tip120 PWM and ATX power supply

hi, a question about current and reading PC PSU spec.

i have 60 meters of led strip i want to fade using PWM. i have the circuit working on small cutouts of this strip. this 60m are devided between 11 PWM pins of arduino mega.
(based on my 5Amp rated bench power supply) each meter of leds draw 12v/0.3Amp so each tip120 should handle (60/11)*0.3=1.6Amp

now my question -
i want to run it all using an ATX power supply. on the ATX it can handle up to 470W, so my understanding it can handle 470W/12V= 40~ amp
but there is spec on the ATX specifing that you can draw up tp 18Amp
from 12v. i suspect this from each RAIL/wire. am i correct?

from wikipedia article on PSU

The ATX12V 2.x and EPS12V power supply standards defer to the IEC 60950 standard, which requires that no more than 240 volt-amps be present between any two accessible points. Thus, each wire must be current-limited to no more than 20 A; typical supplies guarantee 18 A without triggering the current limit. Power supplies capable of delivering more than 18 A at 12 V connect wires in groups to two or more current sensors which will shut down the supply if excess current flows. Unlike a fuse or circuit breaker, these limits reset as soon as the overload is removed.

If you read the label on the power supply if states that it's 470w for 3.3v, 5v and 12v combined..... Handily it also gives a value of 200w for the 3.3v and 5v loads. So you have 270w remaining for the 12v load oe 22.5A. This has then been limited down to 18A.

so if i have at total 18A draw (60*0.3) will my PSU shut off?
i was under the impression this limit is per wire, i'll try and get a beefier PSU.
and if there is any other powering scheme that you might think of, please share.


  1. ye, thats the label of my power supply
  2. i know it a hack, do you have a suggestion for a power supply i can use? rated for this much amp?

keep in mind that most atx psu's require a minimum load on their 5 volt lines. if you load the 12 volt line without having at least 1 or 2 amps on the 5 volt line they may shut off. most people get around this by putting an incandescent light on the 5v rail.

@wirdo, thanks i'll keep it in mind. this PSU works when i connect an arduino to it, so i guess its an older model.
@richard, 18A but i find your attitude off. please dont replay if you find my posts overwhelming.