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Heatbed keeps destroying RAMPS at different places

Posted by Brikowski 
Heatbed keeps destroying RAMPS at different places
February 08, 2018 02:02PM
Hi, I recently built my first reprap (Rebel/Prusa i3 clone) and it seems to work fine (haven't really printed anything yet), but every time I heat up the MK2b heat bed, magic smoke starts to go out of the RAMPS 1.4 controller board.

The first time it happened was when I had heated up the bed for the second time or so. I figured that the smoke was coming from the green wire adapter thingy that connects ramps to the power supply. The green casing was partially melted on one side. I couldn't think of any flaw I had made (I had the cable screws tightened really well), so I just bought a new RAMPS and tried it again.

But this time it was similar, all worked well until I powered the heat bed on, after just like 20 seconds I could smell burning plastic and saw smoke coming from one of the mosfets (the one closest to the yellow capacitors). The mosfet was still hot even after I had unplugged the power supply. So this time, my heat bed burnt a different part.

Do you have any ideas what the problem can be please? I've read that ramps is crap, but this is probably my error somewhere. My only idea is that perhaps the wires I use to connect the supply-ramps-heatbed are too thin (certified for <10A), but I can't see how that could have burnt the mosfet.

Edited 1 time(s). Last edit at 02/08/2018 02:42PM by Brikowski.
VDX
Re: Heatbed keeps destroying RAMPS at different places
February 08, 2018 03:27PM
... you're drawing too much current for the components!

Best use SSR's (Solid-State-Relays) and thicker wires to avoid catching fire ...


Viktor
Re: Heatbed keeps destroying RAMPS at different places
February 08, 2018 05:52PM
standard 200x200 pcb heated bed is 11amps so "(certified for <10A)" isnt going to cut it.


The bed mosfet on all cheap ramps is crap, should be replaced with something better or use an external mosfet.
I use IRLB 8743 mofets or [www.aliexpress.com] as an off board mosfet.

Edited 1 time(s). Last edit at 02/08/2018 05:53PM by Dust.
Re: Heatbed keeps destroying RAMPS at different places
February 08, 2018 10:55PM
Thanks, so do you think that just using thicker cables will solve it? I mean, I've burnt two ramps boards as soon as I turned the heat bed on, so I doubt that this is a ramps design issue. I can switch to SSRs and better mosfets later, but right now there must be a deeper flaw somewhere.

I'm actually not sure that thin cables could have increased the current draw?
Re: Heatbed keeps destroying RAMPS at different places
February 08, 2018 11:10PM
standard 200x200 heated bed is only 1.1 ohm, so any issues with cables and the cables or connectors heat up vs the bed.

You may have shorted the heated bed out... if you have metal screws or springs directly on the heated bed, it doesnt take much to scratch threw the thin coating and short things out.
Strongly advise you have insulating washers everywhere near the heated bed. So springs and screws are not directly on the bed.

Edited 1 time(s). Last edit at 02/08/2018 11:11PM by Dust.
Re: Heatbed keeps destroying RAMPS at different places
February 09, 2018 12:03AM
...better check the wires on the heated bed-side. You might have a shortcut there?
VDX
Re: Heatbed keeps destroying RAMPS at different places
February 09, 2018 03:09AM
... this cheapish screw-terminals on the RAMPS clones weren't meant for more than 5Amps continuous!

Problems with soldered wire ends are common too ...


Viktor
Re: Heatbed keeps destroying RAMPS at different places
February 09, 2018 04:12AM
Thank you all! There were no shorts on the heatbed, resistance was fine, soldering too... I simply changed all the high-amps cables for 15A certified and all seems to be fine. No hot mosfets or connectors.

Time for printing, hopefully.
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