Electronics and DCC

I am building a modest layout (11x18) http://cwhowell2.com/modelrr/modelrr.htm

I have added a lighting buss using #14 solid wire.  I plan to use all LEDs and low voltage bulbs but have no idea what to use as a power supply.  I'd prefer a plug-in solution and would rather not have to build something from components.  Any ideas?  Thanks - Bill

Have you considered one of the heavy-duty transformers designed for low-voltage outdoor lighting circuits?  They are about 100 or 120 VA output at 12Vac.  To convert the output to DC, all you need is a high-power bridge rectifier.

Rather than a single humongutransformer, I power the several zones of my double garage filler with smaller power supplies, including toy train transformers and somewhat more powerful supplies based on 12.6V 3A center-tapped filament transformers.  The latter are especially useful for low-voltage lighting, since they have effectively two separate 6.3V 3A circuits.  Since there is no valid reason for scenic lighting to be powered from a layout-wide bus, I don't.

Chuck (Modeling Central Japan in September, 1964)

 

I'd suggest a 12V DC supply.  The question is how many LEDs do you think you'll be powering ?  Assume 20ma for each.  So for a 1A supply it will power about 50.  The largest "walwarts" are in the 1.5 - 2A range.  You can get a 12V DC in this range for $15 from places like Jameco Electronics.  Beyond that you can go to a larger power supply in the 3-4A range.  You can find them for around $30-40.  You can also split your 14ga bus up into multiple lighting districts and power each one from a walwart.  For each LED you'll need to use a resistor to drop the voltage.  I'd start with 470 ohm 1/4 watt resistors and see if the LEDs are too bright or too dim.  I wouldn't suggest trying 1.5V DC supplies just so you don't have to use resistors.  You may find yourself needs the 12V for other things like panel indicators or other lighting fixtures that won't run opff of 1.5V .

 

You can also run the LEDs in a series/parallel arrangement to save a lot of wasted power and thus need smaller power supplies.

Karl

larak:

You can also run the LEDs in a series/parallel arrangement to save a lot of wasted power and thus need smaller power supplies.

Karl

Agreed.  It just makes troubleshooting more tedious but you can power more LEDs off the same supply.

 

I picked up a couple 12 volt AC transformers at Radioshack really cheap. One is 4 amps. As for the LEDs, they may be more trouble than they are worth. Regular bulbs give more natural light, and don't need resistors. Given the few hours that most of us run our trains, you aren't going to save enough money on electricity to bother with, using LEDs.

I've been saving the power supplies to my old cell phones and accessories. Most are around 5 Volts and 500-800ma. I don't think I really need the extra voltage of a 12V PS, but I'm no electrical genius.

hobo9941:

I picked up a couple 12 volt AC transformers at Radioshack really cheap. One is 4 amps. As for the LEDs, they may be more trouble than they are worth. Regular bulbs give more natural light, and don't need resistors. Given the few hours that most of us run our trains, you aren't going to save enough money on electricity to bother with, using LEDs.

If those Shack transformers are filament type, they have the center taps and two 6.3v circuits I mentioned earlier.

I use 2.5v clear miniature Christmas lamps in strings of three (for white light) or four (for yellowish light) across that 6.3v.  I got a whole case of 300-lamp strings for about /lamp the week after Christmas, so a multi-lamp string costs less than the resistor, never mind the LED.

Chuck (Modeling Central Japan in September, 1964 - as cheaply inexpensively as possible)

gpa:

I've been saving the power supplies to my old cell phones and accessories. Most are around 5 Volts and 500-800ma. I don't think I really need the extra voltage of a 12V PS, but I'm no electrical genius.

No, you don't need the extra voltage. You can operate LEDs from any D.C. voltage. It's just the size of the resistor that would change with different voltages. You should be able to operate 20-25 LEDs from those power supplies. A 150 ohm 1/4 watt resistor should be about right for a 5 volt supply.

The rule of thumb arithmetic for LEDs is quite simple. To determine resistor value, subtract 2 from the power supply voltage. Then divide that answer by 0.02. Use the next largest standard size resistor. 1/4 watt resistors are fine. To find out how many LEDs you can operate from a power supply, divide the millliamps rating of the power supply by 20. then take 80% of that. That's the number of LEDs it will handle with a bit left over for safety so as not to stress the power supply. Most of those chargers are built right up against their limits so it's not a good idea to load them right up.

Hope this helps.

  Most LED's will handle no more that 2-3 volts and draw about 20 ma.  I have used a old cell phone charger 'wall wart' that puts out 3.3 volts DC - it has a 350 ma capacity and I have about 15 'sunny glow' LED's wired in parallel to it.  The 'load' drags down the voltage output to about 2.2 volts DC - perfect!

  If you are going to use an old 12 volt power supply: you will need to put a 1000 ohm resister in series with each LED to protect them.  I would use parallel wiring - series is just to troublesome when something does not work!  BTW, a lot of the new LED Christmas lights have a 'cone' LED that shines light over a wide area, rather than the directed light we see in loco headlights.  These are perfect for building lighting!

Jim