Just added heated bed...I think I broke it.

Hi all,
I built a 3d printer a couple of months ago.
Everything has been going great.
the other day I added a heated bed.
I printed a part using it, and everything worked great except I started losing steps.
the heated bed was drawing so many amps it was pulling the volts down causing the steppers to lose steps.

so, I started changing power supplies to better supply the proper voltages to both ramps inputs.

well, I hooked up the 5amp input backwards. it of course didn't work, so I corrected that problem.
when I started printing my next part I started having usb connection problems.

all would work fine until I turned the heated bed on. then repetier would lose connection to the printer.

Last night I setup better power supplies. I have an old server psu on the 11amp input. It is a 45amp psu.
when I turned everything on, it worked.

I homed the Y axis to test things and it worked great until it hit the e-stop. I had them hooked up wrong so it sat there and ground into the side until I got it turned off.

I think that let me magic smoke out.

with only usb power to the arduino, I can talk to it, but if I turn a power supply on, I lose usb connection.

I've look at the ramps and the arduino, and I can't see any evidence of the burnt part.

I did replace D2. I removed one lead and tested, and it was bad.

does anyone know where I should start looking?

the heater that I added has ohms of 0.6 which am pretty sure is too low. how can I add another 0.6 or so ohms to bring it to where it needs to be?

thanks for any and all responses.

Edited 1 time(s). Last edit at 07/18/2015 10:09AM by n2Jesus.

D2 is the reverse polarity protection, its saraficial, so yes it will have annihilated itself when you pluged it in backwards

Even with D2 it often will not blow fast enough and often pololu modules will also be faulty, or the voltage regualtor

most 12v heated beds are around 1.1 ohm, but a lot of meters cannot acuratly mesure that low.

First test just the mega, You have tried with usb powered and said that works, but this doesnt test the voltage regulator, Upload the blink sketch (will overwrite current firmware) Check it works when mega is usb powered. Now try it with no usb cable and a power supply in the power jack. Something in the 7 to 12 volts range.

I suspect this will fail. If so you have a dead voltage regulator on the mega.

If it doesnt fail here then do the following till you find the part that does cause it to fail.

Put your reprap firmware back on the mega.
Strip the ramps back to bare. plug that into the mega. (Only using usb power) Test that it is talking ok.
Then add thirmistors, and re test check your getting temp readings from the bed and hotend.
Then add endstops and test those work as well. (using m119 command)
Then add the 12v 5amp power and retest
Add in the polulu modules, one at a time and re test each one.
Finally add in 12v 11amp power and retest.

Edited 1 time(s). Last edit at 07/18/2015 11:07AM by Dust.

Thanks for the great advice. I'm out of town right now. I'll try all you suggested when I get home.

ok...
blink worked under usb power.
blink worked under 9volts through power jack

thermistors are giving data back to repetier
end stops work

applied only 12volts 5amp power -- usb connection lost.

unplugged usb and re-connected. windows said it didn't recognize the device.

killed the power and used usb, worked fine.

any thoughts?

what if I clipped D1? I don't use an lcd screen.

long term goal... I've got another ramps coming. my main goal is to not blow it up also.
when it gets here do you think it is safe to plug it into the arduino?

thanks for your help.

well, I clipped D1.

problem solved (hah, hah)

it solve the problem short term. I can move and print again.

however... when I enable the heated bed, I lose usb connection sometime after the bed reaches operating temp.

I have verified that it will print with the bed off.

my next step, I think is to add a very small amount of resistance to the heated bed so it doesn't draw 45+ amps.

any advice is greatly appreciated.

I suspect you have damaged the voltage regulor on the mega..

D1 connects the 12v from the ramps to the mega vin ping wich then goes threw the voltage regulator

now ift its not putting out true 5v, say 4.5 you will have issues.

you need a multimeter and see what the various 5v lines say when the usb cable is disconnected

You cant add a small amount of resitance like that, you will be trying to pull ~10amps threw the resitor (will burn the resitor)

Quote
n2Jesus
the heater that I added has ohms of 0.6 which am pretty sure is too low. how can I add another 0.6 or so ohms to bring it to where it needs to be?

If you measures are correct, you can reduce the max PWM setting in firmware.

#define MAX_BED_POWER 128 // limits duty cycle to bed; 255=full current

-Olaf

so, I changed out all of the electronics. new arduino, ramps, stepper drivers.

I built it up one piece at a time testing as I went. all worked well.

as soon as I turned on the bed, I lost usb connection.

I reduced the max bed power to 1 and I still lose connection.

It has to do with current flow because if I disconnect the bed wires, and then turn the bed on in repetier, everything works fine.

it's only when it's drawing amps.

any thoughts?

I think my next move will be to use the bed connections to power a relay to turn the bed on and off.

does anyone have any advice on doing that?

thanks.

0.6 ohms, voltage = 12V => current = 12/0.6 = 20 Amps.
20A * 12V = 240 Watts.

That's not excessive if you have a largish heated bed. 300x300 heated beds are up to about 300W (25A@12V).

I think you're saying that you're running things off the 11A output of the ATX supply, or even the 5A output? In either case, 20A is more current than the supply can deliver, so the voltage will sag, the arduino will stop working, and you get your USB comms error.

Fundamentally, the heated bed is an on/off device... turning the max bed power down to 1 will just mean that the arduino turns the bed on full power (still at 20A) for 1/256 of the time and off for the rest. That 1/256 of the time would be enough to overload the supply and kill the arduino.

If I've misunderstood about the 11A and you believe that the power supply should be able to deliver 45A at 12V, it could be that your earlier misconnection cooked something inside the power supply, so that it can no longer deliver 45A. My suggestion would be to connect up your heated bed directly to the power supply and measure the voltage across it. Turn it off when you have your reading, otherwise the bed will just continue to heat up! If it's not a good solid 12V, you need to replace the supply. Measure the current drawn by your heated bed at 12V and check that it is 20A or thereabouts, or whatever you expect from the rated power of the bed. If not, you need to replace the supply.