Mosfet controlled heatbed always on

Hello,
I built a reprap and recently decided to change the heatbed power (it wasn't enough to make abs stick). I'm now using an external power supply I'd like to control with a mosfet (Ref K2645). Sooo I soldered everything (schematic is attached) but the mosfet keeps letting the current flow. I checked with a multimeter: a little current (5mA) is going trough D08+ output even if i don't set the bed on with pronterface. I higlighted in blue on the schematic the path I think this current takes but I'm not sure as I haven't done electronics before .
Thanks for your help

Your second mosfet is inverting the signal. So the heat bed will be on when the firmware intends it to be off, and vice versa.You could use an NPN transistor or 2N7000 mosfet, and a pullup resistor, to invert the signal before you feed it to your power mosfet. EDIT - that was wrong, see the next 2 replies.

Edited 1 time(s). Last edit at 05/14/2015 11:55AM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

D8+ is always on, which means your secondary MOSFET is also always on. RAMPS boards switch power by switching the path to ground. The easiest way to do what you're trying to accomplish is a solid state relay.

Is there a reason you're not just using the MOSFET on the RAMPS board? Comparing spec sheets, even though there are better MOSFETs than the one normally used on RAMPS, it's still better than the K2645. It's only rated at 9 amps continuous current with a typical Rds(on) of 1.0 ohms. The STP55NF06L on RAMPS is rated at 55 amps and Rds(on) of .016 ohms.

Unless you're power supply must remain electrically isolated, just hook up the external supply to the heat bed supply connector, and bridge the two supply grounds to tie the ground planes together.

Quote
cdru
D8+ is always on, which means your secondary MOSFET is also always on.

Oops, I didn't notice that he has the second mosfet driven from the +ve side of the output when I wrote my response. I assumed it was driven from the -ve side.

I disagree about using a solid state relay. The DC-DC ones generally drop around 2V, which at 10 to 20A load current means 20 to 40W power dissipation, and so the SSR requires a massive heatsink. OTOH the DC-AC ones work very nicely for driving AC mains voltage heated beds, which is what I use on my large delta.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks for your help,

dc42, I tried turning the bed on with the firmware, it doesn't turn off, so the signal might not be inverted. okay, no problem

cdru, the problem is I don't have one power supply sufficent to power the whole bed so I try to use two less powerful which can both handle half of the bed (two different circuits). That's why I can't use the board mosfet as I need two separated switches. At first, I took a relay but I have to use pwm and the relay doesn't seem happy about it .
Would it be possible to use another pin of the board to send the pwm signal?

Thanks again

Edited 2 time(s). Last edit at 05/14/2015 12:20PM by Zeprazy.

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Zeprazy
cdru, the problem is I don't have one power supply sufficent to power the whole bed so I try to use two less powerful which can both handle half of the bed (two different circuits). That's why I can't use the board mosfet as I need two separated switches. At first, I took a relay but I have to use pwm and the relay doesn't seem happy about it .
Would it be possible to use another pin of the board to send the pwm signal?

Do you have an abnormally large bed that you need heated? It sounds like you're making things a lot more complicated than what you need to. Just find a power supply large enough. ATX or an enclosed SMPS one, Amazon or eBay can get a serviceable one for not that much.

Yeah sure, i will modify the bed resistors circuit instead of the mosfet switch. Thanks for the help