# ATX power supply for tortoise use



## Hotrackguy (May 11, 2018)

My layout is set and I'm now working on a control panel to display a diagram of said layout. My plan is to have toggle switches and LEDs for all of my turnouts (actuated by Tortoise slow motion switch machines). I have an ATX power supply I plan to use to power the switch machines.

My power supply is 500watts / 26amps. I have tested it hooked up to 2 Tortoise machines at the same time from the 12volt output (with 2 dual color LEDs each) and everything works just fine. Will it be enough to run all 12 of my Tortoise machines (with a total of 32 LEDs constantly lit [1 for the PS STBY, 1 for the PS PWR. 6 for manual throws to indicate switch position and the other 24 for the switch machines]) or will I need to break it up into two sections and get another power supply?

Thanks in advance - Dave


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## MichaelE (Mar 7, 2018)

ATX power suppies are a god send and provide cheap power for model railroads among other things.

Your power supply should work just fine, and you might find a use for the 5vdc and 3vdc rails too.

I'm using an ATX for a battery charger for my Kenwood TH-D74 radio. Works great.


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## Lemonhawk (Sep 24, 2013)

Hmm, I bought a 3d printer a few years ago and am powering it with an ATX power supply. There were lots of warnings at the time about doing this, in fact there were several reports of the printers catching fire, and all were apparently powered by ATX power supplies! Notice, however that I am still using the ATX power supply on my printer! My theory is that they were using a cheap ATX power supply. Remember that a PC uses gobs of 3.3 and 5 volts but little 12 volts, so they may not be good at producing 12 volts. The 12 volts for my 8 turnouts with Switch Master motors, all the indicator LEDS and power to the 2 DS64's is supplied by a large 3.5 amp wall wort. Much simpler and probably a better PS for the 12 volts required by the railroad.


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## wvgca (Jan 21, 2013)

uh .. better off with a wall wart ... they are cheap, usually only put out one voltage , and a lot smaller and safer to hook up ..
you can get one for a couple of bucks out of china complete with a UL sticker...


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## DonR (Oct 18, 2012)

Yes, I agree with W\VGCA, 26 amps is a major overkill and possibly an
electrical or fire hazard. You would be much safer and spend little or no
money with a wall wart that has a 12V DC output. 1 amp would be sufficient.

Don


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## MichaelE (Mar 7, 2018)

26 amps is overkill for a computer too that doesn't draw a quarter of that even with a high end video card on any one leg. That is the power supply's total capacity for all legs. Typically, one 12v leg and three or four 5v and 3.3v legs.

The 12vdc leg is not capable of 26 amps.

Your ATX supply will work just fine and is not a fire hazard.


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## flyboy2610 (Jan 20, 2010)

I converted one for my layout.
Computer power supply for layout power | ModelRailroadForums.com 

It works fine.


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## Hotrackguy (May 11, 2018)

Wow... Lot's of experience and thoughts on this topic (I shouldn't have expected any less). I appreciate all the support and think I will go with the power supply for now (with a fire extinguisher nearby, just in case) cause it's what I have all ready to go. I'll let you know if I run into any problems.

Thanks again - Dave


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## kenf (Dec 7, 2019)

Lemonhawk said:


> The 12 volts for my 8 turnouts with Switch Master motors, all the indicator LEDS and power to the 2 DS64's is supplied by a large 3.5 amp wall wort.


Hey Lemonhawk. Could you please post a picture of how you did this? I have a thread in the DCC forum on exactly this topic.


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## gregc (Apr 25, 2015)

Hotrackguy said:


> My power supply is 500watts / 26amps.
> Will it be enough to run all 12 of my Tortoise machines (with a total of 32 LEDs constantly lit


considering that the max LED currnt is 20 ma and that a tortoise machine requires no more power than an LED your 26 A supply is good for 1,300 LED/Tortoise machines


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## Lemonhawk (Sep 24, 2013)

The "advantage" of that high current ATX power supply is that any short will cause a noticeable bang and lots of smoke as it fries everything near the short! You might consider using a lower current power supply or at least some fast acting fuses to protect the wiring at least and reduce the fire hazard. I think even a 1 amp wall wort would be enough power for all your torti!


