In the previous article, we discussed the signal behavior of a resistor. We also talked about various applications of resistors. A resistor is supposed to offer fixed resistance to the flow of current. That is what any ideal resistor (theoretically) is supposed to do. The ideal resistor is also expected to only provide resistance sans any capacitance or inductance in the circuit. In an effort to achieve ideal resistance, resistors are constructed in many ways. Therefore, there are many kinds of resistors.
Firstly, the resistors can be classified based on the nature of the resistance they provide. Further, they are sub-categorized on the basis of their construction. So, the resistors are broadly classified into the following categories:
1) Fixed Value Resistors – These resistors provide a fixed resistance all the time. Their resistance cannot be changed in any way. There are mainly four types of fixed value resistors:
a) Composition (Carbon or Ceramic) Resistors
b) Film Type Resistors
c) Wire-wound Resistors
d) Foil Resistors
e) Semiconductor Resistors
2) Variable Resistors – The resistance of these resistors can be changed manually or digitally. As they have provision for varying their resistance, these are called variable resistors. There are the following types of variable resistors:
b) Digital Potentiometer
3) Dependant Resistors – The resistance of these resistors is dependent on other physical quantity like light, voltage, temperature, magnetism or mechanical stress. There are the following types of dependent resistors:
a) Light Dependent Resistors
b) Voltage Dependent Resistors
d) Magnetic Dependent Resistors
e) Strain Gauges
4) Power Resistors – The purpose of these resistors is not to provide resistance in a circuit but to dissipate power (in the form of heat). There are the following types of power resistors:
a) Wirewound Resistors
b) Grid Resistors
c) SMD Resistors
d) Water Resistors
e) Liquid Rheostat
Most of the time, you might be using fixed value carbon composition resistors or trimpot or potentiometer in ‘common’ circuits. But, selecting a resistor for a specific circuit, use or application may not be that handy. The resistors exhibit lots of not-ideal characteristics that need careful considerations while designing circuits. These not-ideal characteristics are usually evaluated and decided by technical specifications, which can be considered Key Performance Indicators (KPIs) of resistors. The two of these KPIs must be essentially checked (even if you know that you are going to use a fixed value carbon composition resistor ultimately): Nominal resistance (of Fixed Resistor) or range of nominal resistance (of Variable Resistor), and Power Rating of the resistor. Further, there can be several KPIs that may need to be checked out depending upon the specific circuit, use, or application. The different types of resistors often fare well in some KPIs compromising with the others. So, the common indicators associated with resistors include the following –
1) Nominal Resistance
2) Power Rating
4) Voltage Rating
5) Voltage Coefficient of Resistance
6) Temperature Rating
7) Temperature Coefficient of Resistance
8) Frequency Response
10) Pulse Stability
Let us first discuss these technical specifications of resistors. These technical insights of resistors will be extremely useful in understanding applications of various types of resistors.
Nominal Resistance – Any resistor is supposed to offer a known resistance or range of resistance (in case of variable resistors). This is called its nominal resistance or simply the value of the resistor. When a resistor is connected in the circuit, it is required to be of a certain value. Either a resistor of that exact value (if available) or a combination of resistors otherwise close to the desired value is then connected in the circuit. The resistance value of the resistor is obviously the first thing to be checked before connecting it in a circuit. The value of the resistor is indicated by either color-coding or labels.
Power Rating – Power Rating is another very important factor that must be essentially considered while selecting a resistor. The resistors are generally available in power ratings of 1/20, 1/16, 1/10, 1/8, 1/4, 1/2, 1, 2, 5, 10, 15, 20, 25, 50, 100, 200, 300 and 600 Watt. The resistors with power rating less than 5 Watt generally do not have power rating explicitly labeled on the body or package. The resistors with power rating more than 5 Watt are categorized as power resistors and have generally power rating explicitly labeled on the package.
In the case of low wattage resistors — having a power rating less than 5 Watt — the power rating is determined by the size of the resistor package. Generally, the higher the power rating, the greater the size of the resistor package. The resistors come available mainly in three packages — axial, SMD, and MELF. The wattage of the axial resistors can be determined by measuring the length or diameter of the resistor package. The length and diameter of the axial resistor package can then be compared from standard power rating table (that will be mentioned in Resistor Selection Cheat Sheet in a future article) to determine the wattage of the resistor. Similarly, the wattage of an SMD resistor package can be determined by measuring length, breadth or width of the package and comparing it with a standard power rating table.
For SMD resistor packages, appropriate solder pad and land pattern must be used according to the imperial code of the SMD package. Like axial packages, the power rating of MELF packages can also be determined by measuring the length and diameter of the MELF package. The power rating of vendor-specific resistor packages (like vendor-specific wirewound resistors) can be determined again by measuring dimensions of the package and comparing them with power rating table provided by the vendor in its datasheet.
