PC Power Supply query regarding 5v load

Hi all,

I've recently completed my first RepRap (or at least completed to the stage where basic objects print nicely).

For my power supply I used a standard ATX supply rated at 450 Watts. My RepRap is a Prusa i3 with a MK2 heatbed.

The power supply I'm using has a blue LED and fan built into the bottom of it. I've noticed when the heatbed and hotend are both on at the same time, the power supply starts groaning ever so slightly, and the blue light inside it has a noticeable flicker.

I've not yet added a 5v load to the power supply, as I'm currently awaiting some molex parts. My plan is to run a Raspberry Pi + Camera, and some LED light bars from it as the 5v load.

Am I right in thinking that the seemingly unstable power I'm seeing at the moment is due to there not being a load on the 5v rail?

Just wanted to make sure it wasn't an obvious sign of imminent failure or something before I go ahead and carry on working on it!

Many thanks

Check how much current the 12V rail (or rails) can provide. Make sure it's at least 16A. If it is, then a load on the 5V rail may help.

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According to the specs:

+12V1 is 14A
+12V2 is 16A

I'd have thought the 16A would be the one for the square 4-pin ATX board that I'm using would't it?

I'll get some spares to pull the 5v from a spare molex to go into a Raspberry Pi - That should be ok to test it out at least to see if that's the cause

Are you running all of RAMPS on the 16A output?

If so, try the following:
14A PSU output for the 5A RAMPS input.
16A PSU output for the 11A RAMPS input.
Adding a load of about 500mA to the 5V output.

Note: If the PSU has a 6 Pin power connector on it, this will most likely be on a different rail to the main PSU current supply as well. It's for power hungry video cards (and the like).

Edited 1 time(s). Last edit at 11/02/2013 07:42PM by Cefiar.

As it is right now it's wired up so that the 11A input is wired to what would have been the ATX 4-pin 12v output (presumably the 16A rail).
The 5A RAMP input is then running from the 12v supply on a molex that was on its own cable. Left over I have a single SATA power port, and another cable with 2x standard molex and 1x smaller floppy power port.

I was going on the assumption that the 16A rail would be the one for the motherboard, thus should have been the 4-pin socket. I've not opened the PSU up (even if I did, not sure if there is a way of determining which has the higher amperage) to find out though.

Your choices do make sense.

If there is a 6 pin PCI-E power cable (like the 4 pin, but 6, sometimes with 2 extra pins almost directly connected to it) then that would most likely be the connector on a separate rail rather than the 4 pin connector.

Just to update this,

I finally got around to sorting it out.

I've made a 5v Molex > MicroUSB cable, to power a RaspberryPi which will eventually be mounted on the i3 and will serve as a wireless printer interface. The second I plugged it in, the whining sound stopped and the heated bed doesn't take as long to get up to temperature.

I've also got a cooling fan for the RAMPS board, and a LED strip - both of which run on 5v so will also be added soon to add more load. Seems to do the trick though, thats for sure.

Thanks for your help NewPerfection and Cefiar