Bed not heating or constantly heating (new MOSFET)

I removed the Q3 mosfet from the Ramps 1.4 board and added an external mosfet as Q3 was overheating way too much. However although the power is being delivered to the mosfet, the the signal was not going through to the Mosfet. Having isolated different factors, the problem seems to be that the signal from the Ramps is not reacting to any commands (I.e. no red LED for "bed heating". I took a the original mosfet (Q3) and connected it to the original space and then turned on the psu... the red LED indeed switched on, but straight away with power, and the bed began heating up (I now have no apparent control over the bed).
Are there any solutions to this?

The D8, D9 and D10 LEDs are on the output of the MOSFET. they only turn on when the mosfet pulls the - pin to gnd.
Without the mosfet the -ve of the LED is disconnected.

When you removed the MOSFET you probably damaged it. They are static sensitive, easy to damage.

Regarding the external MOSFET, how did you connect it without a D8 MOSFET? Most external MOSFET are 12v triggered, designed to be plugged into the existing MOSFET.

If you did wire the external MOSFET gate to the D8 io line without the original MOSFET and the MOSFET is trigger-able with 5v, It should work, but the D8 LED will not light ever.

What external MOSFET did you use?

I connected the MOSFET's D.C supply to one of the RAMPS power inputs and then the signal wire to D8, so that should be all fine. I just cant seem to get the actual signal to go through without Q3 - perhaps it's the case of completing some circuit that runs through Q3?

I used one of these standard external ones: YOTINO 3D Printer Heat Bed Power Module Mosfet Module Add-on Hot Bed Power Expansion Board MOS Tube High Current Load Module for 3D Printer Signal Connection Cable Included [www.amazon.co.uk]

That External MOSFET is 12v triggered. It will not trigger on 5v.

D8 - pin is not connected to anything but the original mosfet output and LED. Without the MOSFET its it not connected
D8 + pin is always connected to 12v. (from the 11amp plug)



IO pins from the controller are on the left, output plugs are on the right.

Edited 2 time(s). Last edit at 11/27/2019 07:33PM by Dust.

The solution is to place back a working mosfet on the ramps and use the heated bed output on ramps to switch the external mosfet on/off.

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http://www.marinusdebeer.nl/

Where can I source one of the original mosfet (Q3)?

The original is a P55NF06L (the L is very important, not the same as a P55NF06)
But I would go with a IRLB8743 (better part, runs cooler)

Search for these on your favorite supplier (aliexpress,ebay, mouser, digikey etc)
It shouldn't matter if is a cheap clone as your not putting much current through it

Edited 1 time(s). Last edit at 11/28/2019 08:28AM by Dust.

Quote
Dust
The original is a P55NF06L (the L is very important, not the same as a P55NF06)
But I would go with a IRLB8743 (better part, runs cooler)

Search for these on your favorite supplier (aliexpress,ebay, mouser, digikey etc)
It shouldn't matter if is a cheap clone as your not putting much current through it

Exactly, because when you use an external MOSFET it doesn't push current from the onboard MOSFET through to the external one, the external board only pulls what it needs for the control signal thus leaving the onboard MOSFET basically unstressed.

Edited 1 time(s). Last edit at 11/28/2019 10:05AM by Ohmarinus.

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http://www.marinusdebeer.nl/

It is still constantly heating with the IRLB8743 attached. Perhaps there's been some damage to the board? Any ideas? Otherwise would there be a way to control my hotbed using a Rasberry Pi (running Octoprint) and the external mosfet whilst maintaining the power supplied to it as it is now?