Understanding Ohmic and Non-Ohmic Devices with a Focus on Diodes

Introduction to Ohmic and Non-Ohmic Devices

Ohmic and non-ohmic devices are classified based on their behavior with respect to Ohm's Law. Ohm's Law states that the current I through a conductor is directly proportional to the voltage V across it, provided the temperature remains constant.

Ohmic Devices

Definition

Ohmic devices are those that strictly follow Ohm's Law. In such devices, the current through the component remains directly proportional to the voltage across it.

Characteristics

The relationship between current and voltage is linear. The resistance R is constant regardless of the voltage and current levels.

Examples

Resistors Wires Most metallic conductors

Non-Ohmic Devices

Definition

Non-ohmic devices do not adhere to Ohm's Law. These devices exhibit a non-linear relationship between the current and voltage.

Characteristics

The resistance of the device can vary depending on the voltage or current. The current-voltage (I-V) characteristic curve does not form a straight line.

Examples

Diodes Transistors Filament bulbs

Diodes as Non-Ohmic Devices

A diode is classified as a non-ohmic device because it does not conform to Ohm's Law due to its inherent non-linear voltage-current (I-V) characteristics. A diode allows current to flow in one direction when forward-biased and blocks it in the reverse direction when reverse-biased. The resistance of a diode significantly changes with the applied voltage, particularly at or near its threshold voltage.

In summary, diodes are non-ohmic devices due to their non-linear current-voltage relationship.

Ohmic and Non-Ohmic Resistance in Conductors

Ohmic Resistance

Conductors that follow Ohm's Law are known as ohmic conductors. These conductors exhibit a linear relationship between voltage and current. Examples include metals such as aluminum (Al), gold (Au), silver (Ag), and copper (Cu).

For ohmic conductors, the graph between voltage V and current I is always a straight line.

Non-Ohmic Resistance

Conductors that do not obey Ohm's Law are non-ohmic conductors, and the resistance they offer is referred to as non-ohmic resistance. Examples include semiconductors such as silicon (Si) and germanium (Ge), and vacuum tubes.

The graph between voltage V and current I for non-ohmic conductors is not a straight line.

Alloys for Standard Resistance

Some alloys are used to create standard resistance coils and resistance boxes due to their high resistivity and low temperature coefficient of resistivity. These materials are highly reliable and consistent.

Manganin: An alloy of manganese (Mn), copper (Cu), and nickel (Ni) Constantan: An alloy of copper (Cu) and nickel (Ni) Nichrome: An alloy of nickel (Ni), iron (Fe), and chromium (Cr)

Conclusion

In conclusion, understanding the difference between ohmic and non-ohmic devices is crucial for electrical and electronic engineers. Diodes are a prime example of a non-ohmic device, while metals like copper and aluminum are ohmic conductors. Accurate understanding and application of these concepts help in designing efficient electronic circuits and devices.