Wanhao Duplicator i3 Mini Heated bed modification

thingiverse

I purchased a Wanhao Duplicator i3 Mini from eBay for about $40, broken. I thought it would be an easy repair since the screen wasn't working. I assumed the faulty LCD panel was the problem and replaced it, but soon discovered that the issue was elsewhere. After some probing with a multimeter, I found the culprit: the 74HC4050D hex level shifter. This chip translates logic state levels between 3.3 volts and 5 volts for the Arduino. It's used not only for driving the LCD screen but also for communicating with the SD card. If your printer's SD card isn't functioning, this is likely the cause. After getting the printer running, I discovered how frustrating it was to deal with a non-heated bed and constant adhesion failures. So, I decided to add a heater to the bed! After researching online, I found that some people had used the "Bed Add" pins from the motherboard to drive a MOSFET off the 5-volt PWM output from the Arduino. However, this required a larger external power supply and a MOSFET that could handle 5 PWM signals. That seemed like too much work for such a small printer. Instead, I opted for a silicone 220-volt AC heated mat driven by a TRIAC module that accepts both 5-volt and 3.3-volt logic levels. If you're concerned about working with AC mains power, consider using a DC-heated bed with an external power supply. I designed and printed a holder for the TRIAC board to keep it safe from live electrical parts. The holder is secure and held in place with 4 M3 bolts and nylock nuts. Simply line up the holder on the outside of your chassis, mark the positions for the holes, drill away, and bolt it into place. I adapted cable chains from previous projects with custom ends for attaching to the chassis. The cable chain ends have a sleeve/collar that prevents wires from chafing on freshly drilled holes in the metal chassis. Line up where you want to position the cable chain, drill more holes (10mm for cable entry and 2.5mm for mounting), run an M3 tap through, attach the cable chain, thread the wires through, and stick a thermal pad to the aluminum print bed. It's a good idea to run a dedicated earthing strap/wire from the heated print bed to the chassis of the machine. I skipped this step because I like living on the edge. Try to stick the pad directly under the printable area of the bed since there is wasted space towards the back of the printer. Now it's time for some wiring. Load custom Marlin firmware and tweak the code, but don't worry – someone else has already explained it well, and they deserve credit. There are plenty of resources on GitHub to help with tweaking the Marlin firmware, so it should be straightforward. The machine prints really well, and the heated bed performs perfectly without any calibration or tuning needed. You'll be amazed at how quickly it heats up compared to a DC-powered bed. You can also see this modification on my blog: https://www.torbenross.com/2020/02/24/wanhao-duplicator-i3-mini-heated-bed-modification/