Adding Mosfet for the bed wiring diagram  

Re: Adding Mosfet for the bed wiring diagram

Does this help

Peter

Re: Adding Mosfet for the bed wiring diagram

Thanks, was hoping for something a bit simpler!!!
Mainly what do I run from the rambo to the mosfet to switch it on and off.

The power and bed are pretty easy!! Just wondering how others may have set this up.

Re: Adding Mosfet for the bed wiring diagram

You connect the bed connectors from the rambo to the green "Bed + -" connectors on the MKS mosfet.

Maybe this will help.

http://zaribo.org/blog/how-to-install-a-mosfet-to-prevent-burned-plugs-on-the-rambo-board/

Re: Adding Mosfet for the bed wiring diagram

That's great. Thank you very much.

Now I have to figure out where to mount the mosfet!!

Re: Adding Mosfet for the bed wiring diagram

The MKS MOSFET I found is huge, is this all there is?

Did a CAD layout and it takes up the entire space under the RAMBo cover. I'll be going Volcano soon and wanted to get a pair but there no space under there. I saw little MOSFET, will they do or are they not worth the trouble?

Sorry about the huge images, these are from Amazon.

Re: Adding Mosfet for the bed wiring diagram

Ultimaker responded with to my desire for heated bed digital signal out. I've doctored PR's own image to show what pins you should use for MKS MOSFET bed signal.

Change of firmware is required, see Ultimaker response below.

Not sure when I will attempt this but in case anyone else wants to drive your bed off an MKS MOSFET over RAMBo, here is how.

Copy-paste of Service Tech from Ultmachine email.
- - -
To drive an external mosfet you can remap the firmware to drive a spare pin on a pin header.
I recommend using the pin PH2 ( digital pin 84 ) that is located on the P3 header.
RepRap MiniRambo Digital Pin Locations
Here is the Mini-Rambo schematic.
The pin map will be in pins_MINIRAMBO.h (or pins.h under a MiniRambo section).

Modify this line so that the digital pin number is one of the digital pins for your signal.
#define HEATER_BED_PIN 84

Upload the modified firmware and it should work.
Basic instructions for compiling Marlin for MiniRambo are located here.

Don't forget the Rambo board addons for Arduino.
In addition to the signal pin, a common ground should also be connected with the external mosfet board.
We don't have a recommended breakout board.
- - -

Re: Adding Mosfet for the bed wiring diagram

That seems like a lot of work opposed to just connecting the MOSFET to the bed outputs. Is there a benefit?

Re: Adding Mosfet for the bed wiring diagram

I'm not an electrical engineer and was hoping a more senior member of this forum could correct or input but just off the cuff logic tells me running a MOSFET directly off heatbed output off Mini-RAMBo IS a MOSFET working a MOSFET. Mini-RAMBo doesn't seem to have a heatbed bypass signal pin or Ultimachine tech would have said so. Since signal routing on an Arduino MEGA based board is one firmware line change, that is what they recommended. Presumably, this firmware change routes signal away from internal MOSFET to 5V signal line used on "digital" MOSFET types such as the big brick in photo above. This would mean the high speed binary signal managing power thus heat of bed goes directly to off-board to MKS MOSFET instead of on-board MOSFET and subsequent stress and threshold lag. Does it matter for 11.6A draw of the MK42 heatbed? Probably not, but I'm seeing if a second 12V power supply will double heatbed warmup speed as being reported on the interwebs.

But as I can't find any specifications from "far east" MOSFET boards, I'm not pursuing this mod.

Re: Adding Mosfet for the bed wiring diagram

You're asking questions way above my level of expertise, but I can say that these boards have inputs for the heatbed voltage to use as a trigger and are used this way by myself and many others. I've not seen reports of anyone having issues when used in this manner.

Re: Adding Mosfet for the bed wiring diagram

The problem is that when you connect directly to an ATMega output pin, you have to ensure that you can't draw more than 50mA otherwise you will blow that output.

Personally I would connect to the bed output, simply because I would not trust an add-on of this nature to do no damage...

Peter

Re: Adding Mosfet for the bed wiring diagram

Like I said, my electro-foo is weak so I'd like a more proficient electronic to let me know if my analogy is wrong.
From here on, MKS MOSFET will refer to any MOSFET capable of both Logic-level and direct signal.

If you signal the MKS MOSFET via "digital" input, which is optimal as designed, you're trusting the manufacturer of said fet to pull no more than signal which I have just learned is max 50mA. 💡 😉 If not, MiniRambo's Atmega2560 or Atmega32u2 will get fried because there is no circuit protection on P3 pin outs. OK.

Right, the analogy. Isn't running MKS MOSFET off Bed out on a MiniRambo like running a dyno on a car running on the flatbed of a semi to control the throttle of the semi? Or, more straight forward, why send all that power out just to get a fraction more when one (me) was trying to stop roughly 12A from leaving the MiniRambo in the first place? Am I to conclude, that the receiving MOSFET gate is NOT taking 12A of power even though that is what the MiniRambo is designed to output on Bed Out line? If 12A is being pushed to MOSFET gate, doesn't that power have to go somewhere?

Also, MKS MOSFET gate gets fried, does it get stuck on, short the input and/or power, or just become a metal sculpture? As any modern MOSFET is the N-channel enhanced type, the assumption is the MOSFET will disappear from the circuit when fail. Right?

Re: Adding Mosfet for the bed wiring diagram

...

Just imagine a simple switch instead of MOSFET. MiniRambo does not push anything anywhere. It simply switches input power to heatbed on or off. The only problem is that the current going to heatbed is quite high (nearly 15A) and causes unwanted heating of MOSFET integrated on MiniRambo (less) and heatbed connectors (more).
By attaching external MOSFET you make the current go outside MiniRambo, which in that case does not switch 12V to heatbed, but only to external MOSFET.
Simple, no rocket science.

Re: Adding Mosfet for the bed wiring diagram

@David
As I'm very curious about electricity and circuit design but am a specialist in user interface and some education in industrial, transportation and entertainment design, 3D printing is more a very exciting hobby for me. Let me try to rephrase in my own words. Internal MOSFET closes the circuit on its end via internal "logic" signal. As a closed circuit in on-state, power draw is dependent on resistance found on circuit. In this case off-board MOSFET should only call for enough to trip it's own gate. At that point, all amperage is handled within the wires of off-board MOSFET. Therefore, neither MiniRambo processor, internal MOSFET nor bed-out terminals see anywhere near amperage to be of concern as is the case for Search: "toasted terminal". Is that what you mean?

So, my desire to offload internal MOSFET is more a risky and unnecessary effort considering firmware flash annoyances and also potentially damaging processor via pin 84 logic sourcing over draw. Considering neither MOSFET in this daisy chain will ever need to signal past 1MHz (MOSFET performance breakdown threshold), there is zero point in driving external MOSFET via logic aka digital input unless my 3D printer happens to have digital out only design (MiniRambo doesn't). Did I get you right?

Re: Adding Mosfet for the bed wiring diagram

Stephen

That's pretty much exactly correct.

The power draw will depend on the "resistance" of the device receiving the power, so the output of the RAMBo will pass only enough current to turn on the external switch. And there will be no burned connectors on the RAMBo.

Peter