If you are building a CNC machine, and what an excellent project to pursue that is, chances are you will soon be looking into which electronics your new creation will contain. On any given CNC machine, “the electronics” portion of the project has to do with elements such as the power supply, a breakout board to interconnect your power stage to your PC computer, some sensors and push buttons and the actual machine muscles, AKA, the motor drivers.
Well, I guess the muscles would be the motors, but feel assured those motors are not going anywhere unless we supply the needed juice and for that we need a power stage. Ahh! But which one to use?
When we look at the great majority of DIY CNC machines out there, the motor by excellence is the stepper motor. Bipolar stepper motors to be more exact. The beauty behind the stepper is that we can tell it to move a given distance by sending pulses and the electronics to actually achieve this are fairly simple. You can do the same with a Brushless DC (BLDC) motor, but the electronics are not that simple. Not yet, at least… Plus, BLDC motors are not that easy to find anyway, whereas we could easily drown on steppers.
It is hard to say which one is my favorite stepper driver today as there are two devices raging a furious battle inside of my brain. My two good friends are better known as DRV8818 and DRV8825. My indecision starts with microstepping resolution. Whereas DRV8825 gives me an astounding 32 degrees of microstepping, DRV8818 will go up to 8 degrees of microstepping. You would imagine this would settle the score pretty quickly, but on my CNC plasma cutter all I need is 4 degrees of microstepping, so we move on.
How about current? Both of these devices are rated at 2.5A sine wave peak, which in essence tell us they have the same capability to offer. However, taking a deeper look under the hood reveals a quite important capability. DRV8818 has the Over Current Protection (OCP) at 3.5A, whereas DRV8825’s OCP is 3A. Once we factor in other important parameters such as RDSOn, Rise and Fall times, it becomes clear it is easier to run the DRV8818 at higher currents than it would be to do the same with DRV8825. As a matter of fact, on personal experiments I have been able to run the DRV8818 at 3A for prolonged periods of time, when employing a special heat sink system which allows for a very efficient thermal impedance.
We could keep on comparing these two devices for a longer write up but there is just one last item I want to cover and that is the amount of external components needed to properly configure each device. As you will soon see, even on this front determining which device is better is not an easy feat. And that is because DRV8818 is a fairly flexible driver allowing us to configure parameters such as TBLANK, TIME OFF, Mixed Decay percentage, etc. To do this you will need a few resistors and capacitors, which is really not a lot of extra components.
The DRV8825, on the other hand, requires none of these external components at the expense of a fixed current chopping frequency and fixed T BLANK. On DRV8825’s defense, however, it includes a 3.3V regulator you can use to configure the current chopping set point. In other words, DRV8825 requires a single voltage rail (namely VM), whereas DRV8818 requires two voltage rails (i.e. VM and VCC). You can then imagine how easy it is to implement a stepper driver with the DRV8825 as compared to the DRV8818.
So which one is better? As always, it all depends and the application should be the entity with the last word. If you need high resolution microstepping I would go with DRV8825. I can imagine this making sense in applications like 3D printers which is why I have designed my 3D printer electronics around this device. If on the other hand you are trying to assemble a more rugged piece of equipment where you will want lots of current, the DRV8818 will run cooler while still pushing higher loads. An application like a CNC router or a large CNC plasma cutter table could definitely use that extra juice. They are both great devices making bipolar stepper control fascinatingly easy, so whichever you choose chances are you will not lose.