One of my plans is to install interior lighting in all my passenger coaches. I have tried some of the Märklin units such as the 73400 but I want something much better and way more economical so I am keen to hear from knowledgeable electronics folk on this topic.....

Due to the large number of coaches, I have decided I want to use track power. I am aware of the battery solutions and don't want to discuss that in this thread.

I plan on connecting the coaches together so that I do not have to have one pickup shoe per coach. This will reduce cost, drag and noise.
The connections between coaches could be permanent, or use some small plugs.

Here are my 'requirements and goals'

• Use the typical off the shelf 12V (or 24V) LED strips that can be cut to length.
  
• Use digital track current as power supply
  
• Reduce the brightness of the LEDs
  
• Not waste valuable digital track current
  
• Be flicker free (At the Mhz level and short power drops)
  
• Not have a large inrush of current when power is initially applied
  
• Not generate unnecessary heat (some coaches have plastic rooves, and I don't want to waste the energy)
  
• Electronics must be small enough to fit inside a coach or Gepäkwagen

I don't think I need to be able to switch them off digitally, but that would be a nice feature to save power in hidden staging yards. An input contact that could be connected to a cheap decoder to switch it will be a very nice feature. (The decoder would only indicate the switching, not supply the power.)

I know how to rectify and smooth the digital current.
I know how to add a capacitor to prevent flicker.
I think I can find a circuit to reduce inrush.

What I would like to solve is how to reduce the brightness of the LEDs for better visual and less power consumption, without generating heat through a resistor load. I think that a pulse width dimmer would be ideal, allowing the LEDs to run at 12V but not all the time, thus avoiding waste through heat.

I am aware of small buck converters that convert DC voltage to lower voltages with good efficiency.
I am wondering what happens if one does not rectify the input current to a buck converter.
I am wondering if adjusting the output voltage of a buck converter instead of using PWD is a good approach.

I am open to designing a small circuit board and having them made, and then adding the components myself. My plan is to add a pickup shoe to a coach containing my circuit board, then take the output of the board through all the coaches in the rake to provide power to their LED strips.

Obviously, such a circuit board will have some upper limit of how many LEDs is will be able to drive. That limit will simply determine the length of the rake powered by one board. If I have to light a train longer than that, I add another pickup shoe and board. The 12V LEDs at full brightness consume about 80mA per 20cm.

Should I look at creating a PWD circuit using something like a 555 timer?
or would a rectifier + buck converter be a better approach? (and more available)

Would 24V LED strips be better than 12V? They would require less reduction in voltage from the rectified track current (25V?) and also perhaps require less current?

ah yes, thanks, I do recall that thread, and I had forgotten that great tick of running the current through two strips. We could run two cars at once so each car has one strip. I will read through it again.

Do you have all 40 on the layout when you power up? If so do you have inrush issues?
How hot do the resistors get?
Are they overly bright?

I can separate the circuitry from the pickup shoe, so small simple circuits in each (or every other) car could all be fed from a single pickup.

I have used many, many of these. Come w/bridge rectifier, rheostat, and capacitors. Hard to build your own at these prices.

https://www.ebay.com/itm...311b9:g:wLUAAOSwaeRbOQcX

Are those $25 each Mark or am I reading it wrong? The topic in the link I posted includes a capacitor to reduce flicker for about a tenth the cost.
Thanks,
Roger

Hi Dale/All,

My comment is to keep it simple and keep away from LED light strips. Have a look at some of my LED lighting projects under my Tips section on my Web page for ideas.

I have had no issues with the strips except the adhesive on the back could be stronger. A little contact adhesive fixes that. I have used two spools so far and have had no failures.

Hi Dale/All,

Hi Dale/All,

The 15Vdc is obtained when I use a 6080 decoder with a bridge rectifier as I have shown in my circuit diagrams for the lighting articles.

This is what I suspect, and why I think the solution is not to use a resistor but a pulse width dimming approach, ideally as part of the converter from 18 to 12V. In other words a small module that takes the digital track current, rectifies it, buffers it and puts out a 12V PWD output. I think that should get rid of all the waste.

