hello, I recently burned my hot bed transistor from a ready made ramps. There was no heat sink and I think it's a mistake.

any way as I'm building a bigger printer, the hotbed will be bigger and I will need twice or more the power.

is there a known and tested way to add extrenal power board ?

I 'm thinking to add a pnp transistor and 2 -3 resistor to drive multi cheaper mos fet with a 12V gate command tension as i feel the 3.3 V from the arduino is not enough to correctly achieve the lowest Rds the mos fet could do. the data sheet of the power fet says the thresold is 1.7 V but the test start à 5V, i think between there is a grey area where you can't draw the full amount of current.

any hints ?

thank you

I use a solid state relay to control my heated bed, it has a low enough current draw and voltage requirement that it can be run directly off of an Arduino if you so desire. My heated bed runs on AC current at about 2.5 amps, 120 V, which gives almost 300 W of heater power. The only disadvantage is that it can't be run using PWM, unless you get a (more expensive) relay that is meant for that. Most firmwares don't use PWM for the heated bed control anyways.

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bigger mosfet and not on the ramps itself is the best way.
i have made this and set the port fore the fet to 1 of the servo ponts and it works ok.

[4.media.hyves-static.net]

thanks

do you have a schematic of your system (don't need to re design some thing allready done ) and the detail of the modification in the firmware

thank you

NewPerfection Wrote:
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> I use a solid state relay to control my heated
> bed, it has a low enough current draw and voltage
> requirement that it can be run directly off of an
> Arduino if you so desire. My heated bed runs on
> AC current at about 2.5 amps, 120 V, which gives
> almost 300 W of heater power. The only
> disadvantage is that it can't be run using PWM,
> unless you get a (more expensive) relay that is
> meant for that. Most firmwares don't use PWM for
> the heated bed control anyways.

unfortnatly my hotbed is pcb type an i prefer not to use the main power in my printer (i don't like the proximity of the main power to the metallic structure )

A SevenSwitch with an appropriate MOSFET should do it.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

Maybe your RAMPS kit came with an inferior MOSFET. I've run the Prusa heated PCB beds before off of RAMPS with no heatsink on the MOSFET and never had it even get warm. If you are building a bigger heated bed though, I assume that you aren't going to be using a PCB heater. My comment still stands, but you can use a beefy 12 V supply to power the bed through a relay instead of AC current. Or do as Traumflug said, and use something like the SevenSwitch with a beefy MOSFET.

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NewPerfection Wrote:
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> Maybe your RAMPS kit came with an inferior MOSFET.

don't know, add a new one from farnell get very hot too so I use it to drive an automotive relay, it's working great as marlin doesn't use pwm for the hotbed (or a very slow frequency one, look more like bang controle)




> I've run the Prusa heated PCB beds before off of
> RAMPS with no heatsink on the MOSFET and never had
> it even get warm. If you are building a bigger
> heated bed though, I assume that you aren't going
> to be using a PCB heater.

yes it will there is 400*300 35µ epoxy PCB single face at farnell for 25€ just need the etching i've the meteriel to do it

with ceram glass bed



My comment still
> stands, but you can use a beefy 12 V supply to
> power the bed through a relay instead of AC
> current. Or do as Traumflug said, and use
> something like the SevenSwitch with a beefy
> MOSFET.

in fact I plan to use SUP90P06-09L-E3 P mosfet 90A 80V 9.3 mohm with -10V on the gate the minimum Rdson could be achieved on the contrary of the one on the ramps, with the 3.3 V from the magarduino the minimum Rdson is not reach (need 5V for that logic mosfet but with 5V logic not 3.3, it works but not well) it's 3.5€ each from farnell

I can't see how a sevent switch can work with to output for the hot bed on the ramp as th mos fet is in the ground side of the wiring (mean when on the mosfet connect the hotbed to the ground ie +12V when off , 0 when on)and the irfz44 need 12 V gate control . you need to get the signal from the arduino but it's 3.3V not enough for the IRFZ. I haven't cheked the gen 7 electronic so may be it's working but no way it wil work straight with ramps output.

As I will use the 12 V straight from the ramps and I want to put 2 in parrallel I think it will work very nicely and for the ramp it only need a 500 ohm resistor in the gate line to be drived by the existing mosfet (a bit of overkill as a driver but I won't unsolder it to put a weaker one !) so Y will only need 2 resistor and the connectors. no need for capacitor, diode led or anything else

I will keep you informed as soon as it's done

[forums.reprap.org]

You re-invented the SevenSwitch

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

im interested to know about your progress on this, hope you got good results!

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Hector Marquez
Production/Gral Manager

Akai Tech Corp
Miami, FL 33166

We are computer Wholeseller of used and refurbish Computer, Notebooks and LCD Monitors

i need to test it but no time i hope soon.