Dale

Just info the subject has been discussed and I think Gunner John was the main guy who did the show and tell when he made the first one I know of.

I just did a advanced search  with:

computer power supply

and lots of information to read if you want to .

I actually copied his and it's here somewhere in the train room and I never used it.

Larry

You can run as many LEDs as you have current for. Be sure to know the specific specs of your computer power supply. Generally they are marketed to the consumer in a single WATTage rating, but you really need to know how many buss rails it has and what current is available on each. Typically there are multiple 12 V rails, 5V rails and 3.3V rails. Once you know the current supplied by each rail, you can then safely use ~80% of that. So take that 80% figure, and then subtract the current of each individual component on that rail. Once you get to Zero, you have reached the maximum capacity of the rail.

FWIW I have a computer power supply listed in the For Sale part of the forum for FREE - pay shipping only. I would love to see it put to good use by a fellow forumite.

For LEDs, most want to see about 4.5v.    You might get by with 5.0 or even 5.5.    But I think also if you have a 3.5 tap, that will work.

the description above for amperage (power draw) is what you need to do.   the LED package should tell you amps, or watts per LED or group.    Usually for LEDs, this in milli-amps (thousandths).    so if the LED draws 5 MMamps,  and your power supply puts out 5 amps, then 80% would be 4 amps which is 4000 milli-amps.    4000 divided by 5 is 800 individual LEDs for such a power supply.

Most LEDs don't like overvoltage at all. It's not worth the gamble to run over until you know the deep specs well enough, it will be bright long before you reach the peak voltage if fed the (tiny) amps it needs anyhow. Offer them extra amps, not extra voltage. 

  Use the resistor(s) to tune brightness properly too and voltage is less of a worry. Start with high resistance/low volt then lower resistance. Near 0Ω but not happy? Then raise resistance again, now raise volts, now lower resistance again, etc etc. (try 10-500Ω variety packs, then buy more of what you need). 1 to 4 Regular Diodes could drop the 5-6v to 4.5v easily too. 

@prrjim posted:

For LEDs, most want to see about 4.5v.    You might get by with 5.0 or even 5.5.    But I think also if you have a 3.5 tap, that will work.

Not even close!  4.5V is way too much for any common LED, the white ones operate in around 3.0 to 3.3 volts max!  Here's a current vs voltage graph for typical LED's of various colors. the gray line is a typical white LED.  You'll notice at 3.0 volts it's right at 20ma, it's nominal current rating.  At 3.5 volts, it's already at 40ma, twice it's maximum current rating.  At 4.0 volts, it's up at 60ma, three times it's current rating.  By 4.5 volts, it's at 90ma, and probably only going to last for seconds before it croaks!  5.0 volts?  Off the chart and flaming!

I use a modified pc power supply on my layout for dc power for LEDs andlamps,  however, I use the LEDs on a re3l with the dropping resistors already built in so I use12volts. This approach in my opinion is much more flexible than trying to drive LEDs directly at a lower voltage.  The supply I picked had 4 isolated 12vdc outputs at I believe 10 amps each.

@gunrunnerjohn posted:

Not even close!  4.5V is way too much for any common LED, the white ones operate in around 3.0 to 3.3 volts max!  Here's a current vs voltage graph for typical LED's of various colors. the gray line is a typical white LED.  You'll notice at 3.0 volts it's right at 20ma, it's nominal current rating.  At 3.5 volts, it's already at 40ma, twice it's maximum current rating.  At 4.0 volts, it's up at 60ma, three times it's current rating.  By 4.5 volts, it's at 90ma, and probably only going to last for seconds before it croaks!  5.0 volts?  Off the chart and flaming!

Definitely hear what John has stated, I have had a few LEDs actually blow apart with to high voltage, never thought it would happen, some others just burned out.

Ray

I use a computer power supply for 12 VDC lighting and 5 VDC signal system power.  It was very easy to do using one of these boards that plugs into the big power supply output.

https://www.ebay.com/itm/24Pin...047675.c100752.m1982

There are other versions available as well, much easier than trying to figure out the output wiring yourself.

https://www.ebay.com/itm/24-20...047675.c100752.m1982

It's no problem to use LED's with higher voltages, it's just MANDATORY to have current limiting resistors matched to the supply voltage.  For common 20ma LED's, the math is real easy, 50 ohms for each volt you need to drop.  So, if you have a 12V power source and a white LED which requires 3V, you need to drop 9 volts, which equates to 450 ohms.  Since 470 ohms is a standard resistor value, that's a perfect match.  If you had a 5V supply, a 100 ohm resistor would be called for to use that white LED.

It's fine to use more resistance to get less intensity from the LED's, but exceeding the maximum sustained ratings of the LED will greatly reduce it's lifespan.

Note that the color of the LED influences the operating voltage, IR LED's take the lowest voltage, and the UV ones operate at the highest voltage.  The increase in voltage runs up the rainbow, very convenient.

