Resistors - Fixed Value

A fixed value resistor has a fixed, defined, non-adjustable electrical resistance. In an ideal world a perfect fixed resistor would have a constant ohmic resistance under all circumstances – independent of factors like voltage, frequency and temperature. In reality all resistors have some stray capacitance and inductance, and a temperature dependency. The different resistor types and materials determine the dependency of the resistance value on these external factors.

How does a fixed resistor work?

For a given voltage applied across its terminals, a fixed resistor’s resistance value in ohms determines the amount of current flowing through it, as described by Ohm’s Law

V=IR

where V = voltage applied, I = current through the resistor, and R = the resistor’s resistance value.

Manufacturers can control the resistance of their products by varying three resistance material parameters: conductivity, path length, and cross sectional area.

Types of fixed resistors:

Carbon composition: This type of resistor uses one of the oldest construction methods on the market. However they are generally more expensive and less used because other types of fixed resistors have better specifications like tolerance, voltage dependence, and stress thresholds.

Wire wound resistors: These also have a long history, but they are still widely used today because they tolerate high power applications, are stable under high temperatures, and provide long-term stability.

Thin film resistors: These come in two varieties, carbon film resistors and metal film resistors, with nearly identical construction. They are ideal for use in applications like medical devices, audio equipment, and testing and measuring devices that need high stability, high precision, and low noise.

Thick film resistors: These fixed resistors are most commonly used in consumer devices. They are the lowest cost and most readily available. Typically they are used in any electrical device that uses a battery or AC power source.

Fusible resistors not only regulate current, but also act as a failsafe, breaking the circuit in the case of a power surge. They are constructed similarly to wire wound resistors and are commonly used in expensive electronic devices like TV sets, amplifiers, and safety monitoring and managing equipment.

Surface mount resistors: Surface mount technology (SMT) is now the major format used for electronic components. They are easier to use in automated manufacturing, and they can provide very high levels of performance. SMT resistors utilise similar technologies to other forms, but in a surface mount package.

Applications for fixed resistors:

1. Reduce the incoming voltage to power supplies
2. Limit current levels in circuits
3. Dissipation of heat energy in heaters
4. Biasing transistors and diodes
5. Pull-up resistors in digital circuits