The function of resistors is to oppose the flow of electric current in a circuit. Therefore their primary parameter is the resistance value. The manufacturing tolerance must be adequately chosen for each specific application. The ultimate resistance value may deviate from the specification because of many reasons. One is the temperature coefficient of resistance, or TCR, which is often specified for precision applications. Stability defines the long term variations of the resistance. After a long duration of electric load, the resistance value will not return to its original value. Electric noise appears in every resistor, and is for low-noise amplifying applications of importance. For high frequency applications, the inductance and capacitance properties play a role. Next to the characteristics related to resistance value, the maximum power and voltage can be specified. The maximum power rating is mainly for power electronics important, while resistors in electronic circuit boards mostly never reach the maximum power rating. For high voltage circuits, the maximum rated voltage must be taken into account. The quality of a resistor in terms of durability and reliability is for some applications more important than for others. An overview of the most common resistor properties and characteristics to describe a resistor are detailed below. Low Temperature Coefficient of Resistance (TCR) The TCR is dependent on the resistive material and the resistor construction. The temperature dependence of electrical resistivity is determined by the material: Number of phonons Coefficient of expansion from the material Power rating The power rating indicates the maximum dissipation that the component is capable of. The rated dissipation is normally specified at room temperature and decreases at higher temperatures. This is called derating. Typically from 70°C derating is specified. Above this temperature, it can only utilize a reduced power level. This is illustrated by a derating curve. The [… read more]

Resistance changes with temperature The temperature coefficient of resistance, or TCR, is one of the main used parameters to characterize a resistor. The TCR defines the change in resistance as a function of the ambient temperature. The common way to express the TCR is in ppm/°C, which stands for parts per million per centigrade degree. The temperature coefficient of resistance is calculated as follows:     Where TCR is in ppm/°C, R1 is in ohms at room temperature, R2 is resistance at operating temperature in ohms, T1 is the room temperature in °C and T2 is the operating temperature in °C. Often instead of TCR, α is used. Positive or Negative Temperature Coefficient of Resistance? Resistors are available with a TCR that is negative, positive, or stable over a certain temperature range. Choosing the right resistor could prevent the need for temperature compensation. In some applications it is desired to have a large TCR, for example to measure temperature. Resistors for these applications are called thermistors, and can have a positive (PTC) or negative temperature coefficient (NTC). Measuring methods for the TCR The temperature coefficient of resistance for a resistor is determined by measuring the resistances values over an appropriate temperature range. The TCR is calculated as the average slope of the resistance value over this interval. This is accurate for linear relations, since the TCR is constant at every temperature. However, many materials have a non linear coefficient. For Nichrome for example, a popular alloy for resistors, the relation between temperature and TCR is not linear. Because the TCR is calculated as average slope, it is therefore very important to specify the TCR as well as the temperature interval. The way to measure TCR is standardized in MIL-STD-202 Method 304. With this method, TCR is calculated for the range between [… read more]

Resistance of a resistor The function of a resistor is to oppose the electric current through it. This is called electrical resistance, and is measured in the unit ohm. The resistance can be calculated with Ohms law, when the current is known and the voltage drop is measured:     The resistance of a resistor is dependent on its material and shape. Some materials have a higher resistivity, causing a higher value. The value is often printed on the resistor with a number or in the form of a color code. What is resistance? The concept of current, voltage and resistance can be explained by a hydraulic analogy. A flow of water through a pipe is restricted by a constriction. This causes a pressure drop after the constriction. The flow of water is equivalent to electric current. The pressure drop is equal to the voltage drop. The constriction is equivalent to the resistor, and has a certain resistance. The resistance is proportional to the voltage or pressure drop for a given current. In the hydraulic example, the resistance can be increased by for example reducing the diameter of the constriction. For a resistor or wire, the resistance is in general dependent on the material and the geometrical shape. The influence of the geometrical shape, can easily be explained by using the hydraulic example. A long and narrow tube will have a higher resistance than a short and wide tube. The resistance property of a material is called resistivity. The electrical resistance of a resistor is proportional to the resistivity of the material. For a rectangular cross-section resistor the resistance R is given by: where ρ is the resistivity of the resistor material (W·m), l is the length of the resistor along direction of current flow (m), and A is the [… read more]

