Let out the Magic Smoke

So, I built a Prusa i3 and am in the testing phase.

I double and triple checked the wiring and got all motors working correctly. Time to test the heaters. So, with Pronterface, I set the nozzle to 185C and the bed to 65C. They both began a nice rise in temp and the nozzle settled in around 178, but the bed raced on up to 75. That is when I turned around to face the printer and saw a thick plume of smoke rising up through the electronics. I pulled the plug and looking at what was burned; it is the transistor above the power plug D8 - on the silkscreen, it says Q3 - which I understood to be the bed heater, which is where it is hooked up.
Almost forgot to mention the board is RAMPS 1.4

Any ideas why it went toast on me? I have to admit that the bed heated up quite a bit quicker than I expected. The power supply is a 12V @ 15A unit I took out of an old server. It checked out fine and delivers pretty clean power.

Thanks for your thoughts!

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Edited 2 time(s). Last edit at 09/11/2014 01:16AM by AjaxOfTheRockies.

I am not sure how common the problem is, but I had the same thing happen with our i3/RAMPS build.
It's a MOSFET that controls the heatbed. The heatbed resistence should be 1.2 or 1.3 ohms so the circut can pull 9A.
We also found the resettable fuse got hot and let out the magic smoke too.
There are a couple of threads on this forum about it.

We replaced the Q3 STP55NF06L with a IRL8743PBF (with heatsink) and use a 40mm fan to cool the MOSFET and fuse.
All good now.

Cheers
Steve

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My updated Instructable on our Prusa i3 Build
[www.instructables.com]

I bought three cheap RAMPS 1.4 boards and each released the magic smoke from the heated bed mosfet. It's a common problem. SteveRoy's recommendation is spot on.

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[3DKarma.com] - suppliers of quality, affordable 3D printer kits and filament for the UK market.

I am using an AOT240L (as suggested on #reprap irc) and even without a heatsink and fan it isn't hot enough to burn my fingers. My heated bed is about 1.4 ohm.

You probably should still add a fan to be sure.

What is "magic" about smoke?

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Waitaki 3D Printer

Quote
waitaki
What is "magic" about smoke?

Computers and other electronics run on magic smoke. Oh people talk about electrons and such but it's really the magic smoke trapped inside the electronics that make them run. When you let it out ..or it escapes in the night while you are not looking then the electronics stop working. Then you have to do the rituals of aree-place and raw-pear to make it work again..

Thanks Mike


P.S. That was humor or what passes for humor in the shop I work at.

Quote
Miertam

Quote
waitaki
What is "magic" about smoke?

[...]


P.S. That was humor or what passes for humor in the shop I work at.

Or around tech-nuts in general. Such as "There are 10 types of people; Those who understand binary and those who don't"


- a -

If it was blue smoke, you can try to repair it with this.
This version is rated 12V, 150A so you should be OK!

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RS-Online Ormerod #263, Kossel mini with Minitronics, Prusa i3 MK2

I don't know if I should start a new thread or attempt to hijack this one.

Anyway, I have a problem with my heated bed. I believe it is drawing to much current from my PSU. After running it for a bit, I saw some "magic smoke" appear from behind the printer. I pulled the plug and saw that one of the PSU plugs had melted. I felt the wires and they were very hot (the wires going to HB-PWR on the RUMBA board). Luckily no melting on the expensive RUMBA board. Am I correct in assuming that this heated bed i purchased (a very cheap no brand) has too little resistance, thus causing high current draw from the PSU?

PSU is a very cheap generic one for a PC although probably does about 400W. I don't think this is the problem though because i have a second printer running fine from a even cheaper PSU, but a more expensive heated bed.

I am thinking about just buying from a more trustworthy source a second heated bed, unless someone could help me solve this problem if there is a simple solution.

Thanks

It might be worth checking the back of your heatbed PCB. I have seen some where the corners (where the mounting screws go) are not isolated from the tracks, hence shorting through the frame or the solder tabs (backside ones) for the wire connections are also not isolated.

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_______________________________________
Waitaki 3D Printer

Quote
stustu242
I don't know if I should start a new thread or attempt to hijack this one.

Anyway, I have a problem with my heated bed. I believe it is drawing to much current from my PSU. After running it for a bit, I saw some "magic smoke" appear from behind the printer. I pulled the plug and saw that one of the PSU plugs had melted. I felt the wires and they were very hot (the wires going to HB-PWR on the RUMBA board). Luckily no melting on the expensive RUMBA board. Am I correct in assuming that this heated bed i purchased (a very cheap no brand) has too little resistance, thus causing high current draw from the PSU?

PSU is a very cheap generic one for a PC although probably does about 400W. I don't think this is the problem though because i have a second printer running fine from a even cheaper PSU, but a more expensive heated bed.

I am thinking about just buying from a more trustworthy source a second heated bed, unless someone could help me solve this problem if there is a simple solution.

Thanks

What was making most of the smoke was the green connector that Q3 was touching. If you don't have a heat sink on Q3, even if the device can take it, if it is touching something plastic, it will burn it.
I would put a heat sink on Q3 if not already and bend it over slightly so it isn't touching.

- a -

Just noticed a typo in my post in this thread.
The replacement MOSFET should be a IRLB8743PBF

Steve

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My updated Instructable on our Prusa i3 Build
[www.instructables.com]