Backstory: I am building with/for friends their first 3D printers. We ordered mostly from cheap sources, so yes, this is the main failure in the first place. Most of the connectors are not made for the (continuous) power that flows for a rep rap hot bed (12V / 10A max). Instead of ordering one of the widely available MOSFET boards, I went to the local electronic store (which is more of a “hoard cove”) and bought some components for a few bucks. That’s why this writeup is not on “how to” use a presoldered board, but to go a bit further – to understand the basics a bit better to help you debugging and improving your printer.I know that this must bore many of you that have a background in electrical engineering and safe setups already, but this is meant for those without background in it and death traps at home. I hope that it encourages you to improve your kit as well – it just costs you below 6€/$ if you only need the electronic components + heatsink. Pictures follow when I make the second one.1) Preparations:General Warning: Never take the specs of anything you bought for granted. Especially from china. It’s nice to safe some bucks, but the chance that you literally get the best “bang for a buck” is high.Measure the resistance of your hot bed and calculate the current that flows through it when it is connected to your supply directly. Max Current = Supply Voltage / Resistance. I had already boards with WAY TOO LOW resistances that would have sucked around 18 Amps, so take this never as granted when you assemble your printer the first time. Doing it while upgrading won’t hurt you either – if your resistance is too high (can happen too), you should think of ordering a new one as well.2) Calculate the power dissipation for your MOSFETSNow that you know your current, just calculate the power dissipation with this formula: Power = Current * Current * On Resistance. If you did not choose a Power MOSFET yet, have a look at the next point. To see how hot your MOSFET will get, find the Thermal resistance (Junction-ambient) in the datasheet. Temp = 25°C + Power * Thermal Resistance [°C/W or K/W] if this gets ABOVE 60°C, consider taking a heat sink. Take its thermal resistance, add the junction-case thermal resistance to the one of the heatsink and calculate again. If it’s still too much heat, consider taking two Power MOSFETs in parallel – this will half the on resistance and the power per MOSFET will be 1/4th.3) Get the following parts:1-2 Power Mosfet (N-Channel) like the STP55NF06L (same as on the RAMPS) or the VNP35N07 (Thermal protection and other nice stuff inside, but higher On resistance)A heatsink – one that helps with mounting and protects the circuit thermal wise as well12V-Version:10Ohm Resistor 1/4W (the higher, the slower the maximum switching speed gets)1MOhm Resistor 1/4W (can be everything bigger 10k, but I just had this one at home)24V-Version:2x 100Ohm Resistor 1/4W (can be a bit higher, since marlin switches the heatbed really slow)Shrink tubing – a MUST1-2m wire with the right diameter. 2,6mm² or AWG 13+ Should do the job for 10Amps1-2m thinner wire for the control signalA Battery connector for model plane batteries. This is meant for detaching your hot bed when needed. I like the XT-60, but anything with the right current rating will do.About choosing a proper MOSFET:Besides the Current limit (30A+) and the OnResistance (<25mOhm) you should also have a look at the maximum gate voltage. Sometimes the gate is also called VIN when you have a more complex MOSFET with safety features like the VNP35N07. The max Gate voltage is the reason why the 24V-version has a 50:50 Voltage divider on the gate since the maximum allowed voltage on the gate is in my case 18V. Just adapt the divider accordingly to you Gate voltage if necessary.4) Solder everything together:Advice: use shrink tubing wherever applicable. This is not only looking good, but also prevents shorts and sparks and all the stuff we don't like. If you have multiple MOSFETs, solder them in parallel. That means Gate to Gate (small wire) / Drain to Drain (power wire) / Source to Source (power wire). Solder/wire the power path as following:Hot bed (+) -> Power Supply (+)Hot bed (-) -> [Drain] of the MOSFET(s)[Source] of the MOSFET(s) -> Power Supply (-)Now you should have a pair of thick power wires going from the hot bed to your Power switch and one from it to the Power Supply. Now it is time for the Gate (you can use the thinner wire here):Control Board (+) -> 10Ohm Resistor (100Ohm in 24V Version) -> [Gate] of the Mosfet(s) -> 1MOhm Resistor (100Ohm in 24V Version) -> Control Board (-)Never connect the Source of the MOSFET with the negative connection of the control board. If the power wire that goes to the negative supply fails, it will cause to flow all the current to flow through the smaller wire to you control board. This is the old problem all over again and worse, the thinner wire won’t handle it for long.5) Try it out before connecting it to your printerMake a test run with the setup. Best case without a connected heat bed and the control wires not connected to your control board. Use maybe a Led + Resistor first instead of the bed. Also control the switch with 12V(24V) directly, so you can debug it easier than using (&risking) the control board. A common mistake is mixing up Drain & Source of the Power MOSFET. The Hot bed is always connected to Drain.6) EnjoyIf it works, connect the control wires to the control board (polarity!!) and the rest as described above… edit: changed formatting and higher resistor value for 12V version http://ift.tt/2siN3GV