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ramps shuts down when power is connected to 11A heatbed terminal

Posted by printerguykw 
ramps shuts down when power is connected to 11A heatbed terminal
September 30, 2017 06:36AM
For the first time, I took my heated bed up to 110C and it held up fine.

A few hours later, I performed a pla print with heated bed at 55C and it came out fine

Then, the following day, when i tried to power on my printer it would not turn on, I disconnected the 12V power input just to the heated bed and and the printer turns on. But as soon as I reconnect the 12V to the 11Amps input on the ramps board, it will not turn on.

My guess is that something shorted out, but not sure where to start troubleshooting.


Thanks
Re: ramps shuts down when power is connected to 11A heatbed terminal
September 30, 2017 05:14PM
Either heated bed is shorted, or heated bed mosfet or you power supply is just not strong enough.

What happen if you just unplug the heated bed from D8?
Re: ramps shuts down when power is connected to 11A heatbed terminal
October 13, 2017 03:13AM
i've been looking at the RAMPS PCB board layout (quite a lot) and i've noticed in the PCB design that the MOSFETs and the power terminals have "thermals" switched on. what that is intended to do, is, to reduce the copper connectivity to pins so that heat (thermal conductivity) doesn't travel up the pin (or down the pin from a hot MOSFET for example). in the case of power pins however this is very bad, as in some cases on RAMPS it limits the (safe) power provision to around 6 amps: the fuse terminals in particular have extremely thin copper tracks leading to them.

now, it's *probably* fine if you're running at 50% capacity (up to 60C), but if running for prolonged periods of time (which you need to do to get to 110C) the amount of current going through those very narrow bits of copper could get them extremely hot.

if anyone is in touch with the developer of RAMPS it might be a good idea to let him know so that this design error can be fixed for future versions. apologies printerguyfw but you bought a board that's not really designed for what you're intending to use it for.
Re: ramps shuts down when power is connected to 11A heatbed terminal
October 13, 2017 03:18AM
here's what i mean. each "thermal" is only rated for about 0.5 amps worth of copper in a 1oz board. there's only 2 layers. sometimes there are tracks about 1A wide connecting the two. often there are only three thermals per terminal per side, meaning a total of 6A. if you're very lucky the extra track width gives you 8A maximum safe power provision (for a 10C temperature rise). google "online PCB current calculator" for more information.


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