For my 3D printer build a large heat bed was needed. I had ~400mm x 300mm borosilicate glass laying about. Two mk2b pcb heat beds are used to heat up the glass. A problem with the ramps 1.4 board is that the power mosfet is only capable of supplying a maximum of 11 amps. A safe supply current will be about 7-8 amps. This is enough to power one mk2b heatbed. Since 2 heatbeds are used i decided to have them in parallel. Since they are in parallel the voltage drop will be 12v as the supply voltage will 12v from a pc power supply. The resistance of a mk2b board is ~1.5 ohms. The parallel resistance of the two is 1.5/2 = 0.75 ohm. This means that the current flow will be 12v/0.75ohm = 16amp. This is well above the safe current value of the ramps 1.4 heatbed power mosfet.
Here is the 2 mk2b heatbeds wired up:
The mk2b has 3 solder point tabs each. Tab 1 is connected to the positive source and tab 2 ,tab 3 to the negative.
Now designing something that can safely switch on and off the supply from the pc power supply is the challenge. I first went about creating a circuit with 4 mosfets in parallel with their gate voltage controlled by the output of the ramps 1.4 heatbed positive out. 10k ohm connected from each mosfet gate to the ground. Mosfet source and drain copper traces were heavily soldered to decrease the resistance to the current.
Here it is half finished. And wait i realized half way through soldering it that there was a much better way of laying out the components on the copper trace board. I rearranged it so that the gate, source and drain were connected to the same copper trace line.
Its hard to tell from my typically rubbish pics to see the mosfets more neatly arranged. Using a 9v battery to raise the gate above 5v which is the voltage gate voltage to switch on the mosfet. Seemed all ok. Connected it to ramps 1.4 and led + 300 ohm resistor to test it. And… It didn’t work. Wasnt switching off. (Used the heat from my finger to warm the thermistor connected to the ramps that controls the switching of the heatbed) I couldnt understand why. The unforeseen problem which i only found out by googling, Is that the ramps 1.4 heatbed positive isnt switched on and off by the power mosfet, it is the negative side that is switched on and off. This meant that the gate voltage was always high. Meaning the mosfets were always on. Few other problems arose such as alot of heat dissipated from the relay mosfets. I used whatever i had laying about. The rds on value of the mosfets were a quite high. Mosfets i used: IRL3303 N Channel Logic Level Power MOSFET 30V 38A TO220. The rds on value is 26mOhms or 0.026 ohms. Since the current flow threw one will be 16/4 = 4 amps. The power dissipation can be got by P=(I^2)R = (4^2)(0.026) = 0.416w per IRL3303 mosfet. Without a heatsink people recommended from forums that a mosfet heat dissipation should reach no more the 0.3w.
Now the big obvious problem like i stated before is the always on voltage coming from the ramps board heated bed positive. For days i was thinking how in the hell would i go about this. Could i do this by connecting something to the ground… etc. After browsing the internet i saw someone mentioning a optocoupler. Bazinga. opcoupler turns on its internal transistor by the current which flows. Perfect as this gets rid of the voltage problem. The transistor in the optocoupler can toggle on or off the voltage at the gate of the mosfets. I decided on purchasing some mosfets with a low rds on and voltage on and a suitable optocoupler.
Here is what i bought:
2 x IRLB8743PBF n-channel mosfet
1 x sfh618a-5 optocoupler
Everything else from stuff laying about.
Here is the circuit schematic:
The red box is just a simulation of the ramps 1.4 heat bed connection. Circuit was drawn in LTspice.
Testing on breadboard:
Calculations were done as i set up the circuit. So little time for write ups with engineering assessments, assignments and exams every week so not bothered putting all calculations down.