The Motor Drivers will have heat sinks so I hope to be able to provide each motor with something around 1.7A of current (I won't need the highest possible torque).
Now what I am not sure about is what power supply I will need for the motors.
Here is what I calculated so far:
From the Motor Data Sheet it says:
Amps/Phase: 2.00
Resistance/Phase(Ohm): 1.4 ±10%
That would mean the Power each driver has to supply is around 5.6 Watt.
I would like to use a 12V power supply and am trying to calculate how much current it needs to be able to supply so I can power both motor drivers from the same power supply.
A 12V power Supply providing 2x 5.6 Watt has to provide just under 1 Amp, so a 12V 2A power supply should be perfectly fine here in theory.
When I do the whole calculation with 1.7A per motor instead of 2.0, it is obviously even less.
Is my math correct, or is there something I'm missing/doing wrong?
I also read in some forums that people are recommending to use a power source with much higher voltage than needed by these calculations, but I don't really understand why. Is 12V okay, or should I look for a 24V power supply (or even higher)?
Also another somewhat related question: In the Pololu DRV8825 documentation they recommend to put an electrolyte capacitor with at least 47µF between VCC and GND of the motor power supply somewhere near the board. I have some 100µF 25V electrolyte capacitors lying around that I plan on using for that. Is it okay if I use one of them like shown in the schematic I made, or do I need one for each DRV8825 board?
One comment on your thoughts on the 12V supply - with power supplies it is better to overrate the supply, leaving a safety margin. Especially when using estimations for the target load current.
The effort isn't wasted - many supplies operating parameters deteriorate near the maximum rated load, and they become susceptible to external factors like environmental temperature, air flow, line fluctuations and so on.
So for example for an expected constant 5A load, you could use an 8A supply. It's a little "seat of the pants" but it helps.
Is it okay if I use one of them like shown in the schematic I made, or do I need one for each DRV8825 board?
Those should be almost on the board itself, not located far away via supply lines. So use two.
Don't run those currents through a breadboard. It's not designed for that. It will get damaged and unreliable.
12 volt 10 amp will be okey. Higher voltages increases stepper performance. Read drivers datasheet for max voktage resp. current. Forced cooling is likely needed near max current.
That's right. High current circuits require adequate conductor size and proper connector types. Such as terminal blocks and crimp connectors, etc. Don't use prototyping kit stuff for this.
Okay, but using a 2A supply for the expected load of 1A would be enough headroom I guess, right?
Great, thanks a lot, I will do that. And the 100µF 25V one would be adequate for a 12V 2A source?
That’s a very good point, thank you! I will connect these parts somehow else then.
So you are saying I need 10A for this, even though I calculated a max draw of about 1A using the numbers from the motors datasheet? Could you explain why I would need a current that high?
2 steppers x 2 windings x 1.7 amp, as You wrote, would be 6.8 amp. Knowing that Chinese supplies often don't manage the max current at nominal voltage margins are needed. For 1 amp coil current, maybe a 5 amp supply will do. The difference in price is small so why take any risk?
The datasheet (I linked it in my first post) doesn’t say that either. You can calculate it from the resistance and max current provided in the datasheet, which would come out at 2.8V.
But from my understanding the motor‘s voltage (as well as max current!) is NOT the same as the voltage the motor driver will need, or is it?
I know it's not accurate. I'm just catching the ball before it goes out of the park. The current is actually controlled by the driver IC. So you need to have a good look at that first, as it is configurable.
Well thanks, yes, that I know, that's how I had planned to limit the current to 1.7A per Motor.
The question was if a 12V 2A power supply is sufficient for that, but unfortunately the answer to that seems to be more difficult than I thought.
I'm sorry, I still can't follow your calculations. Where does the 1 amp coil current come from? And why is a 5 amp supply sufficient, when you told me I would need a 10 amp supply before?
Please correct me if I'm wrong, but from my understanding, the motor driver works essentially as a step-down power converter, so I doubt that the motor drivers need a power supply that can support the same current that the motors will need, since the motors would operate at a significantly lower voltage. That is why I did the calculation using power consumption in Watts, not directly comparing Amps. Was that not the correct approach?
Also the max current the two coils combined won't ever be 2x max current of a single coil, right? The maximum total needed current per motor should be sqrt(2) * coil max current, right? That is significantly less than 2* coil max current.
@anon63607684 : Your understanding is correct. The needed psu power must be calculated by the needed power, not by simply adding the coil current. But you must take into account, that usually both coils are energized at the same time. So the needed power is two times the power of one coil.
And of course the driver has losses and does not work with 100% efficiency.
No, it works essentially as a linear current source, which depends on dissipating power in the control circuit, to limit the load current. A step down converter uses a transformer and high frequency switching to radically reduce that dissipation (power waste).
These small drivers would not be able to dissipate the power loss when operating as linear regulators. Take a look at the data sheet of the DRV8825. In chapter 8 you will find a detailed explanation of the operation.
8.3.2 Current Regulation
The current through the motor windings is regulated by a fixed-frequency PWM current regulation, or current chopping. When an H-bridge is enabled, current rises through the winding at a rate dependent on the DC voltage and inductance of the winding. Once the current hits the current chopping threshold, the bridge disables the current until the beginning of the next PWM cycle.
...
It also explains the current flow where you can see why the bulk capacitor very close to the driver is important. The coil current flows back and forth between the windings and the capacitor
...
In fast decay mode, once the PWM chopping current level has been reached, the H-bridge reverses state to allow winding current to flow in a reverse direction. As the winding current approaches 0, the bridge is disabled to prevent any reverse current flow. Fast decay mode is shown in Figure 7 as case 2. In slow decay mode, winding current is recirculated by enabling both of the low-side FETs in the bridge. This is shown in Figure 7 as case 3.
...
Thanks for all the responses, some of which were actually helpful.
Thank you!! I was already starting to doubt myself...
Right. I'm going with one power source per motor now and I'll use one 12V 2A power supply for each motor. That way there should not be any problems, right?
As I said before, my source for information, among others, is this. There it says
the driver and coil basically act like a switching step-down power supply.
Pololu is the company that designed this motor controller, so calling this "home brewed ideas" is a bit of a stretch. But if I misunderstood something in their product description, I'm happy to learn!
No need to get angry. I literally said "please correct me if I'm wrong" - turned out I wasn't even that far off.
But I guess that's one way to say that you don't want to (or can't) explain your calculations.
