2.4.1.1 Introduction

Resistors are a fundamental building block of electrical circuits.

Their main purpose is to limit the flow of current to other parts of a circuit and also to convert electrical energy to heat.

2.4.1.2 Ohms Law

Ohms law was developed by Georg Simon Ohm (1787 - 1854). Georg was a German physicist and he found a relation between voltage and current involving the resistance of a circuit. The voltage developed across a resistor is directly proportional to the current going through the resistor.

$$v=R·i$$

Where:

v = voltage across the resistor.

R = Resistance (Ω ohms).

i = current going through the resistor (amps).

Another term which is occasionally used is conductance. This is the inverse of resistance.

$$G=\frac{1}{R}$$

Where:

G = conductance (siemens).

It should be noted too that commonly in the USA, conductance is also mho which is ohm spelled backwards. Its symbol is the omega symbol but inverted ℧

2.4.1.3 Power

Resistors dissipate their energy as heat.

Instantaneous power is the rate at which energy is dissipated and represented by the following formulae.

$$p=v·i$$

Since we know Ohm's law, we can substitute v

$$p=\mathrm{(R}·\mathrm{i)}·i$$
$$p=R·i^2$$

The above formulae shows that even though the resistors exhibits a linear relationship with current and voltage, it does not exhibit a linear relationship with power. In fact it is parabolic as shown below.

2.4.1.4 Types

There are many types of resistors. A selection of resistors types are below:

• Metal Foil - High precision usually < 0.01% over a wide range of conditions.
• Thin Film - High precision usually < 0.1%.
• Metal Film - Common on projects requiring 1% precision.
• Carbon Film - Common on low cost projects requiring 5% precision.
• Wire wound - Common on high power projects. Often wired around a ceramic former.