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What capacitors do can be confusing. Here's a way to think about them that makes sense:

Capacitors oppose a change in VOLTAGE

What this means for DC Circuits:

  • A capacitor will try to keep the voltage constant, when there are changing currents in a circuit
  • This is what is called a "Bypass Capacitor", because it conducts any changes in voltage to ground.
  • Bypass capacitors DO conduct current when some circuit tries to make the voltage change, such as changing currents drawn by Arduino chips on their circuit board. That's why there are "Bypass Capacitors" on the Arduino boards.
  • If there is a constant changing AC signal "trying to change the voltage" like in a power supply, a capacitor can act as a "Filter Capacitor", and "smooths" the voltage so that it is more like pure DC.

What this means for AC Circuits:

  • Capacitors act as "Coupling Capacitors" between AC circuits, by charging up once to the average DC voltage, but then coupling changing voltages (signals) to the following circuit.
  • Capacitors can be used in circuit where they are constantly charged and discharged to produce needed waveforms like Triangle or Sawtooth shaped changes of voltage. Often this is used for timing purposes like in a 555 timer, or changing pulse widths in a PWM circuit.

What might you use Capacitors for with typical Arduino/Microcomputer circuits??

  • Added "bypass capacitors" to smooth power supplied to external circuits. Locate them close to the circuit.
  • Same thing when putting IC Chips on a breadboard. Connect "Bypass Capacitors" from the Power/Vcc pin of the chip to the chip ground pin, very close to the chip. A good value is .1uF
  • Timing capacitor for a 555 timer to set it's frequency, along with a resistor. Maybe setting 38KHz to drive IR LEDs..