Added ATX PC Power supply, mosfets now crazy hot

I just added a heated bed along with a new power supply (EVGA 430W) and using the 16_32_090615 firmware I changed EFF to EFB along with enabled my heat bed temp sensor (using type 11 as suggested by the place I bought it from) and uploaded that without issues to the Arduino board with only USB connected at this point.

Following many of the guides online I isolated the +12V pins and GND pins (they were originally attached to the CPU plugs, 8 wires total, I’m only using 4 of them) along with wiring the green PS_ON to GND to make the power supply turn on when the exterior switch on the power supply is turned on. All that seems to work just fine, the power supply comes on, the +12V lines all register 12.18V with a multimeter.

I then connect those 4 wires up to the green RAMPS plug as +12V,GND,+12V,GND (first +12V is the one closest to D8).
Plug all that in and power everything up, RAMPS board lights up and connects to computer just fine, stepper motors work fine.
Enabled heated bed and it’s light comes on and starts heating up the bed, temp sensor registers the temp change, Extruder heats up also when enabled and temp registers properly for the extruder also.
D8 == Heated Bed
D9 = Empty
D10 = Extruder 1 (I only have one Extruder)

Only problem is one or more of Mosfets heat up almost immediately and at one point the extruder overheated, I think I caught it in time though and powered everything off again. After letting things cool down I powered it all back up again and the Mosfets instantly heat up each time.

One of the wiring guides for RAMPS1.4 and PC power supply suggests a 47 Ohm resistor on the green PS_ON line to achieve soft power on which is the only thing I didn’t do as I didn’t see that suggestion until later when troubleshooting. Most of the other guides I looked at online just mentioned connecting the green PS_ON to a single GND line and calling it done.

With all that info any suggestions on what I did wrong or what I should do now?
I’m guessing at least one of the Mosfets need to be replaced now but what did I do to destroy it?

Are your temps reading correct at the hot end? My 24v E3D V6 Hotend heats to 300deg c in less than 45 seconds. If the thermocouple wasn’t reading right and it overheated, it would be a pile of molten slag with minutes.

Your temp reading (at the hot end) should increase rapidly once the heater turns on. If you don’t see that at the controller you may have a thermistor issue.

Yeah temps at both the hot end and the bed seem fine as I turn them on or off.
I’m more concerned that the Mosfets heat up so quickly, basically one or more of them instantly heats up and starts to smell which has never happened before. The Mosfets heat up without me turning on the bed or the extruder, it happens as soon as the +12V power is turned on.

Make sure none of the mosfets are touching each other. Other than that I’m not sure what can cause that. It shouldn’t do anything until you tell the heaters to turn on. If the indicator LED’s are on without turning on the heaters you have a bad mosfet, and will need to replace it. They are only $1-$2.

I have fried a few of them. It usually happens when voltage testing. Hit that probe on the wrong thing and boom, bad mosfet.

The beta firmware is setup for a heated bed.

If you do need to replace any of the MOSFETS I suggest that you use one with a lower Ron resistance like an IRLB3036PBF, A few dollars more but will run cooler and be able to keep any heated bed hotter.

Also be careful when removing the old ones. It is easy to overheat and pull up the solder pads. Be sure to cut out the old one first before trying to desolder.

Thanks for the link to those Mosfet’s, I ordered them and got them installed without too much trouble.
My system is back up and working again and the heated bed works now.
The bed is super slow to heat up but it works fine, my power supply has plenty of output power and amps so I don’t think that’s the issue with the slow heat up times. Looks like there is some tuning process I can run the bed through to get better values for the firmware which may or may not help.

Thanks again for all the suggestions and help!

Kent, The slow bed heat up time is just a fact of life with a 12V supply and the resistance of the bed that you are using. You can only pump in so many Watts at 12V. My Robo3D takes a few minutes to heat up to 110C for ABS with it’s 12V supply. I know that other’s have upgraded to 24V but I did not think it was worth it because I don’t use ABS much anymore. I usually keep my bed at 80C or so for PETG or 50C for PLA.

I did the Marlin PID tuning process but don’t think it sped up heating much.

By slow I mean 20 minutes or longer to reach 70 degrees and I couldn’t get it to go any higher than that even after 30 minutes.

Some of those power supplies are low amps per 12v rail. I think it is more on the 5v side for some reason. Even if it says 700w it doesn’t mean on the 12v rail.
I could be wrong but I burned out 2 of them so far, that is why I switched to the style I ship with the kits.

That’s the best I could come up with when I fried the second one. Something about them does not work well with the heated beds for sure.

Wow so I’m an idiot, I didn’t realize that the second negative spot on my heated bed had to be hooked up.
I looked at the directions on the board which indicated 1 was positive and 2 and 3 were negative and took that to mean I could hook up either not both.
Turns out 2 is the top half of the board and 3 is the bottom half which is why my top half was crazy hot and my bottom half was cold hence the slow heat up times and never being able to reach certain temps.

I soldered 2 and 3 together with a short piece of wire and bam the bed heats up quick now and evenly.

Always learning something new. :slight_smile: