LEDs For Downtown Buildings

I've always believed that fast-reacting (especially silicon-based) devices such as Diodes and LEDs should be rated for the PEAK voltage. While using RMS for the resistor's power dissipation is appropriate, if you use it for sizing the resistor value then the LED will be receiving more current than it was designed for during 50% of the cycle. The mean value would be slightly worse at 56%.

Here's a nice document that covers both DC and AC LED applications as they relate to model trains. Just dug this up, or I would have shared it earlier.

http://rmdtoytrains.org/TipsTe...ues/Using%20LEDs.pdf

That is a nice LED application note, but his method of calculating the resistor power value is wrong. He used peak voltage x peak current, but he should have used the RMS values since they determine how much heat is generated. His 1/2 watt resistor would drop to only a 1/4 watt resistor.

When you work with "peak" values, you need to know what is meant. Sometimes it is "exceeding this value will kill the device", and other times it means you hit that value some of the time without hurting the device, as in the 10% duty cycle example I posted earlier. Also, it may be an electrical breakdown in the device, or it may be heating limited.

I got my LED power supply built (finally). Had to buy components off eBay so I had to wait on those.

It's an old power supply from a Dell computer. For info sake, I wired 3.5v, 5v and 12v outputs from it. My intention is to use the 5v supply with 20mA LEDs and of course 270ohm resistors. According to my brother the electronics wizard, I should be able to power something up toward 400 LEDs off this single power supply.

Anyway, it works slick, 'as advertised', in the YouTube vids. Now I just have to build my layout and eventually that will power all my LEDs.

FWIW, I'm going to build a second, I guess 'standby unit', just cuz I've got the components.

Later...

quote:

Originally posted by Bob Delbridge:
The LED "Wizard" link I have:

LED WIZARD

Bob, thanks for posting that link. For the first time, I am getting ready to wire my own LEDs and this will help me big-time. I'm planning some structures, for a Winter scene, with track passing through...there will be an arch/portal on each end with 33 LEDs on each. One building will have a total of about 78-90 LEDs. I have plenty of wall warts around for power.

A much more efficient method would be to run two-three of the LED's in series and use a smaller resistor.  That way, you'll need far less total power to light all the LED's.

I'm curious how you managed to burn up LED's using a series connection with one resistor?  I've seen 100's of resistors fail (due to overloading), and in all but one case, they failed open, thus not exposing anything else to excess voltage or current.  It's very rare for a resistor to fail shorted, that's just not the normal failure mode for either a carbon or composition resistor.

 

Of course, using one resistor with each LED will work, as long as you have the power available to use for heating up the resistors.

David or John

when i connect the resistor to the light as show in the diagram can the connection and resistor get covered in heat shrink or just the connected area get heat shrink and leave the center or the resistor open.

The resistor always has 1.2 volts across it, regardless of the amount of current.  For a 1/4W resistor you can pump about 200mA through it (.2A x 1.2V) before reaching the resistor's maximum rating IN FREE AIR.  If you put the resistor in shrink tubing, I would suggest derating it by about half, but that still will allow you about 100mA.  I don't think you will be running the LEDs that hard.

Sorry, I thought this post was regarding the LM317 as a current source.  For a 20mA max current in the resistor to a single LED, you can have a voltage source of up to about 12VDC without frying a 1/4W resistor.  Putting the resistor inside shrink tubing will reduce this rating.

This is just me but I always slide a piece of 3/32 heat shrink over the ground or common side and a 1/4 of heat shrink over the whole thing after that.
Theres no real good way to insulate the hot side because of the big bulge of the resistor
Like Dale said resistors build heat but I don't think it'll make to much difference for this application.

Now if you put your resistor far enough away from the LED you can use 3/32 heat shrink on all of it and not have to cover the resistor.
The 3/32 heat shrink is used assuming your using the solid phone wire anything larger and the 3/32 will be to small

Thanks Dale for pointing that out I forgot that but I seem to forget a lot the older I get.

