Power Supply Shutting Off

I am in the process of building my first reprap (a prusa i3), and have it pretty much all built and wired. I'm having a bit of a problem with the power supply though. When i first turned it on i was able to move the motors with Ponterface, but as soon as i tried to turn on the hotend the power supply shut off. Now the power supply's fan just twitches, but no power is supplied.

I'm using an ATX 2 PC Power Supply, and connected the sensor wire to ground per the wiki page. I found the second sense wire, but it was pre-connected to a 3.3V wire. Does anyone know what might be going wrong? Thanks in advance!

Is the wire that was connected to 3.3v still connected to 3.3v? It's is the wire used to detect the voltage at the plug and adjust accordingly. If it is not connected to 3.3v it will trigger the over or under voltage detection. Which will cause the problem you are having.

It could also be caused by a short. Make sure that no differently colored wires are touching.

What power supply are you using? If you unplug the power supply from the mains, wait a couple of minutes and plug it in again, will it turn on again? Some ATX power supplies do not like to operate with next to no load on 5V and large loads on 12V. In this case, their internal voltage regulation will fail and trigger an over- or undervoltage shutdown. Removing power and then trying to turn the power supply on again should fix this. If this is indeed your problem, you will need to have a small load on the 5V rail.

I have been testing using an ATX power supply, and had the same problem.

I initially thought a minimum load was needed on 12v, but connecting a 50w lamp to the 12v, only help a little.

What fixed it was putting load on the 5V. In my case I took the 50w lamp and wired it to the 5v output. The lamp then only takes around 10W but this seems to be enough to allow my hot end to work ok. I have not tested using a heated bed yet. Hopefully 10w load I.e 2A on the 5V output is enough to allow the power supply to regulate its voltage, with large 12V loads like a heat bed

Edited 1 time(s). Last edit at 01/13/2014 02:57PM by rogerclark.

The other thing I have seen is too much current draw on a single rail. Some ATX power supplies are 2 rail others are single rail.
How many AMP on +12V is your supply rated for? 16 Amps in the minimum with a heated bed but 20AMPs is far better.
Some powersupplies that have high wattage ratings overall (450-500W and up) actually do not have sufficient capacity
on the 12V.

I had the same problem and it turned out to be a bad heater. I swapped out heaters and have never had the problem since.
BTW this happened when my build was new as well.

Thank you all for the replys. Lazzymonk, the 3.3V sensing wire is definitely still connected. It came hardwired to the 3.3V line, so i just left it alone.

I'm using a XFX Core Edition Pro 550W power supply, which claims it can supply 44A to the 12V line. It has something called "EasyRail Technology", but from the description it actually sounds like it would help instead of hurt. In any case i have 2 cables worth of 12V lines (6 individual yellow and 6 black wires) connected to the boards input, so I should have enough current.

Based on everyone's suggestions it sounds like the issue is that there is no load on the 5V line. I'll try hooking something up and seeing how it goes. How picky are power supplies on this? Can I just throw anything on like a 12V PC fan (its not the right voltage, but it will be a load at least), or do i need to try to find something with a precise resistance?

Power supplies aren't too picky about their load. However, a fan might not be enough load on the 5V rail, so you might need to think of something that uses more power.

I played around with this issue some more, and it seems as though my heat bed is bad. As soon as i disconnected it, the power supply starts up fine. I'm even able to run the hotend without a 5V load, although it sounds like the load will help regulate the 12V voltage.

I'll need to do some more digging to see if i can find a short, otherwise ill just have to get a new bed. Thanks for your help!

What is the resistance of the bed? Dont assume that it is faulty. It might be that the power supply cant handle it.

I think the resistance is 0 ohms. My multimeter read around 1-2ohms, but thats about the same as the resistance of the multimeter. I might borrow a better quality one from work tomorrow to double check though

You need to measure it accurately. It should be between 1 and 1.6 ohms. Any lower and it will likely draw far to much current shutting down the power supply. Much above that and it will not be able to achieve higher temperatures needed for things like abs

Found the issue! It turns out that the issue was with the heatbed (I'm using an MK2a). I don't know if this is a design issue or if i just got a bad part, but it seems as though the 2 solder pads on the underside of the board are connected to the copper base layer, and so are wired together. The solder pads on the top surface are fine though, so re-wiring the board should fix the issue. Thank you everyone for the help

I hope I'm not out of line posting on this somewhat old thread, but I don't want to create a thread that is not directly related to 3D printing yet still relates to power supply issues.

My hobby/hacking use of my old Seasonic S12-500 is to supply power for a whole bunch of various components like an odroid, microcontroller, motors and sensors.
I soldered columns of pins to a perfboard and connected the 24-pin ATX header to it.
I then connected a switch to the green pin 16 and ground.
I tested an LED/resistor on 5V and it remained lit, HOWEVER I need to connect my 5V/2.5A Asus router (because I'm not ready for the odroid), and it is that which makes my ATX PS shut off (protection I assume, as under/over voltage and short circuit protection are features).

If that part wasn't interesting enough, I also have another 12V/1A Asus router that I'd prefer not to use but connected to my ATX PS and it remains up and is still running as I type this.
In various sites such as this thread in particular, I've read of load being needed in some cases:
[www.aitechsolutions.net]
and
[electronicmadness.blogspot.com]
I tried connecting a 12V/0.1A fan along with the troubling 5V router and instead of shutting off in 1-2 seconds, it takes about 5-8 seconds I guess before it shuts off.
But I'm thinking "load" is not a requirement in this case considering the LED lit and the 12V fan would run and the 12V router runs as well (with no other load).
I should probably also mention that I'm using what I think is 24awg wire from CAT6 cable.

What could be going on that causes the 5V router to shut off the PS, but not the 12V?

Thoughts? Insight?
Thanks