PSU shuts down trying to power heated bed

Hey folks, I need a little help here.
Something strange is going on, I'm not sure if it's a problem with my RAMPS board or my PSU. First, my stats:

RAMPS v1.2
12v 20amp generic PSU
Prusa's PCB Heatbed from Ultimachine.com
Latest Sprinter

Now, some back story. I recently took everything apart for some maintenance and upgrades. I replaced some RP parts and added a SDRAMPS board to the RAMPS. Since I'm using RAMPS v1.2 this meant I had to desolder a few pins to get the SD board in the right spot. I damaged some of the contacts in doing so but I think I managed to get all the connections working just fine. However, when I hooked the RAMPS back up to the PSU I made the mistake of connecting the + from the PSU to the - on the RAMPS and the - from the PSU to the + on the RAMPS. When I plugged in the PSU the LED on the PSU lit up bright orange (normally it's green) and it stayed this way for a few seconds before I flipped the power off.

Of course I wondered if this had damaged either my PSU or my board or both, but when I reconnected things later (after correcting the +/- issue) almost everything seemed fine. All the motors are getting power and are operable. The thermistors are reading correctly. Communication is working with the SDRAMPS board. I am able to supply power to heat the nozzle.

The only thing that doesn't work now is supplying power to the heated bed. When I try to turn up the temp on the bed, after about a second the green led switches off on the PSU (and it no longer supplies power to the RAMPS) and I turn it off immediately to avoid damage. I've tried a few things to try to fix the problem. I've tried to turn the pot on the PSU (V+ ADJ) up and down but it didn't change a thing. I've tried to remove one of the V+ wires going to the heated bed (I had two wires going there to supply more voltage) but again the problem still occurs. I've tried measuring the voltage going to the bed when I turn up the temp (put the + on my multimeter to the + on the bed and the - to the -) and the voltage reads 0! I measure mosfet D10 + and - when I turned up the temp on the nozzle and got a reading there just fine.

I am no electronics ninja and I have a few questions about this situation:

What exactly is happening to the PSU here? Am I doing damage?

Is the RAMPS circuit from the power hookup to D8- (where I'm connecting - from the heated bed, as recommended in the wiki) a complex one? I assumed it was pretty simple, but is it somehow routing through the corner where the SDRAMPS is? If so, I would assume the problem is there.

Could I just hook the bed up to D9- instead to test? EDIT: I have now tested this on D9 and D10 and in both cases I have the same problem.

Is my PSU just busted? How can I test it?

This is my second PSU. My first one just stopped supplying power one day and the green led stopped lighting up. Is there something I need to be doing to protect my PSUs or are they just crap quality? I got them from China on Ebay for ~$25

I would appreciate any help or advice anyone can give on this matter. Thank you so much for taking the time to read this and reply.

EDIT: I am heating up the nozzle now and it is taking WAY longer than normal, and it seems that it may plateau around 170. Does this look like something is wrong with the PSU? If so, I think I will get a 30 amp PSU in the hopes that it may be able to heat up my bed a bit more. Since it will cost a little more though, I want to make sure I understand proper power supply so I don't ruin it as well. Any tips?

EDIT: Now I've discovered that none of the axes receive any power when the SDRAMPS is plugged in. I'm starting to be pretty sure that the Power supply is just fizzling out and unable to supply the amount of voltage(amps?) all the different components need.

Edited 4 time(s). Last edit at 01/23/2012 12:49AM by unicoder.

sounds like your well under powered.

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[mike-mack.blogspot.com]

Have you read the Wiki on how to convert a PC ATX power supply to a RepRap power supply? See: [reprap.org] and [www.wikihow.com]

My guess is that you haven't hooked up enough 12V lines from the power supply to the heated bed circuit. Assuming your heated bed isn't shorting out (overloading the 12V supply), and depending on what type of heated bed you are using, it should be drawing about 10A - 15A @ 12V (say 120W - 200W) - that is WAAAAY too much load for a single 12V line to deliver, so you need to bundle several of the 12V lines together to avoid overloading the power supply.

(And / or you may not have a load on the 5V line. Standard PC power supply MUST have a load on the 5V circuit to deliver power.)
Edit: Oops - just re-read the OP - I see you are using a "20A generic PSU", by which I assume you mean a ready-made desktop power supply. My above comments relate to using a PC power supply. Check the resistance of the heated bed, and then estimate the current draw (using I = V / R). My guess is that if your heated bed resistance is too low (possible short circuit?) it will draw the best part of 20A on its own; add to that the demand of the RAMPS and steppers, and you are probably overloading your PSU.

I am using a 500W ATX power supply [julianh72.blogspot.com] and it works fine (including powering my heated bed) - I believe 250W or better should be fine.

Hope this helps!

Edited 1 time(s). Last edit at 01/22/2012 08:36PM by julianh72.

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Follow my Mendel Prusa build here: [julianh72.blogspot.com]

Thank you both for your comments. I should have also mentioned I guess that before doing this maintenance I used this very same power supply for hours and hours of successful printing with the heated bed.
That said I did measure the resistance of my heated bed to be about 2 Ohms, so I guess that means it will draw 6 Amps? The nozzle heater draws far less so I suppose that explains why I can power it but not the bed.
So it seems that my power supply has petered out is now unable to supply even 6 Amps. My final question is, is my power supply repairable? If anyone thinks they can I would be happy to ship it to that person, along with my old (also not working) power supply.

Does your PS have a 5v output? Sometimes it helps to load the 5V lines to get a good output on the 12V. It's worth a shot? Maybe take some automotive bulbs (1156) and wire them up to the 5v line. Yes, they won't light bright or be an ideal load, but it should be a start. If that helps, then do the math and put about a 4-5 amp load on it using other bulbs or resistors etc...

I've had this same behavior from ATX supplies that have gone bad. They work well, then you try and connect a high current device and it trips the protection and from that point on, it is unable to provide even the lesser amount of current that was normally ok.

Go buy yourself a 500W ps, and follow the instructions online and you should have no problem.

They are pretty cheap nowadays and it's probably cheaper than getting your old one repaired.

Before you go out and buy another psu, lemmie just clarify something, does the psu turn itself off after ramps starts to heat the bed? As this could mean u have a short between the live and ground wire. Not that I've done that before or anything...

I have a similar problem with two of my PSUs. If I connect the bed directly the supply runs fine and the bed heats up very quickly.

With PWM, though, the supply turns off when I command more than 70/255 (I've configured Teacup to accept direct PWM commands for this). At this point the bed receives just 0.3V on average, so it stays almost cold.

Is it possible this 65 kHz PWM stuff just collides with some circuitry inside these older PSUs?

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Generation 7 Electronics	Teacup Firmware	RepRap DIY