What is Ohm’s law? Ohm’s law states that the electrical current through a conductor is proportional to the potential difference across it. Furthermore, the electrical resistance of the conductor is constant. This leads to the mathematical equation: where I is the current in amperes, V the voltage in volts and R the resistance in ohms. To illustrate: a resistor of one ohm subjected to a current of 1A has a voltage difference of 1V across its terminals. The equation is named after Georg Ohm. In 1827 he published his findings that form the basis of the formula that is used today. He performed a large series of experiments that showed the relation between applied voltage and current through a conductor. The law is therefore empirical. Although Ohm’s law is one of the fundamentals of electrical engineering, at the time of publication it was received with criticism. The ohm is adopted as the official SI unit for electrical resistance. Gustav Kirchhoff (known from Kirchhoff’s circuit laws) made a generalization that is more used in physics: where σ is the conductivity parameter (material specific), J is the current density in a location of that material, and E the electric field in that location. Ohm’s law and resistors Resistors are passive elements that introduce resistance to the flow of electric current in a circuit. A resistor that functions according to Ohm’s law is called an Ohmic resistor. When current passes through an Ohmic resistor, the voltage drop across the terminals is proportionally to the magnitude of resistance. Ohm’s formula stays also valid for circuits with varying voltage or current, so it can be used for AC circuits as well. For capacitors and inductors the law can of course not be used, since their I-V curve is inherently not linear (not Ohmic). Ohm’s formula [… read more]

What is a Foil Resistor? The metal foil resistor has the best precision and stability properties of all resistor types. The foil is made of an alloy of usually Nichrome with additives. It is mounted on a ceramic carrier with high heat conductivity. The foil has a thickness of only several micrometers. The desired resistance value is achieved by a photoetched resistive pattern in the foil. The metal foil resistor has a low Temperature Coefficient of Resistance (TCR), good long term stability, low noise, low capacitance, fast thermal stabilization and no inductance. The low TCR is one of the most important parameters that influence the stability. This means that the resistance value will vary only a small amount as the ambient temperature and the resistor’s internal temperature changes. Over a range from 0 till 60 deg C, a typical value for TCR is around 1 ppm per deg C. This depends on the construction (thermo-mechanical effects) and the properties of the foil material. In the planar foil the pattern is made parallel to reduce inductance (max 0.08 microH). Foil Resistor Definition A foil resistor is a high precision component to limit electric current. The opposition to current flow is provided by a very thin piece of metal. Metal Foil Resistor Characteristics The excellent properties for precision and stability are due to a combination of several characteristics, which are following from the design principles of the metal foil resistor. Temperature Coefficient, Resistance (TCR) – Foil resistors achieve a low TCR by taking advantage of two characteristics of the foil. The resistance of the foil naturally increases as temperature increases. The resistor is manufactured so that rising temperature causes compression of the foil. This makes the resistance drop as temperature rises. The total effect is one of very little resistance change as [… read more]

What is a variable resistor? A variable resistor is a resistor of which the electric resistance value can be adjusted. A variable resistor is in essence an electro-mechanical transducer and normally works by sliding a contact (wiper) over a resistive element. When a variable resistor is used as a potential divider by using 3 terminals it is called a potentiometer. When only two terminals are used, it functions as a variable resistance and is called a rheostat. Electronically controlled variable resistors exist, which can be controlled electronically instead of by mechanical action. These resistors are called digital potentiometers. Variable resistor definition A resistor of which the ohmic resistance value can be adjusted. Either mechanically (potentiometer, rheostat) or electronically (digital potentiometer). Types of variable resistors Potentiometer The potentiometer is the most common variable resistor. It functions as a potential divider and is used to generate a voltage signal depending on the position of the potentiometer. This signal can be used for a very wide variety of applications including: Amplifier gain control(audio volume), measurement of distance or angles, tuning of circuits and much more. When variable resistors are used to tune or calibrate a circuit or application, trimmer potentiometers or trimpots are used, this are mostly small potentiometers mounted on the circuit board, which can be adjusted using a screwdriver. Rheostat Rheostats are very similar in construction to potentiometers, but are not used as a potential divider, but as a variable resistance. They use only 2 terminals instead of the 3 terminals potentiometers use. One connection is made at one end of the resistive element, the other at the wiper of the variable resistor. In the past rheostats were  used as power control devices in series with the load, such as a light bulb. Nowadays rheostats are not used as power control anymore [… read more]