I created these notes to help better understand the role of the components we are soldering on the MAKERbuino. They also contribute towards an understanding of the schematic. Please feel free to suggest changes.
The Three 100uF Electrolytic Capacitors
As described previously, with reference to the two 22pF capacitors, a capacitor accumulates and stores charge when current is applied to it and then discharges when less / no current is applied. It is similar to a battery but functions over much shorter periods.
Capacitors have many uses in electronics. One use is for smoothing out electronic signal and power by being a reservoir of power. When inserted between a positive and negative power line, for example, it can supplement power when there is a temporary drain in power helping to provide a stable power supply to the circuit when current draw or current supply is inconsistent (sometimes referred to as noise). This strategy is called decoupling and a capacitor used for this purpose is called a ‘decoupling capacitor’.
All three of the 100uF capacitors appear to be used for decoupling. C4 is placed right next to the input of the voltage regulator. Its purpose is to reduce the noise on the incoming power line (specifically to overcome any impedance to the current flowing in). Without this capacitor the voltage regulator can become unstable. The size of the capacitor required depends on where the power is coming from. This is clearly explained in the datasheet of the voltage regulator used.
C5 is placed right next to the output of the voltage regulator. This provides a buffer/reservoir of power at the output voltage.
C7 is placed right next to the input power pins of the LCD. This provides a buffer/reservoir of power for the LCD which suggests it is a culprit for irregular current draw. This prevents the LCD’s erratic power consumption to disturb the rest of the circuit but more importantly to give the LCD extra power when it needs it. In the latter sense, omitting the capacitor can result in irregularities on the display.