The goal of this project is to fit the DRV8818 driver circuit onto the standard 0.8" X 0.6" PCB size used in RAMPS 3D printers. DRV8825 drivers are a popular choice for desktop 3D printers, because they can provide up to 2.5 amps peak current. The DRV8818 is a similar IC capable of driving up to 3.5 amps, but the circuit is too large to fit on PCB using regular methods. Also, without a propper heatsink it will overheat.
I managed to fit DRV8818 circuitry, a PowerPeg heatsink, ESD protection, and DIP headers onto this tiny PCB. This driver can operate steady at 3.4 amps peak without overheating! With a larger driver new printer styles are possible using NEMA 23 and 34 motors.
I diced the 4 foot bars into 0.8" pieces outside in the garage. Then I was able to machine the smaller pieces indoors without making too much mess. In this video I add the PowerPeg receptacle, and some recessions that fit the PCB.
If youre wondering what machine is in the video, check it out here.
This video shows the trim pot configuration. The components are mounted on the underside of PCB, and are adjusted through a hole. Test pads on the top side alow for quick measurement of Vref and Vdecay for tuning.
A unique problem with this driver is the wide range of adjustability. The trimmers rotate 270 degrees, and the current range is 0-3500mA. That means 1 degree of rotation equals 13mA. Dialing in the correct current requires a steady hand.
To help I added resistors in series with the trimmers to narrow the range. (schematic) So this prototype is adjustable from 800mA - 3400mA, and the resolution was improved to 9.5mA/degree.
There will be a substancial ammount of precious metal on this board, between the thermal connector, heatsink, high-current headers, and PCB traces. That's the first thing I thought once I finished the CAD model. These drivers last a long time under normal circumstances, but they are notorious for EXPLODING if the cables come loose. Its not right to use such expensive parts unless it is built to last. Call me a hippie, but the idea of PowerPegs thrown in the trash made me cringe... I digress.
Back EMF from the motor can cause permanent damage if the wires are accidently disconnected while the motor is powered. This happens sometimes with loose connectors on shaky 3D printers. So in the first prototype I experimented with a 30 volt TVS installed at the output. In theory this will prevent this sort of damage.
The first prototype functioned normally. I did try yanking the cable on a NEMA17 motor, and no damage! Eventually I will do extensive testing , and capture some waveforms with the oscilloscpe. Stay tuned for that...
After the first prototype success I thought the entire device should have ESD protection. The problem was: not enough space. Here is an image of the first prototype layout.
As you can see not much room for five more diodes. Luckly there were three header pins not being used. So I decided to omit them from the layout to make room for more diodes.
So now every pin on the device is protected from voltage surges. This includes step, direction, reset, enable, V+, A, -A, B and -B.