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## johnfm3 (Sep 30, 2016)

Hotrackguy said:


> ... or will I need to break it up into two sections and get another power supply?
> 
> Thanks in advance - Dave


Good Morning,
As someone who is using a 1200w HP server power supply, I would like to share my experiance.

I bough following...
HP 1200w PS
Breakout Board

So at 12v, I am capable of running 100A. Due to the HP Server Grade hardware, I am about 99.3% of that capability. Which is pretty dang good. Currently, this is running 6 dual coil snap switches. Complete Overkill. Keep in mind, even though the Power Supply is capable of supplying 100A (in my case), Current still equals the Voltage divided by the Resistance. I would be surprised if I am using .5A at 12v at any given time. I have this power supply in place for future upgrades I want. And as you can see, the breakout board allows for 6 circuits.

At 500W, you are capable of producing 41A at 12v against a .25 ohm load. Needless to say, if you attach the power supply to a single LED, its toast. The reality is, that the power supply will only provide as much power as you demand from it. And power is calculated with the following to formula's
V = I x R
W = V x I

There are a few others, but those 2 will get you what you need. 4 LEDs in series is only 600ohms, and will only demand .02A at 12v from the power supply with NO additional resister. In my case I still have a free 99.8A to go.

In all, I will say I love how this setup works. And I am guaranteed clean 12v power with near no deviation allowing me to be assured that my structures which I build in need of resistance will work consistently as designed. The best part of the solution I went with, I dont have to modify the power supply or the wiring harness. Buy the PS, plug the breakout board to it, wire it to the layout and supply house voltage. If I want to replace the PS, its a Plug n Play change.

Good luck in your modeling,
John


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## johnfm3 (Sep 30, 2016)

BTW: A fuse block between a Power Supply and the load should be done no matter what you use.


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## gunrunnerjohn (Nov 10, 2010)

johnfm3 said:


> There are a few others, but those 2 will get you what you need. 4 LEDs in series is only 600ohms, and will only demand .02A at 12v from the power supply with NO additional resister. In my case I still have a free 99.8A to go.


FWIW, I'd have a current limiting resistor even if you think the voltage is right. The current/voltage curve of an LED is very steep, .1 volts can change the current 2:1 in some cases. 

Let's look at one white LED current/voltage curve.

You'll notice at 3.7 volts we are the rated 20ma for the LED. By the time that the voltage is at 2.8 volts, we're already around 50ma, two and a half times the current rating! By the time the voltage gets to 3.0 volts, the LED current is a destructive 130ma, and the LED has minutes to live, if that.

With the simple addition of a 100 ohm current limiting resistor in the string of LED's, by the time the current reaches 20ma, the resistor is dropping 2 volts. This allows pretty much full intensity of the LED's, but limits the destructive current a very small voltage overage.

If you've ever looked at the 12V LED strips, the reasoning of adding the 150 ohm resistor and three LED's in the group becomes pretty clear. The resistor gives the current elasticity to protect the LED's from a slight over-voltage that would result in significant current over their specifications.


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## johnfm3 (Sep 30, 2016)

gunrunnerjohn said:


> FWIW, I'd have a current limiting resistor even if you think the voltage is right. The current/voltage curve of an LED is very steep, .1 volts can change the current 2:1 in some cases.
> 
> Let's look at one white LED current/voltage curve.
> 
> ...


Where did you find this chart? My test on my electronics board and 30v @ 10A adjustable regulated power supply supports the math that the Voltage/Current/Resistance relationship stayed consistant from 2.6V when the LED first lit to 3.9V when it died. And when I had 4 in series, the math still worked with 20mA. And when I had 4 in parelle at 3v, the current was 80mA (4 x 20mA).


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## johnfm3 (Sep 30, 2016)

A current limiting resister is a big issue when you have un regulated voltage which can change more than .1V +/-. Or if your going to be running over the recommended voltage per LED.


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## Lemonhawk (Sep 24, 2013)

misprint 2.7 volts, not 3.7 for 20 ma. Also there may be a little transient in the voltage at the start in the PS the could wipe out the LED's. Independent of all this I would never use such a powerful PS just for low current LED's. I'll stick with using 1 to 1.5 amp wall Worts and lots of resistance. If I want control then I'll use an Arduino and RGB LED's strings that support a 24 bit data packet, ultimate in individual control of brightness and color and no worries about resistors.


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