I'm preparing to add some lighting and have on hand warm white 5mm LEDs stripped from christmas lights, resistors, a 12v regulated, 1A power supply, and also strip LEDs that can be cut into 3-LED strips with the right resistor for 12v DC. Some final questions:
1) I presume I should add a fuse so it blows when I goof instead of ruining the walwort. I see Radio Shack has fuse holders and 3/4 amp fast blow fuses. Is that what I should add?
2) How do I know how many LEDs I can expect to put on each walwort system (with a 3/4 amp fuse). I think I've seen that LEDs are typically active at 20-30ma. That would say about 25 per power supply with 3/4 amp fuse? Or should I measure one (if my meter can) with a 1k resistor hooked up to the 12v supply to get a better idea?
3) Same question, how many can I use, with the strip LEDs. As I understand those, there are three in series plus a resistor (in series with the LEDS?). Anyway, I have no idea how much current each small strip of three may demand, but maybe only 20-30ma per set of 3, since they are in series? Again, should I just measure it?
4) I presume I can mix single LEDs with their 1k resistors with 3-LED strips in parallel (for other structures) on the same lighting bus, since they both require 12v and will be in parallel.
Thanks for any advice as I get set to move forward.
Paul
Modeling HO with a transition era UP bent
1) that should be good
2) if you know the LED's forward voltage, you can calculate the current with a 1K resistor - with a typical white LED and a 1K resistor at 12V, it's about 10ma per LED. Not knowing the specs - hook one up and measure it with the meter. The current spec on an LED is the MAX it can take. How much it actually draws depends on the forward voltage of the LED and how big a resistor you use.
3) again, measure them. Cut at the proper points, you end up with 3 LEDs in series with the proper resistor to feed the string from 12V. However, the LED light strips generally run the LEDs much closer to maximum current, for the brightest light output.
4) as long as you are wiring in parallel, yes. Add up all the currents of each parallel circuit, and keep it below the 750ma of the fuse, and you will be all set.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Re 3), you can add additional resistance to the 12 volt 3-LED light strip segments.As Randy noted, they'll be awfully bright without it. If you add enough resistance, it turns out each individual "LED" is actually three smaller SMD LEDs behind a diffuser.
I haven't tried to separate the individual LEDs segments so that it could be used as a standalone light fixture, but it's a nice effect that might prove useful because it looks like a 3-lightbulb lamp fixture.
Mike Lehman
Urbana, IL
Thanks for the info. I've got the fuse holders and some 1/2a fuses for initial setup, with 3/4a fuses on order. I measured the current use at 12v for the christmas 5mm LED with 1k resistor that drew 9ma, while the 3-LED strip with internal resistor drew 25ma.
I installed two LEDs in the roof of my Cornerstone Industry Office and quickly learned some other important things. I had the interior walls painted with gray primer. The building sitting on the earth brown painted plywood did not look very "lit". I think that's not so much a need for more light output but more an issue of interior colors (walls, floor, etc. I added a basswood light colored floor which added lots of brightness, but that makes it look ok from head on but when looking down at a normal viewing angle the floor looks rather bright, so needs to be a more normal floor color. So at least on other structures I'll consider what color floor and interior walls works well with combinations of light output. Fortunately I have not jet added window materials (other than to the office) so I can brush paint the interior wall colors over the gray primer to a more typical, lighter wall color. I have a number of structures that have just one or two small windows that won't take too much optimizing, so I'll try those next.
On the 2-story Laube Linen structure I'll need to add a floor for the 2nd story, some interior walls, a few window blackouts and shades, etc. Not sure how to add 2nd floors and make them removeable if needed (e.g., to fix a window that might fall loose) so any hints there are welcome. Maybe just barely cement the 2nd floor in so it can be popped loose if needed.
If the LED is giving too much glare off a floor or just in general in a "room" I often take my punch tool, take thin white plastic sheeting and punch out a "lampshade". I use a dot of CA to glue it to the bottom of the (presumably downward-pointing) LED. If you need more extreme control, I've done the same thing with black construction paper. And you can always add more resistance in the form of additional resistors.
Another dressier method useful where the lamp is in sight is to take a slice of white plastic and slide the LED into that. It's largish for HO for most fixtures, but can also be used inside a building on a LED to mellow it and reduce glare.
Speaking of paper again, rooms that are dull can be brightened by cutting white paper to line them, which is often easier than painting them once aasssembled.
For a removable floor, working from underneath glue a thickish piece of plastic or angle at the floor level on each of two opposite walls of the inside of the structure. Cut a piece of stiff cardboard or thickish plastic so that it's too long to fall through when sitting level on top of the angles on each side, but can be tilted up at one end to slip out of the bottom opening. It just lies there, but it's often possible to install figures, furniture, etc on top of it and still get it to tilt into place, depends on how much overhead clearance you have.
You're doing the right thing with the fuse. I've got my light circuits fused, too, and it's saved the power supply on more than one occasion.
I would use a meter to check current. To measure amps, the meter needs to be in series with the circuit. This is awkward if you've soldered everything, but one trick is to remove the fuse and put the meter probes across the fuse holder.
It takes an iron man to play with a toy iron horse.