Those LED strips usually seem to have three LEDS in series with a resistor that is sized for 12V operation. The power 'rails' run all the way down the strip, positive one side and negative the other, but only every third LED actually makes a connection to it. This is so that LED strings can be cut to length without the end user having to do anything special other than use a pair of scissors.

So by removing the resistor on one set of three LEDs and cutting the track to the power rail on the adjacent set of three LEDs and connecting the disconnected LED to where the resistor used to be you should get a much more efficient use of the current. At digital supply voltages you may be able to connect three sets of LEDs, i.e. nine LEDs in total, in a string and still have it work. To adjust the brightness you could even remove the resistor from the strip and then you can fine tune the resistor value externally if it isn't bright enough, although that would surprise me as the white LEDs are very bright even at very low currents.

Further confirmation that a switching converter is the direction to go...
see https://www.instructables.com/id/Poormans-Buck/

The PWM regulator can be easily purchased from eBay. Search keyword: "mini DC regulator". Look for the one which using PWM chip MP2307. I found that this chip is small and reliable to work on intermittent rail supply (dirty track), but its max input voltage is just 23 volt, therefore is not suitable for Marklin analog with high reversing voltage. I bought it from eBay, 10 pieces US$7 (less than $1 per piece)

Hmm yes well... at least it does include a decoder at that price...

But a single function decoder can be had for less that $15 and the rest is less than a dollar...

I do add a capacitor at the output of bridge rectifier (the input of PWM regulator).
Capacitor value: 47uF 25V Panasonic, from Digikey.com at 27 cents each. This capacitor size is just around 7mm x 6mm, small enough to fit into the coach's corner.
Theoretically, bigger value is better, but the size will be larger too, and if you have many coaches on the layout, then the combined in-rush current during power-up may trip the controller.

"Buck converter" is the term widely used by common public.
Technically, this device is called "switching regulator". Since the output voltage (or current) has to be regulated, therefore it need a method to modulate the switching. Common switching modulation method are: PWM and PFM (see the basic here: https://micro.rohm.com/en/techweb/knowledge/dcdc/s-dcdc/02-s-dcdc/90 )
My device is using PWM method (switching frequency is fixed at 340 KHz), therefore it is simply called "PWM regulator" (to differentiate against other type).
Most of switching power supply, use PWM modulation with fixed switching frequency so that the inductor/transformer can be optimized into single frequency.

well $15 decoders can have 6 functions, but still irrelevant if I only need one function.

PWM regulator: 36c
capacitor: 27c
LED strip: 50c
some wire: 5c ?

I use Layouts4U units that incorporate a reed switch. At £6 per unit, they are very economical. Each coach has a battery and you turn the lights on or off with a magnet. The batteries are very cheap on Amazon - 10 packs of CR2032 are just £4.99.

Note: that this is fine if you do not have boosters. If you have more than one power district, this will likely connect the center rail of two districts when the train runs from one to the other.

I would first look at additional capacitors, to handle short power interruptions before adding a second slider.

I replaced the old Marklin slider with hole on my Rheingold F-train set with a Brawa slider. It actually removed 95% of flicker - it's pretty rock solid now.

Bad flickering probably indicates poor slider contact, likely between the leaf spring part and the slider rather than the slider to the studs.

No
It is like to use the ground (brown) instead the U+ 18 (Orange) in a loco light

Costas

if we step back from the double conversion and power cap and go back to the simpler:

Center rail pickup -> Bridge rectifier -> DC-DC regulator -> Cap -> LED strip

Can the ground of the LED strip go to the common rail ground?

thanks again, I never knew that.
I had built full bridge rectified circuits that use a separate 16VAC supply and sharing the same ground, but of course that is NOT using digital power from the rails.

So:

Pickup shoe - half wave rectifier -> capacitor -> DC-DC regulator -> LED strip(s) -> rails