Resistor Selection Circuit Clinic 1


Starting a series of articles to explain the circuit design considerations and troubleshooting to cure defects. When we see a circuit, sometimes we think that how these components are selected? Why some additional components are included to make the circuit over designed? Why a specific value component is selected. All these are some of the common doubts arise when we see a circuit. Circuit design is an art which has many aims. First is its proper working and second is its compactness. In addition to this, we have to use rare components not easily seen in common circuits for the purpose of introducing their working to hobbyists. Another important thing to remember while selecting a component, especially ICs, transistors, sensors etc, is the ultimate result of the circuit to satisfy our needs.

A circuit can be designed in different ways according to the idea of the designer. Some designers design the circuit using only a few components with a concept that this is sufficient and it is easy to describe. The circuit will work but we cannot study anything from it. If the circuit is slightly complicated, the performance will be more accurate and a student can analyze the working of each section in the circuit. See, a hobbyist or a circuit designer is not designing the circuit for the purpose of commercial sale but to understand the basics of electronics. For commercial applications, the design is entirely different and the only aim is to satisfy the needs, low cost and low size. You can see the power supply in china make low cost LED light. They are using only a capacitor and a diode to convert AC to DC. So, design the circuit as per your idea, interest and purpose.

Do not try to follow others because each person has his own ideas and the way of designing. Do not under estimate other’s designs because it is their design according to their mind. But the ultimate aim should be proper working of the circuit. We can light an LED with a series resistor starting from 100 ohms to 1 kilo ohms with a 12 volt supply. Take your own value depending on the forward voltage of the LED. Only difference is change in brightness and current consumption. If you use a high value resistor, brightness will be reduced. So it is your choice according the requirement.

If a circuit is not working, don’t throw it away. Find out the error and make it working. There lies your skill and ability. Experimentation is always better than too much theory and calculations using paper and pen.


The most abundant component present in a circuit is the resistor. It is a simple passive component made up of typically ceramic material. The only aim of the resistor is to limit voltage and current from a particular section of the circuit to another. So first calculate how much voltage / current should flow. You can use Ohms law for the purpose.


This is very important when sensitive CMOS components are used. Slight changes in voltage or current can be allowed because the value of the resistor will not be accurate. For example 1K resistor may be 900 ohms or 1.1 K in real working. In less important parts of the circuit, you can select a general value between 1K and 10K if the voltage is not a problem. The most important thing is the feeding current into the input of components. For example transistors usually conduct only when the base voltage is above 0.6 / 0.7 volts. So the voltage should not be less than this value and should not be very high. You can determine the voltage passing through the base resistor using ohms law.

Another important thing is the current handling capacity of resistor. Most of the circuits use power supply between 5 volt and 12 volts with a current of less than 500 milli ampere. So ¼ watt resistor is sufficient. If the current is between 500 mA and 1 Ampere use ½ watt resistor depending on the voltage / current levels at the particular section. If the current is above 1 ampere it is better to use 1 watt resistor as a safety measure.


You can easily identify the resistor because the colour coding is a part of school studies. Prepare a chart of the colour code of common resistors for easy identification.


You can do one trick. First see the third colour. It gives an approximate value range. Then identify the value of second colour and then that of the first colour.


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