Electronics Basics - Arduino Sketches: Tools and Techniques for Programming Wizardry - page 48

and components use 3.3 V DC for power. The Uno has a separate regulator to
power devices that require this voltage.
Providing too much or too little voltage to a component may damage it or
destroy it.
Amperage
Amperage describes the amount of current in a circuit, which is the rate at which
electric charge l ows past a point in a circuit. It is measured in Amperes, or
Amps. You'll be using components that use fractions of an amp in the projects
in this topic. It's common to use the analogy of water l owing through a pipe
to illustrate the concept of electricity in a circuit. In this analogy, if voltage is
the pressure forcing water through the pipe, amperage would be the amount
of water l owing past a specii c point in the pipe. The faster the water, the more
current. Contrary to voltage, with amperage it is better to provide more than
is required by the system because the system uses only the amount it needs.
To illustrate how current is used, imagine a simple circuit with a battery and
a lamp. The lamp is the load in the circuit, and the battery is the power supply.
The lamp runs off of 5 V DC and 20 milliamps (0.02 amps). The battery can
supply up to one amp of current at 5 V DC. All the voltage will be used up, but
the lamp will only use the amount of current it needs to turn on. Unlike extra
voltage in a circuit, surplus amperage doesn't get used. If too little amperage is
available, components will not work as expected: lights will dim, microcontrollers
will reset, and all sorts of problems can result. Typically it's a good idea to use a
power supply that provides at least two times the amperage your circuit needs.
Resistance
Resistance describes the ability for something to resist the l ow of electrical
energy. Materials with very high resistance are often used as insulators, like
rubber and plastics. It's often necessary to regulate the l ow of electrical energy
through a circuit by increasing or decreasing resistance. For example, most
LEDs used as indicators in hobbyist projects use less than the 5 V DC that the
Arduino supplies. Placing a resistive element in series with the Arduino and
the LED will decrease the voltage so the LED will function properly.
The practical unit of resistance is called the Ohm and is represented by the
Greek letter omega (Ω). A resistance of 1 ohm is considered to be extremely weak,
while a resistance of 1 million ohms is considered to be an effective insulator.
Even if it's not stated in the documentation, all components have some resistance,
even wires for carrying electricity.