The Learning Circuit 17: Resistors

According to Ohm’s Law, in every circuit there is a balance between voltage, current, and resistance.  Most electrical components require a certain voltage and current to function properly. By adding resistors, you can control the voltage and current in various parts of a circuit.  Resistance in an electrical circuit is when a material prevents some or all of the electrical current from flowing through it, converting that restricted energy into heat.  Every material has some resistance.  In resistors, this principle is used to create a controlled amount of resistance that can be used to regulate circuits.  Resistors can be made of a variety of materials.  Carbon composite resistors are made by combining a conductive material, usually finely ground carbon or graphite, with a non-conducting material like ceramic. Once widely used, they have mostly been replaced by more efficient and precise resistor types. While cheap to produce, their lack of use means they are often more expensive.  Carbon film, metal film, and metal oxide film are examples of thin film resistors.  Thin film resistors are generally made by depositing a conductive material onto an insulating ceramic rod or substrate.  A laser trimmed pattern is cut into the film in order to increase its conductive or resistive path. The resistance value is controlled by varying the thickness of the film.  Thin film resistors are the most common type of through-hole resistors.  Surface mount resistors are created using the same method as thin film resistor.  Surface mount resistors feature relatively high tolerances, low temperature coefficients, low noise, and work well in high frequency applications.

Another type, thick film or cermet, uses a paste combining ceramic and metal; designed to have the optimal properties of both materials. They have good temperature stability, low noise, and good voltage ratings but have low surge current properties. This method is most commonly used to make potentiometers and surface mount resistors. Wire round resistors are made with wire, with higher than normal resistance.  This gives them a high level of reliability and comparatively low level of temperature coefficient, making them well-suited to handle high powers. Given the size of the wire used to make them, they tend to be quite large. They are often used in circuit breakers or as fuses.

Depending on your circuit, you don’t necessarily have to have a resistor that’s the exact value you want.  Often, you can get away with a resistor that’s just close to your desired value. The temperature coefficient gives you the amount of resistance change that occurs when the temperature of the resistor changes.  The size of the resistor is often relative to its power rating in watts.  If you need a resistor that can handle a lot of power, you’re likely stuck with a larger resistor. If you’re working with low power circuits, you have a lot more options.  SMD resistors have gotten so small, it's not a question of how small a resistor you can get, but how small of a resistor you can solder to your board reliably. Not all resistors have fixed values. Variable resistors have a maximum resistance and can be adjusted to have a resistance between 0 or near zero and their maximum resistance.  Some respond to environmental stimuli such as flex sensors that bend, force sensing resistors that respond to pressure, thermistors that respond to heat, and photo resistors or LDRs that respond to light. There are others that respond to moisture, gas, and other environmental stimuli. The most common variable resistors are potentiometers. Inside, potentiometers have a band of resistive material. The control shaft connects to a wiper that contacts the resistive band and rotates, varying the amount of resistive material between two leads. Another type of potentiometer is a trimmer pot, also known as trim pots or presets . Trimmer Pots, also known as trim pots or presets, are a type of potentiometer that are normally mounted on PCBs and adjusted by using a screwdriver. They are meant for occasional tuning and calibration. Rheostats are similar to potentiometers except they are mostly constructed as wire round resistors.  The wiper moves back and forth along the wire coil to vary the amount of resistance.