The RJ Journal - Electronics

So what are resistors used for in electronic circuits? Resistors are mainly used for two things - limiting current (series resistor) or reducing voltage (voltage divider). Resistors are also used to clamp inputs to a known high or low voltage when the input is otherwise unconnected or driven by an output that actively drives to only low or high (a transistor or mechanical switch, for example). A resistor used in this way is called a pull up or pull down resistor. Together with capacitors and inductors, resistors are used to create filters. More of this in the chapter about capacitors and inductors. Resistors can also be used together with capacitors in timing circuits.

Resistor types

Resistors comes in different types and shapes. The most common types are carbon composition, film resistors and wire wound resistors.

Carbon composition resistors have the entire body of the resistor made out of a resistive composition. A composition which consists of carbon and an insulating filler, usually ceramic, where the resistance is depending on the ratio between the carbon and the filler. These resistors where the most commonly used up until the 1970s. They have medium to low accuracy (tolerance), high noise, medium to high tolerances and high temperature instability. Since the whole body is used as the resistive element, which is a relatively large mass, carbon composition resistors handles short pulses of high energy very good. They also have a low inductance compared to axial film type resistors and wirewound resistors. Today they are relatively expensive and not used as a general purpose resistor.

Film resistors can in turn be divided into carbon film, metal film, metal oxide, thick film and thin film types. All film type resistors have a resistive coating on a substrate mostly made of ceramic. Depending on type, the film can then either be etched to a spiral or a pattern on the surface or covering the entire surface.

Carbon film resistors are the successors to carbon composition resistors. Since the resistive material is similar to the one used in carbon composition resistors, they share many of their characteristics. Generally they have somewhat better accuracy and temperature stability than carbon composite resistors. Because of the carbon they are still noisy. The spiral etching in the film makes them somewhat inductive. They are still very cheap and can be used as a general purpose resistor. They can also achieve very high resistance values.

Metal film resistors are made in a similar way as the carbon film resistors and have even better characteristics regarding accuracy and temperature stability. They have very low noise and good frequency characteristics. They are used in applications that needs better performance and precision than the carbon film can deliver. They can achieve lower maximum resitance values than carbon film, though.

Metal oxide resistors are similar and look the same as metal film resistors. However, their entire body is deposited with a metal oxide and coated with an insulating, heat resistive layer, without the spiral etching, which makes them very low inductive with good high frequency characteristics. They can also withstand a higher temperature than the other type of film resistors which means that they can take more power. Because of this, they can also radiate more heat, which has to be taken into account when designing them onto a circuit board.

Thick film resistors are the most common type for SMD resistors but they can also be made on a rod shaped body. These resistors are made using a traditional screen-printing technology, where the print is the resistive material. They are then trimmed to an accurate value with abrasion or laser. They have very good accuracy and low noise. They have pretty low temperature stability though.

Thin film resistors looks the same as thick film resistors. However, they are made with a different technology, where the resistive layer first is deposited to the surface of the substrate. The final pattern, which determines the resistance value, is created with an etching process, much like the way printed circuit boards are made. The final resistor value is then trimmed with abrasion or laser, like with thick film resistors. This method gives the best accuracy and highest temperature stability. To no surprise, this resistor type is also more expensive than the other types.

Wire wound resistors are, as the name implies, a resistive wire wound around a plastic, ceramic or fiberglass core. These can be made for very high powers and are then enclosed in a metal or ceramic outer case. The ones with a metal case has to be mounted on a metal plate or a heat sink if they are to be operated at their higher power rating. They can be made very accurate with high temperature stability. Since they are wound, they are very inductive and not suited for high frequencies.

Nonlinear resistors

There exists other types of resistors, some of which will change their resistance depending on parameters in their environments. These are called nonlinear resistors.

VDR resistor - VDR stands for voltage dependent resistor. These resistors decreases their resistance when the voltage is above a certain threshold. These type of resistors are used as protection against power surges and is then connected in parallel with the circuit it is protecting. A VDR is also called a varistor.

NTC resistor - This is a resistor with a negative temperature coefficient, it has less resistance at higher temperatures. These are used as temperature sensors, as inrush current limiters and for temperature compensation in some circuits. They are also called thermistors.

PTC resistor - This is just like an NTC resistor, with the difference that they have a positive temperature coefficient. They can also be used as temperature sensors and for temperature compensating in some circuits and also as thermal fuses in transformers for example.

LDR resistor - This is a resistor which is light sensitive and can be said to convert brightness (or light) to a resistance. LDR stands for light dependant resistor.

Potentiometers

Until now we have only mentioned fixed value resistors. There is also variable resistors which are called potentiometers or pots for short. Their symbol is slightly different from a fixed value resistor and they are often prefixed with a P instead of an R in schematic diagrams.

The middle connection to the resistor body is called the wiper and it can be slid across the whole resistor. The resistor body of the potentiometer has a fixed resistor value and the resistance between the sides of the resistor and the wiper varies when it is moved. The sum of the resistance between one side of the resistor and the wiper and between the other side of resistor and the wiper is always the same and equal to the total resistance of the resistor body. In an electronic circuit, the potentiometer can be used either as a variable resistor or as a voltage divider. The wiper position can either be set with an axle and a knob or a slider.

For a variable resistor only two terminals are used, the wiper and one of the resistor sides. Sometimes the other side of the resistor is also connected to the wiper in this configuration. A variable resistor is also known as a rheostat. The variable resistor configuration will be used when the current in a circuit is to be varied. This could be used to dim a (low current) lamp, vary the speed of a (low current) motor or to vary the charging time for a capacitor in a timing circuit, for example.

When the resistor is used as a voltage divider the voltage that is to be divided is connected across the resistor ends and the divided voltage is taken out on the wiper, just as with the fixed voltage divider in the example circuit above. In this configuration the bottom end of the resistor is normally connected to 0V and the top end of the resistor is connected to a voltage that either comes from an output in another part of the circuit, a preamplifier for example, or to an output from some external circuit, such as the audio output signal from a tuner. The wiper is then connected to the input of the circuit which uses the scaled down voltage. With the potentiometer this input voltage can be varied between 0% and 100%. If the signal is an audio signal, the potentiometer becomes a volume control. Sometimes we don't want to be able to vary the signal all the way between 0% and 100%. In this case we just add resistors between the input and the top side of the potentiometer resistor and/or between 0V and the bottom side of the potentiometer resistor. Potentiometers used in this way to control audio volume is often doubled since audio often comes in stereo - 1 knob controls 2 potentiometers. The potentiometers is then said to be ganged. For audio applications the potentiometer is also often logarithmic instead of linear because our ear senses audio volume in a logarithmic way. In a logarithmic potentiometer the ratio between the high and low side resistance varies logarithmically when the wiper is varied linearly.

Another type of potentiometer is the trim potentiometer that is used to trim resistances and voltages in sensitive circuits. This is needed because of the tolerances in fixed resistors. The trim potentiometers are mostly set when the circuit is tested during manufacturing and are not supposed to changed thereafter.

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