Hello. I can not figure out why this is happening.
I have a 430w ATX PSU that I converted for bench use. My conversion consisted of clipping the 24 pin plug off and separating the +12v, 5v 3.3v and ground wires.
I then grounded the PS-ON, clipped and capped the 5vsb and -12v. At this point the fan will turn on without a dummy load. I only want to use the 12v and so I checked it and it is 12.1 stable.
I ran one ground wire to ground on the mega, and one yellow 12v wire to the vin on the mega without the usb plugged in. So I flipped the power switch on the psu and the mega powered up normally. I checked the amps and it drew 10ma. So just to be clear, the mega by itself is connected to the atx via 12v vin.
I can connect a 9v ac/dc wall adapter in place of the atx and I can connect the usb without damage to the mega. The ATX is the problem, yet I have no idea why.
I shut off the psu and disconnected it from the psu and connected it to the usb and the mega powers on normally.
Here is my problem.
I connected the mega back to the atx via vin. The +12v is connected to vin and the black atx is connected to ground on the mega. The mega powers on normally. Once I plug in the mega powered by the atx to the usb in order to connect to the pc. My amps jump to 11 and the mega smoked and burned the traces near the usb.
I can not use arduino with the atx via vin or even the japan jack and plug in the usb without killing the arduino. I have tried an authentic and generic arduino with the same result. I have also tried using a dummy load on the 12v without success.
I have even cut the 5v wire on the usb cable thinking that eliminating the 5v from the pc usb would solve the issue. The board still smoked. I do not understand why I can not use the atx psu to power the mega and not be able to plug in the usb without it smoking the mega.
Powering the mega via atx using vin works great until you plug in the usb, then it's all over for the mega.
You seem to have kind of a ground loop, where both your PC and ATX PSU are connected somehow. I guess that some voltage difference exists between both ground wires - check yourself.
At this point in time, i’d say you’re really lucky it was the MEGA that was cooked, not your PC.
Sadly, you’ll need to check as DrDiettrich noted - BEFORE connecting anything, because the ‘suck it and see’ method isn’t working.
ALL Arduino projects require a fundamental understanding of DC circuits.
Missing from a lot of posts on this forum.
Thanks for the response. I can connect a 9v ac/dc wall adapter into vin just like I did with the atx, and plug in the usb. So with the wall adapter no issues, the only issue is the atx and I can't figure out why.
I don't understand why the 12v from the atx is causing an instant ramp to 11 amps when the usb is plulgged in, and the 9v ac/dc does not. This is a new atx psu, not some old pos. Like I said, I do not have anything connected to the mega, just imputing 12v via vin and a single ground.
Only connect the grounds of the PC/USB and PSU. What happens?
seems to me you would choose the 5 VDC power for a Mega. anything more just heats up the regulator
The 11 amp figure is the Arduino acting as a fuse - between the ATX 12v and the other connections.
There is more to this than meets the eye...
Possibly the ATX 0V is not at ground, or the USB source is floating independent of that common 0V / ground reference.*
If you have a multimeter, confirm the 0V wires are all low-ohms (< 1 ohm) between all points - before you power on anything.
If that’s the case, then without connecting the MEGA, power on, and verify all voltages are where they should be.
11A is a decent short circuit. Are you sure of your wiring, solder joints and connections?
- remember that separate power supplies are nor ‘aware’ of each other - hence there is a need for a common ground/0V reference tokeep them on the same page.. 12V on the ATX is only 12V wrt the 0V on the ATX.
For it to be 12v wrt the USB and Arduino, the two supplies need to share a common reference - usually 0V/ground.
DrDiettrich:
Only connect the grounds of the PC/USB and PSU. What happens?
Ok, I had a feeling that would happen. So I used a 26 awg wire in case it melted. So ya, the problem is with the ground.
I'm not sure how to resolve the issue or get the pc usb and the atx on the same ground.
I spoke wrong in the first post. I connected the PS-ON to a black ground wire from the bundle of grounds wire from the 24pin and peripherals.
Use the +12v from the external atx, and a ground from the pc psu ground?
We need to identify if anything is grounded.
Separately, Measure ohms between the USB 0V and ‘earth’, and then between the ATX’s 0V and earth.
I’m guessing one of them will not be tied.
Hard to say what's wrong. What kind of AC do you have, 50 or 60 Hz?
If you have an earth/ground pin on your mains connectors, try to connect the PC or PSU gnd to that line. This should do no harm with properly insulated power supplies.
WOW, just checked the continuity on the atx between the yellow 12v lines and the earth ground on the ac in connector, and I have tone. 3.3v and 5v have zero continuity to earth ground via the ac connector, but 12v does.
Problem solved. I can't believe it, I have obviously checked for continuity before but a bit of solder on the bottom of the atx was grounding to the metal frame. I re-soldered and insulated the bottom better, and can now plug in 12v vin and the usb with no issue.
The mega draws 80ma at 12.1 steady... 12v to ground short.. UNBELIEVABLE
...and thankfully, you’re still alive.
Not being dramatic, but this is exactly the type of problem tha kills people with cheap knock-off phone chargers.
Followers - be careful!
lastchancename:
...and thankfully, you’re still alive.
Not being dramatic, but this is exactly the type of problem tha kills people with cheap knock-off phone chargers.
Followers - be careful!
I take precautions, a fire extinguisher 10 feet away from my bench just in case. I also discharge the caps, and clip a 6ohm 100w resistor to + and - after unplugging the psu from the wall, then flip it back on for a while and wear rubber gloves also just to be extra careful.
I didn't ever get the usb fully in the jack before fixing the short, just touching it was enough to eat the mega. So I then unplugged everything, never touching the steel case or have anything around it during testing.
So yeah, even dangerous as it was. I made it as safe as possible. Never assume it's going to work first try. No corners cut.
+1 to that, but keep kids, pets and wives away too.
lastchancename:
+1 to that, but keep kids, pets and wives away too.
Oh man, you had to throw the wife in there.. glad I didn't say it. Ha!
Hi,
Can I suggest you go looking for a proper powersupply that has CURRENT LIMIT control before your ATX zaps anything more.
Either look for a circuit or purchase a benchtop supply.
While you have no CURRENT LIMIT, any short you produce will have the full wrath of the ATX current pushed through it, this will inevitably let the smoke out.
Tom... 
Trust me I get it. I will eventually get a different unit but for now this will work.
Thank you all for your help.