Searchlight Tower Conversion to LEDs

In case it's not obvious, you want to wire the LED's with opposite polarity if you want to run on AC, that way they'll protect each other from high reverse voltage.  It's a lot more common to have AC floating around your layout, so that may be a better option.

If you add a cheap bridge rectifier, it'll run on both AC and DC, in that case you'd use your first drawing and just add the bridge.

The revised drawing WILL NOT protect the LEDs from reverse voltage.  You must tie the LEDs directly together back-to-back and use a SINGLE dropping resistor.

I don't understand why you are using two separate parallel legs, rather than one leg with the two LEDs in series.

Right Dale, I didn't look close enough!  In any case, I'd add the bridge rectifier if I were going to the trouble, then it could run on AC or DC without any risk.

Bob

I will let Dale M look at the other diagrams. But if you put the 2 LEDs in series with the bridge,only one resistor is needed and it will shed less heat. This is not a big deal with the 20ma standard LEDs but if you go with the 100 ma ones I linked the series wiring becomes very advantageous.  

Dale H

The AC/no-bridge diagram should have only one resistor in series with two back-to-back LEDs.  Move the resistor to one of the feed legs.

On the electrical forum I was looking for something to lower the voltage to a Christmas mini light for a neat road sign.  What kind of resistor would I use to lower the voltage to no more than 5 volts.  Everything is running off track voltage at 15 to 18 volts.

I'd probably consider a bridge rectifier and a three terminal voltage regulator for this application.  What kind of current does the road sign need?

AC current, I presume.  No more than 5 volts.  After that, the bulb blows.

Current, not voltage.   I'm assuming a bulb will be happy with AC or DC.

A 7805 circuit could work. However without knowing the amperage draw of the lights (not sure whether they are LEDs or not)  It is impossible to figure the load on the regulator. The 7805 is good for 1 amp if a heat sink is used. At 18 volts input the wattage  used by the regulator would be 13 times the amperage drawn. I would not draw any more than .2 amps without a heat sink. The regulator would be practical for LEDs.

Here is a 7805 circuit driving 6  LEDs mounted without a PC board. Since the current draw is low (.1 amps) a heat sink is not needed. The regulator at most would shed 1.5 watts of heat.

http://www.jcstudiosinc.com/Bl...8&categoryId=426

Another way is to put a string of diodes in series from the track voltage.

Dropping voltage with diodes is described in the below link.  If in series to light bulbs, a single string of diodes could drop voltage, and supply pulsed half waved DC for the light bulbs. 7 diodes in series would drop 18 VAC to about 5  VDC (half waved pulsed current) RMS . I think this is right if I calculate correctly. The diodes drop fairly consistently regardless of the load. There is some variation but that is not important to this application.

http://www.jcstudiosinc.com/Bl...3&categoryId=426

Dale H

The one with the bridge rectifier is good for AC or DC, it's a universal power model.

Got it John. The sketch is now ready for public consumption. Thanks John, Dale & Dale for sharing your knowledge. More folks like me are going to love this forum more because of you all.

One last change.  You need to indicate the + and - pins on the LED's.  Is that what the red and green are supposed to indicate?  What about the blue wire?

If the wire color is the key, perhaps a mention of that.

BTW, what program are you using to do the drawings, I like the look.  It's not a schematic, but rather something that folks might more identify with.

Nice job Bobby.

Love the drawing, that looks very useful.  I'd like to get that program, whatever it is.

Yes, the drawings are top-notch. 

Some suggestions for your write-up:

- A comparison of the power savings of your LED method vs. incandescent would be interesting

- Don't know how tight the wiring is, but AWG 26 or 28 have the current carrying capacity and might be easier to dress

- I'd also go with the series method for the DC-only circuit. It would consume half the power for the same overall brightness.

- Perhaps you're still working out the resistor values, but values need to be adjusted for the different circuits.  The resistor should be much smaller for the series method since you only need to drop the voltage down to ~7V (2 LEDs) instead of ~3.5V (1 LED). 

- For the back-to-back AC-only method, I think it's worth pointing out that each LED is on during alternate halves of the AC cycle.  In these situations, most LEDs can be driven at double the current (e.g., 40 mA) during half-cycle when on such that the average current is 20 mA.  If you've included this adjustment, it's worth pointing out.

- Does the Radio Shack LED have some kind of polarity indication such as a flat side on the package indicating "-"? That might be nice to include on your drawings.

Well, the power savings will be SIGNIFICANT!

Truthfully, I'd probably dispense with the resistor and put in a CL-2 constant current chip.  That would make the voltage universal and the brightness would be constant over a wide range.  The CL-2 is only about half a buck, so it's not exactly cost prohibitive.

Hi John,

I guess there are many solutions to lighting LEDs, but for us novices chips are in another galaxy. As for the program I use to draw sketches, it's MS Paint. Paint takes some practice and its tools are somewhat inefficient, but when Paint was new it was the cat's meow.

WOW!  You did that in MS Paint?  I'm stunned.

In my engineering days I had to acquire illustrating skills, so Paint is just another blank page for sketching, but with a mouse instead of a pencil.

Do you use clipart to create those?  I typically use Visio for my graphic stuff.  In any case, a very nice illustration job.

John,

My artwork is 100% freehand. No clip art.

OK, I'm officially in awe!  No way I do that kind of stuff without some graphic assist.

I am also very impressed with you artwork. You should sell your services to Lionel, MTH and the others!

Ron

Bob Did this for me taken from a rather sloppy hand drawn schematic I sent him. The second picture is the train stop circuit made up mounted on a piece of plywood. An explanation of the circuit is here

http://www.jcstudiosinc.com/BlogShowThread?id=800&categoryId=426

This man certainly has a second career if he wants one!

I prefer a schematic myself, but many folks really need something like this for wiring projects.

John - I am one of those folks who the illustrations benefit. I do not have any sort of electronics education. I can assemble anything and follow directions to a T (ask my wife ).

Anyhow, I love the stuff Dale H creates but I find it hard to follow the schematics. But the time I figure out what the symbols mean and translate to a shopping list I have forgotten what I was doing. What Bobby does with the illustrations is perferct for me. I cannot tell you how great this is.

Having illustrations to go with all the great info you guys provide is priceless!!!!

Thank you all who contribute.

I know many folks really identify with that kind of diagrams, and those are really done very well.  I'm blown away he does all that with no clipart, very talented!

Thank you all. I am glad that some folks benefit from my sketches. There will be more as I learn from this forum.

Hi Guys,

It was only recently that  I realized that there was an electrical forum. This thread is really good. I, too, am in awe of the artwork and the advice from members. The Dales have helped me in the past and gunrunnerjohn as recently as yesterday.

Good work guys!

Cheers

Denny

Denny, it is good to have you back as an active participant.

The keep moving it around to confuse us.

Hi Dale,

It is good to be back. I travel all over the world and find it difficult to reengage in the hobby when I am totally jet lagged. I was fortunate to spend 4 weeks in Wisconsin recently and managed to build a second control panel. My enthusiasm is high at the moment so I hope that I can keep the momentum going. I am in Germany as I write this and am leaving for China tomorrow only to return to Wisconsin on Thursday. I have high hope of connecting that control panel to the layout when I return. I cannot let it take me two weeks to reengage again. If it does then I am on the road again and miss my window of opportunity.

I really appreciate the contributions that you guys make to us neophites. And to think that you have been doing it for many years!

Cheers

Denny

World traveler!   I'd love to get over to China someday, just to see it.