This is the first of four installments on resistors. In this lesson we’ll cover what resistors are, what they do, and provide an overview of the many types, sizes, values, and ratings of common resistors. In lesson two we’ll cover resistor circuit basics. In lesson three we’ll cover potentiometers, and in lesson four we’ll cover other types of variable resistors such as temperature and light sensing resistors.
Resistors are the most used components in electronics appearing in almost every circuit.
Even when a circuit has no physical resistors, it still has resistance. Parasitic resistance as well as parasitic impedance, another form of resistance, at one level or another, occur naturally in all components including wires and circuit board traces.
Resistors, as the name implies, resist the flow of current. We use them in circuits to limit or control the amount of current reaching other components or to create a drop in voltage. In this way, a resistor is analogous to a valve (resistor) in a piping system (circuit) that reduces the flow of liquid (current) or creates a drop in pressure (voltage).
Unlike many other electronic components, a resistor has no polarity and can be inserted into a circuit in either direction.
Resistors come in both fixed and variable types. A variable resistor is called a potentiometer. Potentiometers allow us to adjust the resistance for functions such as volume level control or calibration.
American vs. European Resistor Symbols
Resistors are available in many different values, power ratings and tolerances. Tolerance indicates how closely the actual value of the resistor (measured with an Ohmmeter) matches the specified resistance. Tolerances can range from less than 0.1% to 20%. It’s important to use accurate resistors (very low % means very accurate) in precision applications. For other applications, such as lighting an LED, tolerance isn’t as important. The lower the resistors tolerance, the higher the cost.
Resistors also come in many shapes and sizes from large power resistors, some intended to be mounted on a heatsink, to tiny chip resistors designed for surface mounting on a printed circuit board (PCB).
A sampling of different resistor types
All resistors dissipate power in the form of heat. The power the resistor dissipates during operation is a product of the voltage and current to which the resistor is subjected. Resistors are power rated in watts. As form follows function, so does power rating follow application. The almost infinite applications in which resistors are used means they are available in a great number of power ratings from less than 1/8 of a watt to 100 watts and more.
Resistors are made from a number of different materials depending on the required application, size, power rating, and tolerance.
Resistors are made from different materials based on application and required specifications
Resistor values are stated in Ohms after George Ohm, a German physicist, teacher, and electrical pioneer who discovered the relationship between voltage, current, resistance, and power. The unit, Ohm, is abbreviated using the Greek capital letter Omega (Ω). Resistors come in standard values. Some types and/or tolerances of resistors come in more off-the-shelf values than others. While you may not find the exact value you calculate, 123Ω for example, you will almost always find one that is close enough for your application.
Standard values for 5% tolerance, 1/4 watt resistors
Resistors are labeled in different ways depending on tolerance, size, manufacturer, etc. The most common methods are colored bands (either 3, 4, 5, or 6), a numeric code, or values written directly on the resistor.
It takes a while, and a bit of practice, before you can easily decipher the code used to indicate resistor values
Common value abbreviations you’ll see used with resistors are k, meaning kilo Ω or x1,000, or M, meaning Mega Ω or x1,000,000. For example, a resistor that is marked 1.5k is actually 1.5 x1000 or 1,500Ω. Another common code system uses the letters R, k, or M as a decimal point. You will see this on schematics as well. The placement of the R, k, or M within the code tells you how to read the value. R33 = .33Ω while 3R3 = 3.3Ω and 33R = 33Ω.
Don’t worry if you don’t get it at first, there are a number of online resistor value calculators that you can use while you are learning. You’ll pick it up and be reading resistor values like a pro in no time.
This online calculator works for resistors with 3, 4, 5, or 6 color bands
When choosing a resistor, it’s important to understand the application. Do you need very accurate (tolerance) resistors? How does the resistor need to behave with respect to temperature? Some resistors, called thermistors, are specifically designed to correlate their resistance to temperature for use in thermostats and sensing applications. Other resistors are sensitive to light and vary their resistance in proportion to the amount of light striking the device.
What about power handling? Power resistors are large so they can withstand heat. It’s critical to understand the power the resistor needs to dissipate. A resistor that is power rated too low for the application can lead to disaster.
All electronic components contain magic smoke that imparts the devices amazing and unique capabilities. Apply more power than the device rating and the magic smoke is released and gone for ever. Trust me, I know from experience having accidentally released much magic smoke over the years. Oh, and it makes the house smell bad as well.
Well, that does it for this segment. If you enjoyed this topic and found it useful then please like and share. Thoughtful, on-topic comments are always appreciated.
The second installment, “Resistor Circuits”, will include video and cover resistor circuit analysis for DC circuits, Ohm’s and Kirchhoff’s Laws, and give a few practical applications that you can put to use immediately.
Watch for it in the next few days.
Dominick
