Not only Ohms
- When considering resistors, resistance is not the only important thing to think about. Like any other component there are a number of important things to consider.
- Here are 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 ideally means downloading a manufacturer’s data sheet for any particular component.
- These are widely available for almost any component listed on any manufacturer’s or component supplier’s web site.
The value of a resistor is dependent 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 having a resistivity not greatly influenced by temperature.
- That is, 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.
- Ideally, resistors should act as pure resistors, without any of the characteristics of other types of component and when they are used in DC circuits they do.
- In AC circuits however some resistors may have characteristics that make them unsuitable for a particular purpose.
- At high frequencies, some resistors also have characteristics of capacitance and/or inductance.
- Because of this they will have a property called reactance, similar to resistance but dependent 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 dependent 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 although inferior to film type resistors in most other respects, act as pure resistors at frequencies in the Megahertz (MHz) range (at least those with a resistance below about 10KΩ) .
- 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 as surface mount resistors remain purely resistive up to hundreds of MHz.
- The resistors with the worst frequency response are not surprisingly wire wound 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 wire wound resistors in any circuit operating at frequencies above a few hundred Hertz (Hz).
- Wire wound 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 wire wound types, they do have a much better frequency response.
- 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 1Watt with some larger carbon and metal resistors available in 1Watt to about 5Watts.
- Wire wound resistors are normally available in power ratings of up to about 25W, and special wire wound types are made by component manufacturers in much higher power ratings, often to the specifications of the customer (the equipment maker).
- Typical maximum temperatures for carbon composition resistors would be around 100 to 120°C and for metal and oxide film types, about 150°C.
- Wire wound resistors can operate at higher temperatures up to around 300°C.
Figure 1.Power Derating Curve
- For power resistors, as an alternative to a specified maximum temperature, manufactures data sheets often specify a “Power de-rating curve” similar to that illustrated in Fig. 1,which shows how the specified power rating of the resistor must be reduced (de-rated) at various temperatures above the normal operating range.
- Resistors are designed to operate within a specified temperature
- Within this range parameters such as tolerance and temperature coefficient are ‘as advertised’ but outside this range they are not
- 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
- Whilst very low temperatures can occur in such circuits as aerospace equipment, high temperatures can be encountered very locally in almost any electrical equipment due to a resistor being mounted close to some other heat generating
- The long term effect on a resistor of being subjected to high operating temperatures is that its resistance value will gradually
- This is especially noticeable on resistors having a high resistance value to start
- 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.
- As the power (P) dissipated as heat, depends on this voltage multiplied by the current (I), which will decrease due to the increase in
- However the current will probably not decrease proportionately because other components in the circuit will also have an effect on the amount of current drawn through the
- Because (P=VI), the power dissipated by the resistor increases, and so will the heat
- Eventually (in the absence of any safety measures) the resistor will burn out and/or damage other components in the circuit.
- The voltage developed across 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 from 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 manufacturer’s 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 certain critical resistors in any piece of equipment may be labelled as a safety component with a small symbol similar to those shown in Fig 2.
Figure 2. Safety Component Symbols
- In these instances, ONLY the manufacturer’s direct replacement is suitable.
- The markings shown are not universally adopted however, so when servicing any electronics equipment, close attention must be paid to manufacturer’s service manuals for the particular equipment being worked on.