What are Capacitors?

This blog post on capacitors is an excerpt from Lesson B-17 of our Intro to Robotics program. Level B is the second level in Intro to Robotics and covers working with electronic components (including a variety of sensors), writing intermediate-level code commands in Python, and using a Raspberry Pi to control your electronics projects with the code you write. It contains 575 pages with 18 lessons and 57 projects and activities. Sample lessons and a full scope and sequence for Level B can be found here.

 

In electronic circuits, capacitors are used to quickly store and discharge energy into other components. The amount of energy a capacitor can store is known as its capacitance. Capacitance is measured in a unit called a Farad, but most of the capacitors you see in projects will be measured in millionths of Farads or micro-Farads. The symbol used to indicate micro-Farads is uF.

The capacitance value determines how much energy can be stored in a capacitor. A higher value indicates that it can store more energy, so a 2200uF capacitor can store around 10 times more energy than a 220uF capacitor.

 

Capacitor Construction

Capacitors store energy by using two foil plates separated by an insulating material like paper or plastic. In tiny capacitors this may appear like a tiny foil and plastic sandwich, but in larger capacitors the sandwich is rolled up. This helps minimize the horizontal size of the component:

Capacitor construction diagram

Smaller capacitors are not often polarized so they can be used in a circuit in either direction. Larger capacitors, known as electrolytic, are polarized and must be placed in the circuit properly to avoid damage to components. The negative of an electrolytic capacitor will be marked with minus signs and often a large gray band.

Capacitor Polarity

 

Current Limiting

A capacitor will try to charge or discharge as fast as the electrons can move in or out of the device. When initially connected to a supply voltage, a fully discharged capacitor will act like a short circuit to your voltage source. Current will rush in to charge the capacitor as fast as possible, and if this amount of current exceeds what your power supply can deliver, the supply could be damaged.

Current Limiting Resistors and Capacitors 

The same type of damage could occur during the discharge of a capacitor. If you connect a full charged capacitor directly to a low GPIO pin, the grounded GPIO pin will attempt to provide a current path for the charge inside the capacitor. If the current in or out of a GPIO pin exceeds 16mA, then it could be permanently damaged.

Instead, a current limiting resistor should be placed in series with the capacitor. Using the supply voltage and the value of the series resistor, you can then calculate the maximum amount of current that will be allowed to flow in and out of that GPIO pin.

This lesson goes on to further discuss the equations to calculate the current that will flow through the circuit and capacitor charge time.

 

This blog post on capacitors is an excerpt from Lesson B-17 of our Intro to Robotics program. Level B is the second level in Intro to Robotics and covers working with electronic components (including a variety of sensors), writing intermediate-level code commands in Python, and using a Raspberry Pi to control your electronics projects with the code you write. It contains 575 pages with 18 lessons and 57 projects and activities. Sample lessons and a full scope and sequence for Level B can be found here.

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