Arduino 101
Basic Circuit Theory

Nick Borko

September 21, 2013

Except where otherwise noted, this work is licensed under
http://creativecommons.org/licenses/by-nc-sa/3.0

Agenda

• Introduction
• Basic Definitions
• Concepts
  • Ohm's Law
  • Kirchhoff's Circuit Laws
  • Power
• Hands on: Blinky Explained

Basic Definitions

• We will discuss concepts in terms of electric engineering, not physics
• Circuit: A collection of electrical components and wires

Basic Definitions

• Current: The rate of flow of electricity
  • Measures the flow of electrons over time, represented as the variable "I"
  • Unit is ampere, or "amp," abbreviated as "A"
• Voltage: The electric potential to produce a current
  • Measures the amount of work required to "push" electrons in a circuit, as "V"
  • Unit is volt, abbreviated as "V"

Basic Definitions

• Resistance: The opposition of electrical flow through an element
  • Measures the "resistance" of current, as "R"
  • Unit is ohm, abbreviated as "Ω"
• Power: The rate at which energy is converted (to something, e.g. heat, light)
  • Measures the amount of electrical energy used over time, as "P"
  • Unit is watt, abbreviated as "W"

Basic Definitions

Concepts

• We will cover some basic concepts for analyzing and designing direct current (DC) electrical circuits
• The goal is to give you a basic understanding of the concepts of resistance, current and power in real world circuits
• Circuit/network analysis and design can easily span 6 college level classes

Concepts: Combining Components

• Multiple current sources can be added together when placed in parallel
• Multiple voltage sources can be added together when placed in series
• Multiple resistances can combined:
  • In series, simply add the resistance
  • In parallel, use the following equation:
    
    R₁ × R₂ × ⋅⋅⋅ × R_n
    R₁ + R₂ + ⋅⋅⋅ + R_n

Concepts: Ohm's Law

• Ohm's Law describes the relationship between voltage, current and resistance
• Represented by the equation: V=I×R
  • V is voltage, in volts
  • I is current, in amperes
  • R is resistance, in ohms
• If you know any two of the variables, you can calculate the third using basic algebra

Concepts: Kirchhoff's Circuit Laws

• Kirchhoff's Voltage Law (KVL) describes the distribution of voltage in a circuit loop
• The sum of all voltage potentials in a circuit loop is 0
  • A voltage source can be thought as contributing a "positive" potential to a circuit
  • All components in a circuit loop have resistance and contribute to voltage drop, or "negative" potential

Concepts: Kirchhoff's Circuit Laws

• Kirchhoff's Current Law (KCL) describes the distribution of current at a circuit node
• The sum of all current entering a node must equal the sum of all current leaving a node
  • This law is the basis of network analysis and most circuit simulation software
  • With KVL, used to help determine the total power used by a circuit

Concepts: Power

• Power measures the energy used by a circuit, calculated by multiplying current and voltage
• Represented by the equation: P=I×V
  • P is power, in watts
  • I is current, in amperes
  • V is voltage, in volts
• Most components have a power rating that cannot be exceeded

What does this have to do with the Blinky?

Blinky Schematic

What's the Point?

• Components have a maximum voltage and current rating
• Operating above these values will destroy the part
• Parts also have "typical" operating values, which should be your target
• Use KVL, KCL and Ohm's Law to pick the right parts

Other Blinky Scenarios

• Smaller resistor used: 100Ω
  • Current = 2.5 / 100 = 25mA
  • You might be able to get away with it
• Larger resistor used: 470Ω
  • Current = 2.5 / 470 = 5.3mA
  • LED will be dim (below operating current)
• Skip the resistor?
  • 4.5V · 40mA from pin to the LED = BURNOUT

What about Power?

• Power used in the Blinky circuit:
  • Calculate power with P=I×V
  • P = 0.0114 × 4.5 = 0.051W = 5.1mW
• Resistors have a power rating
  • Your blue resistors are 0.25W (1/4W, 250mW)
  • Beige resistors are typically 0.125W (1/8W)
• Exceeding the power rating of a component will result in smoke

Don't Destroy Your Arduino

• Output current of an I/O pin is 40mA
  • You can probably pull up to 50mA safely
  • Total output cannot exceed 200mA on all pins
• Maximum input voltage is 20V
  • The on-board voltage regulator cannot dissipate that much heat
  • Recommended input voltage is 7-12V
• The maximum power an Arduino can drive is 5V × 0.2A = 1W

Conclusion

• Basic Definitions
• Concepts
  • Ohm's Law
  • Kirchhoff's Circuit Laws
  • Power
• Hands on: Blinky Explained
• Next Steps: Blink some more!

This presentation is available online at:
http://nborko.github.io/arduino101/

All source files used for this workshop are available online at:
https://github.com/nborko/arduino101