Decoupling capacitor placements and ratings - how to choose?

Projects General Guidance
1

Hello,

I am building a circuit with 2 hybrid stepper motors each powered by a stepper motor driver A4988 and both drivers are connected to an Arduino UNO R3. Before soldering the components together on a PCB, i would like to clear my doubts, confirm what i'm doing and prevent any disasters or improve it, if possible. I have done some research about the importance of using capacitors, but i still have some unanswered questions.

Here are the specs for each hybrid stepper motor:
Rated Current (Amps/Phase): 1.68
Recommended voltage: 12 - 24 V
Holding Torque (kg.cm): 4.4
Resistance (Ohms/Phase): 1.65
Inductance (mH/Phase): 3.6
Inertia (g.cm2): 54
Detent Torque (g.cm): 150
Weight (kg): 0.28

Here are two variants of the section of the circuit schematic relating to the stepper motors and drivers only:

A4988 driver capacitors.png
A4988 driver capacitors.png837×599 36 KB

I have a few questions since during the circuit design, i've been puzzling over several aspects of the decoupling capacitor placements and their respective capacitance and voltage ratings.

  1. I have made two versions of the schematic in the picture above but i can't figure out which one is better? And why? In design B, i placed a 100nF ceramic capacitor between the 5V and GND of each stepper driver - is it better from a design perspective? Although the right-side design uses one extra 100nF capacitor, i am trying to make the best practical circuit possible and explain it coherently.

  2. Why connect a 35 V electrolytic capacitor when only a 9V supply is used to power the motors? How is this 35 V calculated? I found this in a schematic during my search but i want to understand why that value was chosen before i build it.

  3. I have read that as a good rule of thumb to always use a small 100nF ceramic capacitor and a bigger 100uF electrolytic capacitor in parallel to the supply and ground. But in this schematic, there is only a 100 uF between the MV and GND, so should i add a 100nF ceramic capacitor in parallel to improve this design??

  4. Why is a 100nF ceramic capacitor used between the logic voltage 5V (supplied from the 5V output from the Arduino UNO R3) and GND? I read that it filters out high frequency signals. But shouldn't the voltage output from the 5 V pin on the Arduino already be stable??

  5. The recommended voltage for the motor is 12 V, so if i were to use a 12 V DC power supply instead of 9 V, what capacitor voltage ratings should i use? How do i calculate this?

  6. I have set the current limit to each stepper motor to about 500 mA (from my online search, i read that a limit of 500 mA is safe so as not to damage the wires), so is that something to be considered when choosing the capacitor ratings??

At this point, i am just blindly using what i found in my search online going through several schematics without really understanding why. I would really appreciate some clarifications. Maybe someone with PCB design experience can enlighten me on this issue?

A4988 driver capacitors.png
A4988 driver capacitors.png837×599 36 KB

2

I used a PROTOONER board version 3.00 that carries 3 (4) A4988 drivers. Using that board I added no extra components. Looking at the PROTOONER board I se a quite large electrolytic capacitor at each driver. If there are any ceramic capacitors as well I can't tell.

3

Study these:
http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html

4

  1. One 100nF ceramic capacitor for each digital IC, placed as close as possible to the Vcc pin. Those that have multiple Vcc pins need multiple capacitors: one for each pin. SMD is better than THT as you can get closer; if you go THT make sure the leads to the caps are as short as possible as well.

  2. Well, 35V is a common value and it's more than 9V. 25V would do just fine, 16V should work as well but it's a bit low. Lower rated voltage allows for smaller, cheaper parts. Go too low and you risk failure. Higher is fine, too. Basically as long as it's more than 9V.

  3. Motor voltage won't benefit much from a 100 nF ceramic cap, but it will benefit from a 100µF electrolytic or tantalum cap.

  4. Voltage should be stable... but there's an Arduino in play, which contains a microcontroller. That's a major source of high frequency noise.

  5. 25V or 35V or anything higher. No calculation involved. A very common parameter in making such a decision is "what do I happen to have in my parts box?". I have a bag 35V, 220µF caps so I'd be using that when building your circuit to stabilise MV.

  6. Higher current demands a larger cap. How much... hard to say. Connect a scope and look at your voltage ripple, if you want to go scientific about it. Increase the value if it's too much; decrease if you want to save a few pennies. At 500 mA a 100µF cap sounds like more than enough to me. For a one-off (or a handful) you normally just take a look at your parts box, see what's suitable, and throw that in.