The lowly resistor is probably the electronic component that is most familiar to most of us. Its one and only purpose is to limit the number of electrons that flow through a circuit.

The “poorly conductive material” used in the vast majority of resistors is a mixture of carbon with various other “stuff” to hold the carbon particles together and provide even greater resistance to the flow of electrons. The resistance can be changed by changing the recipe for the mix or by changing the physical size of the resistor. A relatively good analogy for a resistor is a kink in a garden hose. The kink in the garden hose limits the flow of water through the hose in much the same way that a resistor limits the flow of electrons.

Continuing the analogy with water a resistor reduces the pressure (pressure in liquids is analogous to voltage in electricity), so resistors can be used to reduce voltage using what is called a voltage divider circuit. This arrangement connects two resistors in series across a voltage supply.

If the resistances are equal, 1/2 of the total voltage will appear across each resistor. If one resistor has two times the resistance of the other, the voltage across the resistor with higher resistance will be twice that of the other. Notice that in both cases above, the total voltage across the two resistors remains the same. But, ohms law, I=E/R in this case, must be satisfied. So, since the total resistance across the voltage source is three ohms in the right example above, the total current flowing through the circuit has changed as well. On the left, the total resistance is 2 ohms (resistances in series add) so the current is 1 amp. In the example on right, the total resistance is 3 ohms so the current is reduced to 2/3 amp.

*Note – A string of incandescent Christmas lights today consists of up to 50 individual light bulbs (think “resistors”) in series. The power line provides 120 volts but each of the 50 bulbs connected in series only sees 2.4 volts due to the voltage dividing principle as they all have the same resistance. Yes, there are 100 and 150 light strings but the bulbs are nearly always wired in groups of 50.*

The amount of resistance of a resistor is measured in Ohms. The higher the ohms value of a resistor, the more the resistor limits the flow of electrons. A 1 ohm resistor doesn’t present much resistance to flow while a 1 megohm resistor (1,000,000 ohms) allows a very limited flow of electrons. Resistors are also rated in watts of dissipation. Current flowing through a resistor creates heat. The resistor must be able to dissipate that heat to prevent overheating and failure (basically it burns up!). So resistors are also rated in watts which means that they are good up to the specified number of watts of dissipation. In the example on the left above, each resistor is dissipating 1 watt of power. On the right, the 1 ohm resistor is dissipating 2/3 watts while the 2 ohm resistor is dissipating approximately .89 watts.

Reducing voltage by limiting the flow of electrons is the primary role of resistors in electronics. They are a part of nearly any electronic circuit.

– JF –