ATX power supply unit (PSU) cuts out when all electronics are connected

Hi everybody,

After a lot of readying finally my first question on this forum. I have build my own reprap 3D printer based on the delta-pi with some modifications. A couple of days ago I finished the hardware and I am actually quite pleased with the result so far. I just have one problem. When I turn on the printer, the PSU cuts out, with what looks like some safety feature.

After disconnecting some components, it turns out the PSU has enough power for the basic electronics (RAMPS 1.4, mega 2560, display, three small fans) and the stepper motors tuned down to about 25%. When I tune the stepper motors higher than 25%, or when I connect the hotend or heated bed, the PSU cuts out. When I then disconnect the hotend and the heated bed and restart the PSU, it works again...

The PSU I used is an ATX version I modified using the manual on this site. I used the 4-pin molex connector (that originally is meant for the CPU) to power the electronics. It should have enough amps for the printer (see attachment).
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I have three questions I hope you can help me answer.

1. I know I have to draw power from the 5v extension in order to get power to the 12V extension. For this I used a 12V 25W halogen light bulb on the 5V extension. Is this enough or should I draw more power from the 5V extension?
2. Do I need to use another connection than the 4-pin molex connector to power the electronics? This regarding the rail 1/rail 2 power from the PSU? And if so, how should I connect it?
3. Or do I just have a bad (low quality) PSU that I need to replace? I have read this is quite common... And if so, what should I look for in a replacement PSU?

I know the PSU might be a little low-powered, but I would expect it to at least power the hotend and the electronics. At this point I'm stuck on options except for buying another PSU, but those cost money and the budget is tight... So any help is welcome!

Edited 2 time(s). Last edit at 12/25/2013 12:18PM by Thijs.

From reading here in many threads, using an atx power supply has many problems.

I use this power supply:
[www.trcelectronics.com]

1) imo, the 12v load put on 5v means is no longer 25w, but you could measure the resistance of the light bulb directly and math out how much it would draw at 5v.
2) I dont see different rails on the picture label. If it would of had different rails it would of said something like 12v1 and 12v2, each on its row in the table and consumption limits for each. So i guess this one doesnt have different rails, and also instead of just using one single connector, you could practically tie more 12v wires together to avoid having all the 18 amps through just 1 single wire.
3) perhaps 18a may simply not be enough, but also perhaps you may have a short on the hotend or bed and then changing the psu wont make a difference; i would check and measure resistances of heated bed and see exactly how much current it draws, also check hotend resistance (for shorts etc), then sum up all the consumers, which should be something like 12A for bed, probably 1,76A for hotend, and roughly around 1A for each driver, so this puts the psu quite near its limit.

The heated bed is by large the biggest consumer. So first thing i would do is disconnect the bed and check if everything else works ok as long as that is off.

You want a supply that's rated to put out at least 25A. Running a supply right at it's maximum output rating is not a good idea. Low end PC supplies like that one are rated for very brief current drain on their rails (a minute or two). The whole unit is rated for 400W max. If you load all the outputs, you get something like 520W. What you *think* they are saying is -- "I can take 400W off any output". What they really are saying is "the real ratings are about 3/4 of what's shown on the label". On a printer you may be pulling a lot of power for 10's of minutes to many hours. These low end PC supplies just aren't up to that.

Thank you all for your help.

Just reporting back that I bought a 33A power supply and that it solved all the power issues. I should have done that right away, as it turned out to be quite affordable through eBay....