Resistor Parameters
Not only Ohms
When considering resistors, its easy to think of resistance as the only thing we need to think about, but that is not the case; like any other component, there are a number of important things to know about. Here we consider a few of the main parameters. For full information on any resistor (or in fact any other component) you should seek out a reliable source of information, which usually means a manufacturers or component supplier's data sheet.
Temperature coefficient
The value of a resistor is dependant on the length, cross sectional area and resistivity of the resistive material it is made from. The quoted value of a resistor however is actually given as "So many ohms at a particular temperature". This is because the temperature of the resistor also affects its value.
The change in resistance due to a change in temperature is normally quite small over a particular temperature range. This is because the manufacturer has chosen a material whose resistivity is not greatly influenced by temperature. We say that the material (and so the resistor) has a low TEMPERATURE COEFFICIENT. In other words, there is only a small change in value per °C. This change in value is normally quoted in parts per million (ppm) so a typical resistor would have, as part of its specification a quoted temperature coefficient such as;
Temperature coefficient: 50ppm/°C
Meaning that the change in value due to a temperature change of 1°C will not be more than 50Ω for every 1MΩ of the resistor's value (or 0.05Ω for every 1KΩ of its value).
The temperature coefficient quoted above would be typical of a metal film resistor. Carbon film types have temperature coefficients typically around 200 to 500ppm/°C
The change in value of a resistor with changing temperature is not very dependent on changes in the dimensions of the component as it expands or contracts due to temperature changes. It is due mainly to a change in the resistivity of the material caused by the activity of the atoms of which the material is made.
Frequency Response
Ideally, resistors should act as pure resistors, without any of the characteristics of other types of component. When resistors are used in DC circuits they do but in AC circuits different designs of resistor may have characteristics that make them unsuitable for a particular design. At higher frequencies, resistors tend to have some of the characteristics of capacitors and/or inductors. In addition to capacitance and inductance properties of these components they each have have a property called reactance, similar to resistance but dependant on the frequency of AC signals passing through the component. The frequency response of a resistor tells us at what frequencies the resistor still acts as a pure resistor, without any significant effects associated with these other types of frequency dependant components. For this reason this parameter is chiefly of interest to people working with high frequency AC circuits, such as radio frequency (RF) engineers.
Carbon composition resistors (at least those with a resistance below about 10KΩ) act as pure resistors at frequencies in the Megahertz (MHz) range.
Film type resistors having a spiral construction do tend to exhibit the properties of inductors (which are basically spirally wound coils of wire) but this is not usually a problem until used at frequencies in the MHz range. Film type resistors that do not have a spiral track, such a surface mount resistors remain purely resistive up to hundreds of MHz.
The resistors with the worst frequency response are not surprisingly wirewound types, as their construction is really a coil of wire - just like an inductor. Therefore the inductance and reactance effects must be considered when using wirewound resistor in any circuit operating at frequencies above a few hundred Hertz (Hz). Wirewound resistors are used for high power applications and are available in resistances up to a few KΩ. At higher resistances high power metal film resistors may be used; although they do not have as high a power rating as some wirewound types, they do have a much better frequency response.
Power Dissipation
This is a measure of the amount of power that a resistor can dissipate without causing it to overheat. resistors are manufactured in standard power ratings and mostly these are in fractions of 1 Watt with some larger carbon and metal resistors available in 1Watt to about 5Watts. Higher power ratings are available. Wirewound resistors are normally available in power ratings of up to about 5W. However special wirewound types are made by component manufacturers in much higher power ratings to the specification of the customer (the equipment maker).
Maximum Temperature
Resistors are designed to operate within specified a specified temperature range. Within this range parameters such as tolerance and temperature coefficient are "as advertised" but outside this range they are not guaranteed. The most likely limit of the temperature range to be achieved in most uses will be the maximum, due to the heat produced by the working circuit in addition to any ambient temperature. Low temperature problems are more likely to occur in aerospace equipment.
High temperatures can be encountered very locally in electrical equipment due to a resistor being mounted close to some other heat generating component. The long term effect on a resistor of being subjected to high operating temperatures is that its resistance value will gradually increase. This is especially noticeable on resistors having a high resistance value to start with. Where resistors are used in high power situations this increase in resistance (R) will lead to an increase in the voltage (V) developed across it as V=IR, and as the power (P), dissipated as heat, depends on this voltage multiplied by the current (I) because (P=VI). As the power dissipated by the resistor increases, so will the heat generated and eventually (in the absence of any safety measures) the resistor will burn out and/or damage other components in the circuit.
Typical maximum temperatures for carbon composition resistor would be around 100 to 120°C and for metal and oxide film types, about 150°C. Wi rewound resistors can operate at higher temperatures up to around 300°C. For power resistors, as an alternative to a specified maximum temperature, manufactures data sheets often specify a "Power de−rating curve" similar to that shown below,which shows how the specified power rating of the resistor must be reduced (de−rated) at various temperatures above the normal operating range.
Maximum Voltage
The voltage voltage developed across the ends of a resistor as current flows through it places an electrical stress on the materials from which the resistor is made. If this voltage exceeds the permitted maximum there is a likelihood of a sudden breakdown of the resistor and a voltage flash over. The maximum voltage varies greatly between different types of resistor form just a few volts for some surface mount types to several thousand volts for some specialist high voltage resistors.
All the above parameters plus others such as the amount of random electrical noise generated may need to be taken into account when selecting a resistor for a particular application. A reliable source of information such as a supplier's catalogue or manufacturers data sheet should be consulted when choosing resistors.
When servicing equipment it is advisable to use replacement components supplied by the original manufacturer as far as is possible. In addition it is common for certain resistors in any piece of equipment to be labeled as a safety component with a small symbol similar to those shown below. In these instances ONLY the manufacturer's direct replacement is suitable.
Safety component symbols.
