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, TVS 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.
After the first prototype failed, I repeated the experiment. The second device failed in the exact same way. I measured a short across VM/GND and 30 ohms across VDD/GND.
I felt discouraged, but the identical symptoms indicated a trend. After reading, thinking, and getting some help from the TI E2E forum, I believe the problem was identified. The device failed because of inadequate bulk capacitance on VM. (You can read up on bulk capicitance in section 9.1 of datasheet.) When capicitance on VM is too low, motor inductance can cause voltage spikes which damage the device.
This is my best guess, because aside from this capicitor, most of the schematic matches the example provided by manufacturer.
The reason I did not add more capicitance in the first place was because the PCB is so small. I looked carefully at the layout... I think one more capicitor can be squeezed. Also I can use different capicitors to bring the total capicitance from 20uf up to 66uf. This will increase the cost of the device a little, but It's worth it.
My previous experience tells me 66uf will be enough. StepperStack had only 37uf on-board, and I have never seen one of those fail in the same way. Plus 3D printers usually have additional capicitance on the motherboard.
Before spending more time and money on a new PCB prototype, I modified one of the V2 models with three 22uf ceramic caps. If this one survives stress testing, then we're in business.
Although it was a discouraging and costly setback, I feel like real progress was made. There was really no way to learn the limits, without trying and failing first.
Yesterday I started collecting thermal data. I was unable to complete testing after 6 hours. I let the temperature go too high (151°C) and the chip was damaged.Now I'm waiting for new parts in the mail. In the meantime here is a sample.
I like Sparkfun's schematics so much, I used it as a template. Except TEM schematics are blue. Man that looks good! #CreativeCommons
It was time to decide on a name. I thought about calling them "Ice Cube" drivers, because of the square shiny heatsink. For now I'm going with simply "TEM 8818 Driver".
A couple of notes:
1) The schematic specifies a 1.5K high-side resistor for Vref trimmer. That means the current can only be adjusted up to 2.6 amps. My prototype has an 866 ohm resistor, allowing for full-range adjustment.
2) While the DRV8818 can opperate from 8-35 VDC, the TVS circuit is rated for 30 volts. So the TEM 8818 Driver can operate from 8-30 volt power supply.
This is a good thing because we know the IC will be safe behind the TVS (in theory). Im looking foward to capturing some surge waveforms with the oscilloscope.
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.