Can't use MOSFET because d8 pins are at 12v relative to ground?

I'm trying to add a MOSFET to my Makerfarm i3v's bed by following Tom's guide. I ended up with this configuration:[i.imgur.com]

When I first turned the printer on with the newly added MOSFET, the bed started heating right away. After further investigation, I discovered that this is because when the d8 pins on the RAMPS board are supposed to be off, both pins are at 12v relative to ground on the power supply.

Is there any way I can modify my MOSFET wiring or Marlin configuration to accommodate this discrepancy? Possibly re-purpose some other pins to control the mosfet?

Thanks!

This is normal, both D8 - and + are at 12v when mosfet is off.

The mosfet on a ramps is a N channel and connects d8- to gnd, thus allowing power to flow.

See ramps diagram for how its wired [reprap.org]

what are you trying to achieve? bypass a damaged d8 and mosfet?
If so

Your mostfet needs to be a logic level mosfet. (see lots of mosfet details [reprap.org] )

Dont connect source to d8-, connect it to GND
Dont connect gate to D8+ connect to any io line you want (you just need to update your pins.h file with the new pin number.)

Or are you trying to drive a bigger mosfet from the existing mosfet output?

Edited 2 time(s). Last edit at 07/17/2017 08:40AM by Dust.

Thanks for the reply!

The end goal is to head my 18A bed with PWM. My understanding is that the d8 mosfet is too small for an 18 amp bed. The mosfet I'm trying to use is the irlb8748, which I'd say if well suited for the purpose (it is logic-level too).

I like your idea of using a different pin to drive the mosfet. However, I'm not really familiar with the RAMPS board, could you recommend a set of pins? Are they all capable of PWM?

Thanks again!

Never mind, I found this thread discussing pin selection. I decided to go with D11, and everything seems to work fine.

Thanks for all the help!

Now let's just hope I don't fry something.