I'm using a Adafruit Servo Driver (PCA9685) and I have to solder a capacitor depending of the number of servos.
We have a spot on the PCB for soldering in an electrolytic capacitor. Based on your usage, you may or may not need a capacitor. If you are driving a lot of servos from a power supply that dips a lot when the servos move, n * 100uF where n is the number of servos is a good place to start – eg 470uF or more for 5 servos. Since its so dependent on servo current draw, the torque on each motor, and what power supply, there is no "one magic capacitor value" we can suggest which is why we don't include a capacitor in the kit.
Right now i'm using only 12 servos (I shoould use a 1200uF capacitor ) but in the future i'm going to use 14 servos (1400uF)
my question is: What happen if I use a bigger capacitor (1400uF) for 12 servos or less?
Best Answer
Agree with previous answers regarding practical effects. Here's some of the theory to help understand what happens when you use a bigger capacitor for this purpose.
The measure of capacitance is the capacity of the device to hold charge for a given voltage (C = Q/V). So if you picture your capacitor as two plates separated by a material that prevents current flowing, the bigger the capacitance, the same voltage held across the plates will result in a greater build up of charge on the plates (imagine that the effective area of the plates is larger).
As you are using your capacitor to act as a short term power source for when the supply dips, a bigger capacitor means a larger pool of charge to draw upon when these temporary dips occur. This is captured in the maths by the time constant tau (https://en.wikipedia.org/wiki/RC_time_constant) which indicates that increasing capacitance slows down the discharge rate. Therefore by having a larger capacitor connected to the supply rails close to the servo, it slows down the exponential decay of the voltage level by providing the current that the power supply has suddenly stopped giving. The more charge it has to give, the longer it can provide the current for.
This is why there is no golden rule for how large your capacitance needs to be. It is a direct measure of how much energy you require and for how long.