@Dale Sr posted:

 

i have gone to using LEDs for all my lighting on my layout. using them for all my buildings and signage ect. i had read that using a power supply for computers  works well. i will be installing over 350 LED and wanted the advise of anyone that has done this, how well it works, how many LED's can you install on one power supply zand any other bugs or trip ups there would be to using them?

First of all, i want to thank you all for your advice. A lot of good info from you all. The LED's I have bought already are pre wired for either  3v or 9-11v. I am having issue it seems as when I turn on more than 20 or 30 LED's at a time they start to go dim or turn off. Any thoughts?

Overextending the power supply abilities would be my first thought. (Although it would have to be a very pitiful power supply )  It comes down to the math mentioned earlier where your current draw needs to be less than the current available.

Maybe a picture of your setup would help, along with the exact specifications of your power supply?

In addition to identifying your present power supply, what are the make/model of the pre-wired LEDs.  I can understand a pre-wired 3V LED intended for operation with a 2x 1.5V battery pack.  But a 9-11V pre-wired LED is somewhat curious as that's not a commonly available DC voltage range.  I'd say the most common DC voltages for hobby type projects are 5V or 12V.

@Dale Sr posted:

first of all, i want to thank you all for your advise. a lot of good info from you all. the LED's i have bought, already are pre wired for either  3v or 9-11v. i am having issue it seems as when i turn on more than 20 or 30 LED's at a time they start to go dim or turn off. any thoughts?

It would have to be a pretty small power supply to be overloaded with 20 LED's!  That would be a max of maybe 400 milliamps unless there are specialty high-intensity/high-current LED's.

the power supply is a off the shelf PC power supply from our local PC shop. it is rated at 280 watts. 5v @ 14.A, 3.3v @ 18A, 12v @ 17A. as far mas the LED's, the 9-11 v is just the everyday stuff on Ebay. i have it set up as 6-8 LED's in each building, all buildings on a separate switch. 8-10 buildings on a separate circuit from the pc power supply. im also running another pc power supply that i dont have the specs for it, other than i know its a 550 watt. i would of thought i had more than enough power to do about anything, but having the same issues with both power supplies. i my have mixed a 3v with another voltage LED? that ill have to check. would that cause the issues ? again. very new to this .

I presume these are ATX power supplies. Did you ground out the PS_ON pin to power on the power supply? It seems you may have simply tapped into the +5 VSB ( standby power)?

if you are talking about the green wire, yes i took the green wire and took it to a black wire. i am using both the 12v and the 5v and it seems to make no difference what one i hook up to. after i turn on 3-4 buildings ( 16-20 LED's), when i turn on the next set in the building they go dim or they all go off.

@Dale Sr posted:

... the 9-11 v is just the everyday stuff on Ebay. i have it set up as 6-8 LED's in each building, all buildings on a separate switch. 8-10 buildings on a separate circuit from the pc power supply. ...

Seems to me that a pre-wired LED would be 12V...as opposed to 9-11V.  That is, pre-wired suggests easy-to-use, plug-and-play, or what have you.  12V DC (or 5V DC) would be easy-to-find.  9-11V no so much

So I looked on eBay and lo and behold there are buckets of 9-11V pre-wired LEDs!

But there's something fishy here.  These pre-wired LEDs have a resistor buried in the wiring as illustrated above.  Is this the style that you have? 

As GRJ notes earlier, there are specialty high-power LEDs.  For example here's one specified for 9-11V:

But these high-power LEDs at 3 Watts would essentially blind you and be inappropriate for O-gauge building lighting!  This class of 9-11V LEDs are meant to be driven by a specialty power-supply that regulates current (aka current-source)...not by a 12V DC computer power supply voltage output.  So unless you have a pre-wired 9-11V high-power LED, this sidebar is not relevant to the matter at hand.

Stan, the other fishy thing is they're awfully expensive for a hank of wire, a one cent resistor, and a 5 cent LED!

FWIW, I have a couple of work lights with those "blinding" LED's, and they would overpower most any lighting task on a model train platform!  They're great for throwing light on the subject, but not for the use here.

Stan, what is the source of the 5 V LED strips that you have in the photograph in this queue.

Search "5V led strip" on the web, eBay, Amazon, wherever and you'll get thousands of hits.  Narrow it down by adding, say, "5050" to specify the size - I believe the pic shows 5050 (i.e., 5.0mm x 5.0mm) LEDs.  Or add "warm white" if that's the color you want.  It's been a couple years but I'm pretty certain I got them from eBay (Asia seller).

Addendum:

Was preparing a post for another building LED lighting thread and came across this OGR thread showing how even Menards is using a 5V LED strip for one of its buildings!  The thread has some additional details on buying the 5V LED strip as the OP needed to repair/replace his 5V strip.

Thanks for the help,stan2004, With your help I found them.