Power supply amperage ??

[Nacimroc]

So I'm trying to figure out how much amperage a regular 400w (ish) power supply would give out. I have a PC power supply for my PC in the cab, but would it have enough power to supply 15 x 12v LED's and 4 x 120mm fans off it, aswell as running the PC. Or do I need another PC power supply to drive them.

(The LED's are the illuminated buttons from ultimarc and the fans are regular old PC fans.)

Thanks

[BobA]

15 leds at avg 20 ma = 300 ma
4 fans at 250 ma to 400 ma each = 1A to 1.6 A

In total you are probably adding 2A max to your 12V rail in your supply.   Your 400W supply can probably cover this addition easily unless it is highly loaded right now.

[Nacimroc]

So if I was to roughly guess (power = voltage x current) a regular PC power supply would output about 2.5 amps ? So If i was drawing almost 2 amps with LED's and lights alone would this leave enough for a PC to run aswell ? Especially on startup?

[BobA]

Voltage x current is 12 x 2 amps or 24 watts.   Your power supply is a 400 Watt unit.  You should have lots of power left.  Look at the specs on the side you your supply it will say what the max amps it can supply at each voltage.
 

[BobA]

These are the typical specs of a 400w ATV supply.    Note there are 2 12V rails capable of supplying a total of over 25 amps.

Typical Power Distribution for a 400 W ATX12V Configuration

Output          Min Current    Max Current     Peak Current
  Volts                (amps)             (amps)               (amps)
+12 V1DC                1                    14                        15
+12 V2DC                1                    13                        16.5
+5 VDC                  0.3                   14
+3.3 VDC                0.5                   20
-12 VDC                  0                      0.3
+5 VSB                    0                      2.5                     3.5

[Nacimroc]

I got a name branded cheapo one with no specs! Didn't realise it had that much power! The 2 rail thing is handy to know instead of loading one to death!

Thanks for all the help!

[MonMotha]

For grins, here's the ratings from an old 490W supply out of a Compaq server from the late 90s:

3.4V 18A
12V 12A
5V 75A

You could weld with this thing!

Due to the way power in computers works these days (CPU/GPU cores, which consume the most power of everything on the system, use locally generated supplies), a lot of the power has migrated to the 12V rails, but 400W is a LOT of power.

Now, do be aware that, if you have a cheap supply, it's probably not actually good to the claimed ratings for any period of time.  I'd derate it by at least 25%.  This means your 400W supply is more like a 300W supply.  Oh boo hoo, it's still got more juice than you'll ever need just for some LEDs and fans and such.