree with Dale here, if you run them at 20ma each, it'll look like the sun has settled inside your cars!

If the resistor is 150 ohms, and you are driving 5 strips from a 20 mA current source, the resistors are dropping only .6 volts.  The input voltage to the strip is less than 9.5V.  Add to this the drop across the constant current source.

Right, the resistor is sized for 12 volts at full current for each three LED segment.  It normally drops three volts.  Divide that by five and...

It remains my opinion that if there's to be a single regulator per car, it should be a voltage regulator such as the LM317T using 2 resistors as shown earlier. In this way, you do not incur the voltage tax across the current-sensing resistor. 

If a current regulator, you assume the fixed current will equally split into your 4, 5 or 6 strips. It depends on each group of 3 LEDs having identical characteristics. Maybe, maybe not.  Plus, if you add or remove a strip, the current per strip changes proportionally.

If a voltage regulator, the resistors in each strip act as ballasts of sorts to demote variations in the LED characteristics. The resistors serve to equalize the currents into the 4, 5 or 6 strips given inevitable variations in LED characteristics. If you add or remove a strip, the current per strip does not change.  

As Dale says, if you only need a few mA per strip, the voltage drop across the 150 ohm resistor becomes less relevant. I appreciate the simplicity and cost of shorting out the resistor with a piece of wire. But if you do choose to fuss with the resistor for improved low-voltage operation try, say, 50 ohms - about 2 cents from Mouser.

You can, if you like, wire that same LM317 as an adjustable voltage regulator and pick a voltage that will light your LED strips to the desired brightness.  I see no problem with that approach either.  You even get to use the same components.

          John,

   Thanks for the info I just ordered a reel.

          Bob K.~

If you want lower resistances, 2 100's in parallel makes 50, etc.

The breadboards are great for experimenting.  There are a few versions available at a reasonable price that include adjustable power supplies.  I think these are worth their weight in gold oil Lionel trains.

So for this particular application, the wattage ratings I spotted can be ignored?

---PCJ

You could add a fire extinguisher just in case.

Ron

The 4A bridge was way overkill!   For LED circuits I use a small DF0005 1A bridge, it's the size of an 8 pin DIP IC, about 3/8" square.

No wonder it was oversize . The smaller one on the RS site had only two comments--both of them negative, so I went with the other one. I'll remember your recommendation when I order the parts that are actually going into the cars.

---PCJ

I buy mine at either Mouser or Digikey.  Either place beats the pants off Radio Shack for prices.

Digikey has slightly better shipping rates for small orders, and Mouser has slightly better prices on a lot of items.  It's a trade-off which one I use, though normally if I need two or three items, I tend to go Digikey.

Well, I finally got around to assembling a test circuit on the breadboard. I figured I'd post a pic to be sure I have everything hooked up correctly:

The components are as described above. The red and green wire leaping off to the right are meant to represent the output to the LED strip.

---PCJ

A schematic would be a lot easier to follow.  Also, those two wires to the right are apparently connected to nothing, the groups at the edge are isolated from each other.

Not industry standard, but this should clarify. I moved the power input and output wires to share the same five-contact groups. The description on the back of the breadboard package suggested that the top and bottom rows were arranged into eight groups of 25 interconnected contacts.

I based this on the first schematic you posted.

---PCJ

My schematic worked, so as long as you have the pins right on the LM317, it should work.  I hope the Plus side of the bridge is connected to the positive terminal of the cap!