This is entry #1 of my 52 Project 2012: Foundational Electronic Components Crash Course series.
What is a resistor?
A resistor is one of the simplest types of electronic components in existence. It simply takes some amount of power in one end, and lets some smaller amount of power out the other end.
How does a resistor work?
Fundamentally, a resistor is very simple. It is a single linear component that is made out of a specially calibrated quantity of material, such as ceramic, that lets current through, but not as efficiently as a plain wire does.
What about the extra power though? It can’t just disappear, obviously. If more goes in one end than comes out the other end, something must be happening in the middle. That something is that the excess electrical power is converted by the resistive material into heat.
You may have noticed that many electronic devices get warm or even extremely hot to the touch when they are turned on. This happens because electronic components inside the devices are not 100% efficient, and the inefficient power usage results in heat. Sometimes this is an intended part of the circuit design, as is the case with resistors. Resistors are used to limit the quantity of power that can go into any particular part of the circuit. The heat, if there is any, is an expected byproduct inherent in the design. However, other components often exhibit this heat byproduct even though we’d prefer them not to, and this occurs simply because component designs cannot escape the 2nd Law of Thermodynamics. There is always some inefficiency.
One interesting example of this effect is the eye on an electric stove, which is usually a coiled up piece of metal. This metal is nothing more than a giant resistor. When you turn the eye on, a bunch of power is pushed through this resistor, and the effect is a whole lot of heat dissipation.
Resistor ratings and sizes
Resistors are rated in overall power dissipation (given in watts), resistance (given in ohms), and tolerance (given as a percentage). The power dissipation is the maximum power that can be safely dissipated by the resistor before there is danger of heat damage to the material. The resistance controls what fraction of the power will be removed and converted into heat. The tolerance is the margin of error of the resistance rating (often 5% or 10% in either direction).
In general, resistors are very inexpensive, unless they (1) have a very high power dissipation and/or (2) are made with an extremely precise tolerance. Resistors are one of the absolute cheapest components you can buy. Common small resistors are rated at 1/8 watt, 1/4 watt, or 1/2 watt, and anywhere from 1 ohm to many millions of ohms. 1/4 watt is not a whole lot of power, but it’s often more than enough for any given part of a small circuit.
What happens to the voltage and current?
Remember that power is made up of voltage and current. If you recall, current in a circuit remains constant throughout all components connected in series, so we know that resistors don’t reduce current. Since we know that they reduce power, that leaves only one option: they reduce the voltage. But by how much, exactly?
That depends on the resistance rating. As noted above, resistors are rated in ohms. The higher this number is, the more “resisting” they do. We know that power is current times voltage, or:
P = I * E
Note that P is the notation used for power, I is the notation used for current, and E is usually used as the notation for voltage, though V is sometimes used. The “E” notation comes from the term often used for voltage by physicists, which is “electromotive force.”
We already discussed that simple relation of current and voltage to calculate power. It turns out that there is another equally simple relation relating current, voltage, and resistance, which is that voltage equals current times resistance:
E = I * R
This is known as Ohm’s law. Stated differently, this is:
I = E / R
Mathematically, it is easy to see that if current (I) remains constant, then if resistance (R) increases, voltage (E) must decrease to maintain the validity of the equation.
For a description of this 52 Project series and links to the all existing entries, go to the 52 Project 2012: Foundational Electronic Components Crash Course page.