Lesson 07 – Resistors

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Lesson 7 – Resistors

As I introduce new terms, I have included a link to Wikipedia. Read ahead a little, and if you still need help you can click on any orange word below for more information than you probably want. 🙂

OK, we talked quite a lot about resistors in the last three lessons on Ohms Law, but I wanted to use this lesson to expand on the different types of resistors. Not just what they are made of, but also to show that there are some that are adjustable, some that respond to light, and some that respond to temperature changes.

Fixed Resistors

First, let’s talk about fixed resistors. A fixed resistor remains at a fairly constant value even when the temperature or voltage changes. There are many ways of manufacturing fixed resistors.

The older resistors, still found in some equipment, were called Carbon Composition Resistors or CCR, (not related to the Creedence Clearwater Revival band, sorry, they are one of my old favorites). They are made of a mixture of carbon and an insulating material, usually ceramic. More carbon means lower resistance since carbon is a conductor, more ceramic meant higher resistance.

Both of the resistors shown below are carbon composition resistors. The older style, on the left, called “dog bone” resistors, had the two leads wrapped around, and soldered to, the uninsulated carbon and ceramic cylinder, and then painted.

The ones on the left came later, with the leads coming out axially, meaning out the ends like an axle. Both of these are found in older devices and equipment like old radios, TVs, etc.

It isn’t really my intention of going into every possible type of fixed resistor, especially when they all do basically the same job, and also since Wikipedia does such a great job of doing that. Instead, I want to show more details of “other than fixed resistors”. Before we move on though, here are a few fixed resistors with links to Wikipedia if you want more details.

A Carbon Pile Resistor was just a stack of carbon discs clamped together. A Carbon Film Resistor has an insulated cylinder with a carbon trace printed spirally around it. A Printed Carbon Resistor is a Carbon Composition Resistor that is printed directly onto a printed circuit board.

There are Thick and Thin Film Resistors, Metal Film Resistors, Wire Wound Resistors, and probably some other ways of manufacturing them by the time you read this. The most popular fixed resistor for experimenters is the Metal Film Resistor, and in manufacturing, they mostly use the Surface-Mount Technology to manufacture Surface-Mount Devices.

Potentiometers

A Potentiometer, sometimes just called a Pot, is a type of adjustable resistor. Below are three photos of what some different types look like. The one on the left is your normal rotary type which would have the shaft protruding thru usually a metal panel with a knob on it, such as a volume control. The middle photo is a collection of various types that would be mounted on a printed circuit board. Some of those have a small screw on them and are multi turn pots. They don’t just go one revolution, some of them take 10 full turns on the screw to go from one end to the other. That allows them to adjust in very small increments. The ones on the right are the type that you slide instead of turning to adjust.

All of them work the same way, They have a resistor with a lead on each end, and a wiper that moves from one end to the other as you adjust it. That wiper makes contact with the resistor at various spots along the way, and is connected to the center lead.

In the next diagram below, on the left, we have a 10 volt battery, a 20 ohm resistor, and an 80 ohm resistor in series. If we hooked 2 voltmeters across the 2 resistors, we would see 2 volts on the top one and 8 volts on the bottom one. We have a fixed voltage divider. We learned in the last three lessons how to calculate those values.

The diagram below on the right is basically the same thing, except the center connection, shown with the arrow, can move up and down the resistor. That changes the value of the resistance as we turn (or slide) the pot.

Picture that arrow on the center of the pot being all the way at the top. The top voltmeter would effectively be shorted out and would read zero. The bottom voltmeter would have the full 10 volts across it. As we moved the pot toward the opposite direction, the voltmeters would show more and more voltage at the top while at the same time less at the bottom.

At the extreme opposite end from where we started, the arrow would now be at the bottom and now the bottom meter would be zero and the top would read 10 volts. So we could set the pot to read 5 and 5, or 4 and 6, or any combination we wanted to as long as they added up to 10 volts. We now have an adjustable voltage divider.

Potentiometers come in various values and wattages, just like regular fixed resistors. In addition, they also come in 2 types based on how they change resistance when you position the knob or slide. A Linear Taper Potentiometer changes resistance as described above, meaning that if the shaft or the slide were set right in the middle, the resistance on each side of the center terminal would be equal.

That type would be good for pretty much every use except for an audio circuit. A volume control, for example would use a logarithmic taper, also called an audio taper. That is because of the way our ears work. At higher volumes, a larger change is needed for us to be able to sense that it is louder. If you are replacing a potentiometer in a circuit, you will need to know which type to order, along with value, wattage, and physical size.

Rheostats

A lot of people use the term rheostat and potentiometer interchangeably. Technically speaking , there are a few differences, but the sometimes you could use either. A potentiometer has 3 terminals whereas a rheostat only has 2.

A potentiometer is used to control voltage, a rheostat is used to control current. If the amount of current was small enough, a potentiometer could be used as a rheostat by only using 2 of its 3 terminals. But for heavier loads, a rheostat is usually wire wound and can handle more current. Below is a photo of a rotary rheostat.

Light Dependent Resistors

A Light Dependent Resistor or LDR is a resistive device that changes resistance based on the amout of light shining on it. It can be used to turn outside lights off when the sun comes up in the morning, and back on again when it gets dark out. It can be used in an alarm system to see when someone or something breaks a beam of light shining across a doorway, for example. They are also called Photoresistors or Photo-Conductive cells. Shown below is a light dependent resistor photo on the left, and its schematic symbol on the right.

Thermistors

A Thermistor is a resistor that changes value, in a controled way, as its tempeature changes. There are basically two types, an NTC and a PTC. The NTC stands for negative temperature coefficient. With that type, as the temperature goes up, its resistance goes down. They are usually used as a temperature sensor.

The other type, a PTC, has a positive temperature coefficient. In that type, the resistance goes up as as the temperature rises. They are typically used as resetable fuses found in electric heaters, dishwashers, etc. Below is a photo of one type of thermistor and its symbol.

Test your knowledge if you feel like it with a little test. No cheating now!

Q1: A potentiometer can have either a linear taper, or ___________ taper.

Q2: A Rheostat normally has ____ terminals.

Two
Three
Four

Q3: A Thermistor whose resistance increases when the temperature increases is a _____________ thermistor.