ATX Power Supply Unit (PSU)
I have replaced the stock Anet PSU with an Ace 750W A-750BR computer power supply unit. I bought the unit for around £30 three years ago. It can deliver 37A on it’s +12V rail. It also has several useful safety features including OVP, UVP, OPP and PFC. Compared to the stock unit, which is rated 20A, this should be more than sufficient.
The +12V rail has about ten yellow wires coming from it. They are used to power the hot end, the heated bed and the mainboard. In order to spread the load across several wires, I soldered them together in groups of 3.
There is also a 5V rail (red) and 5VSB rail (purple). I used the 5VSB (standby) to power a Raspberry Pi running OctoPrint via it’s GPIO pins. The 5VSB stays on when the PSU is switched off. This prevents the Pi having to boot up each time and helps avoid any corruption due to sudden loss of power.
The 5VSB rail for this unit can output 2.5A. It might be worth checking your unit’s spec.
MOSFETs are responsible for switching the power to the heated elements on and off. Although the MOSFETs on the mainboard are adequate, the design of the board means that all of the high current has to travel through the board and in particular, those green connectors at the bottom. There have been several reports of those green connectors melting under the high current and leading to Anets going up in flames.
These ‘external’ MOSFETS are about £5 each and allow the components to draw their power directly from the PSU, rather than via the mainboard. You can use one for the hot end and another for the heated bed, but it is the bed that is particularly important, as this draws a lot more current.
The wiring is very simple. A +12V supply (and GND) from the PSU goes into the MOSFET via the DC IN and DC OUT connectors. The other two connectors go straight to the heated bed (or hot end).
The white 2-pin connector on the MOSFET plugs into the appropriate green connector on the mainboard (depending on whether you are using it for the bed or hot end). When the component needs to heat up, a small current goes through the white wires and tells the MOSFET to ‘open the gate’ and let power through to the bed/hot end.
Soldering wires to the bed
Wires from the mainboard connect to the heated bed via a 4-pin plastic connector. The metal in this connector is quite thin and could melt the plastic when the current gets particularly high.
A simple fix for this is to cut the connector off and solder the wires directly to the pads on the underside of the bed. You can use the connector to work out which wire should be soldered to each pad.