David

Boxcar,
Here's some uses for the home made LED's.
Now if someone doesn't have the skills to make these all is not totally lost.
You can buy them ready made from folks like Evans Designs

This is  a coal tower and Atlas switch tower at night

Daylight , Those are home made lights 3MM warm white Christmas lights

Williams NS Exec's LED headlight, Class lights and ditch lights

Atlas station

LED's don't make any heat so you can let them lay against plastic without worrying about the bulb melting anything .
All these LED's courtesy of Wal- Mart and  Lowes  just after Christmas
I got some this last year that were 1 buck for a 100 light string on close out.



David

Nice lighting David, obviously they didn't suffer with more resistors!

Thanks John,
I think it takes a little time to get used to using these things. I hope they come up with one with a wider light pattern and I'm sure thats down the road.
They don't pull any load and last a long time, definitely worth learning to use.

David

There are some wide angle LED's, I found some a while back, haven't had a chance to use them yet.  Here's a sample test and what they look like.  First test is the wide angle LED, the second is the standard 3mm LED.  Both were running about 17MA, 20 volts with a 1K resistor.  They're being held about 6" above the bench for this test.

 

 

 

David and John nice pictures. John I came across the led you show in your picture here is the link 

http://www.superbrightleds.com...mm_High_Flux,18,1221:

If I went with these would I still need to use resistors if i wire a few in series or just added a resistor to each one. should I use an led that has a 180 degree viewing and what would be the minimum viewing angle you would use. my house are all plasticville

I think 1 light in each building should be good.

here is a video of my layout http://www.youtube.com/watch?v=Yf00WBvZMhM

For general lighting 140 degree is fine. I use 3mm 40 degree ones for headlamps. Make sure you get warm white unless you like the bluish tone. I purchase mine directly from China for 35 cents each.  For a Plasticvillle house I would mount 2 on a roof underside and let them shine down.

 

Dale H

 

I've experimented with standard 5mm LED's grinding the tip off, but I don't get the same results as the ones I posted.

I plan on powering the leds with an old ho transformer that puts out 19.9vdc.          the leds i plan to get have on hang up for me so for. the majority of the leds have a FV of 3.3v and a cfv of 30

and the other few have a fv of 2.0v and cfv of 30.  if i understand this right I should use 2 different resistor 1 kind for the 3.3v and a different one for the 2.2volt.

I used this calculator http://led.linear1.org/1led.wiz

according to this calculator the 3.3v leds should get a resistor 560ohms 1 w

and for the 2.2v it would get a resistor of 680ohms 2w.

I'd put one resistor for each led, wire them in  parallel and power 27 on one circuit.

would this be correct.

          thank you,

          Boxcar

I plan on powering the leds with an old ho transformer that puts out 19.9vdc.          the leds i plan to get have on hang up for me so for. the majority of the leds have a FV of 3.3v and a cfv of 30

and the other few have a fv of 2.0v and cfv of 30.  if i understand this right I should use 2 different resistor 1 kind for the 3.3v and a different one for the 2.2volt.

I used this calculator http://led.linear1.org/1led.wiz

according to this calculator the 3.3v leds should get a resistor 560ohms 1 w

and for the 2.2v it would get a resistor of 680ohms 2w.

I'd put one resistor for each led, wire them in  parallel and power 27 on one circuit.

would this be correct.

          thank you,

          Boxcar

Did you know that you can knock the tip off and re polish the plastic with a buffing wheel and some jewelers rouge .
I bought some 2mm LEDs from a company to use with my DCC decoders for headlights
and they actually recommended doing that to shorten it if needed.
Well I needed to and it worked.

David

Hey BC,
Why don't you fix that HO transformer so the handle is at around 12 to 14 volts and make a stop so it can't go any higher.
That way you could use a 750  Ohm 1/4 watt resistor and believe me they'll still be plenty bright .

David

I do that all the time.  I just did that for the LED's I installed in a Thomas locomotive to get them the right length.  It's a painless job, I hold them against the grinding wheel for about half a second, then hit them with the buffer with polishing compound and they're good.  Takes me a minute or so to do several.