A resistor is one of the most fundamental components in electronics. Its basic function is to control/reduce the flow of current. This ohmic blueprint on resistors has all the important points which you should know.

After reading this post you will be able to learn:

  1. Different types of Resistors
  2. Difference between Potentiometer and Rheostat
  3. Resistor Color Codes
  4. Prefixes
  5. Size of Components and its relationship with wattage rating

The ability of resistor to current flow is known as resistance. SI unit of resistance is ohms.

Types of Resistors

Various types of resistors are used in different electrical applications. All these types, however, fall in one of these two categories: The fixed resistors and variable resistors.

Fixed Resistors (FR)

FRs have a constant value of resistance. This value always remains the same within the tolerance levels. The carbon composition models are inexpensive and easily available. However, they have a large range of tolerance usually ranging between 2-10%.

Other FR models include metal film and metal oxide resistors. Both these models have very small tolerance value and they provide highly precise values. The cons of such model, however, is their high price.

Wire wound resistors is another class of FRs. These models can be employed when a large amount of heat is present and normal FRs are expected to die.

Variable Resistors (VR)

As the name explains, VRs provide a variable resistance. VRs have three terminals. This section provides information on two different models of VRs.

Generic Potentiometer (GP)

The generic potentiometer is also known as the classic potentiometer. Its usual size ranges between 1-1.5 inches. A wiper (knob) is attached on GPs which can be adjusted for changing resistance.

Trimpots (TP)

TPs can be thought of as a modern class of GPs. TPs require a screw drive for adjusting the value of resistance.

difference-between-potentiometer-and-trimpot

Working of Generic Potentiometers and Trimpots

The working of GPs and TPs is exactly similar. There are two working modes: As Potentiometer and as Rheostat

Potentiometer

In this configuration, the terminals b,c connect with an external circuit for providing variable resistance. The knob/setting of the component can be adjusted to change the value of resistance. Change in resistance changes the output voltage.

Rheostat configuration

Two connections a and b join with an external circuit. The knob/settings can be adjusted to change the value of resistance. The change in resistance changes the amount of current flowing through the circuit.

difference-between-potentiometer-and-rheostat

In other words, the potentiometer mode can be thought of as a variable voltage providing device and rheostat mode can be though as a variable current providing the device.

Other Classes

Photoresistor: It is a light controlled VR whose value of resistance (VOR) decrease with the increase in the intensity of light.

Thermistor: The temperature dependent VR whose VOR changes with a change in temperature. Thermistors are classified in two classes: NTC and PTC.NTC stands for negative temperature coefficient

NTC stands for the negative temperature coefficient thermistor and resistance of such components decrease with increase in temperature.

PTC stands for the positive temperature coefficient thermistor and r of such devices increases with increase in temperature.

Use of Prefixes

Resistors are measured in ohms (Ω) and they could have a very small value like 1Ω or a very large value such as 10,00,000Ω. In scientific work, the notation large numbers like 10,00,000 are raised to the power of 10. For example, we can write 10,00,000 as 105. In Electrical Engineering the prefixes are used instead. For example, 10,00,000 can be expressed as 1 Mega ohm (1MΩ). A list of most commonly used prefixes is provided here for sake of reference. You can also use a prefixes calculator for conversions.

prefixes-table-for-electrical-engineers

Resistor Color Coding

The tolerance and resistance values of Wire wound and power resistors are printed over their bodies. On the other hand, small sized resistors such as Carbon and metal film resistors have circular bands printed over their bodies. The number of bands on resistors can be 4, 5 or 6.

4 Band Color Code

The first band represent the first and second digit of value. The third band is multiplier and value obtained from first and second are multiplied with this band. The fourth and last one represents tolerance value. The tolerance value is a range of values within which the resistance can oscillate.

Example: 5kΩ ±5%.

The first band was Green which provides first value 5, the second band was black which provides second digit 0. The third one is a red multiplier of 100. This says 50 * 100 = 5kΩ and finally a tolerance of ±5%.

4-band-color-code-reference-chart

5 Band Resistor Color Code

The 5 Band code is same to 4 with a difference in an extra value.

5-band-resistor-color-code-reference-chart

You can use these both charts or can use the following calculator for finding values directly.

However, there are 7 Different values which you should memorise.

Size of Component

The size of resistors depends on the power rating. The figure below gives a comparative analysis of size and wattage rating:

size-of-resistors-and-wattage-rating

So that was all about our Ohmic Guide. Hope you enjoyed it and learn a lot from it. Please share your views in comments.