The power rating is the second most important factor that must be considered. The resistors can only dissipate power in the form of heat. Any excessive heating of the resistor can permanently damage it or change its actual resistance irreversibly. It is always safe to pick up a resistor with a power rating at least twice the maximum power it may encounter in the circuit. There can be other factors like the enclosure of the resistors, grouping, operating temperature range of the circuit and other possible environmental factors where a resistor of even higher power rating (four or five times of maximum power the resistor may encounter in the circuit) may need to be selected. For most of the applications, 1/4, 1/2 or 1 watt of carbon composition, SMD or film resistors or, in some cases 1 to 5 Watt of wire-wound resistors, are sufficient. In very rare applications, power resistors of rating ranging from 10 to 600 Watt may be required.
The power rating of the resistors is generally specified (either explicitly or otherwise) as maximum power that resistor can tolerate at 25° C temperature. With the rise in ambient temperature, the power capacity of the resistors decreases. The fall in power capacity of resistors against rising ambient temperature is specified by a derating curve by the manufacturers. The derating curve is generally plotted for temperature ranging from full-load ambient temperature (generally 25° C) to maximum no-load temperature that the resistor can withstand. In the derating curve, the ambient temperature is plotted on the horizontal axis in degree Celsius and power capacity is plotted on the vertical axis as percent rated load ranging from 100 percent (full load capacity at 25° C) to 0 percent (no-load capacity at maximum temperature resistor can tolerate). This is generally a linear plot sloping down from full load capacity (100 percent at 25° C) to no-load capacity (0 percent at maximum allowable ambient temperature).
Graph showing Power Derating Curve of a Resistor Series, (Source: Digi-Key)
Most of the resistors are designed to tolerate ambient temperature from 30° C to 40°C. If a specific circuit is to be designed for operations in temperatures above 40° C, the derating curve must be essentially checked. Then, it will be safe to pick up a resistor of power rating at least thrice or four times the maximum power it may encounter in the circuit. A high power rated resistor can even operate at higher temperatures without breaking when it is used below its full load power capacity.
It is important that resistors are generally grouped into series by manufacturers by their power rating as seen in the manufacturer’s datasheets for 1/4 Watt, 1/2 Watt, 1 Watt resistor series and so on.
Tolerance – The resistors may not deliver the specified nominal resistance even at 25° C. Their actual resistance may deviate from their nominal value due to environmental factors. This deviation is generally known by quality testing of resistor package by the manufacturer and is explicitly indicated as tolerance either in color code or label of the resistor package. The typical tolerance levels are 1%, 2%, 5%, 10% and 20% for various types of resistors. There are also precision resistors (metal film, wirewound and foil resistor types) that have tolerance as low as 0.1% to 0.0005%. If a 100Ω resistor has supposed 5% tolerance, then its actual resistance can be anywhere between 95Ω to 105Ω. Generally, low-value resistors in any series (like 1/4 watt resistor series from a manufacturer) have a higher tolerance while high value resistors in the same series have a lower tolerance.
Electrical Characteristics from datasheet of 1/4 Watt Metal Resistors from NTE Electronics
Voltage Rating – The voltage rating is the maximum DC or RMS voltage that a resistor can tolerate. The voltage rating is generally specified by the manufacturer for resistor series. The resistor series is specified by the power rating such as 1/4, 1/2, or 1 Watt resistors and so on. Any voltage beyond this rating can cause a surge current or leakage current in the resistor permanently damaging it or even damaging the nearby components. Most of the 1/4, 1/2, and 1 Watt resistor series have a voltage rating from 250 V to 350 V. Generally, the higher the power rating, the higher the voltage rating. There can be multiple voltage ratings available for different resistor series of same power rating from a manufacturer. For instance, a 1 W resistor series can be available for voltage rating from 250 V to as high as 1000 V.
Voltage Coefficient of Resistance – The resistance of a resistor may change due to applied voltage. This is indicated by the voltage coefficient of the resistor series. The voltage coefficient is the percentage change in resistance of the resistor per volt. This is usually a very low value that can be generally ignored until the applied voltage is below the voltage rating of the resistor.
Temperature Rating – The temperature rating is also specified for resistor series by the manufacturers instead of individual resistors. It is often specified as operating temperature range. For example, in the above screenshot, a 1/4 W Metal Resistor series has an operating temperature range from -55°C to 155°C. Some manufacturers also specify full-load maximum temperature and no-load maximum temperature. The full-load maximum temperature is the maximum temperature at which the resistor can be used dissipating maximum power. The no-load maximum temperature is the maximum temperature at which resistors can be stored without damage. The no-load maximum temperature is usually double the full-load maximum temperature. So, resistors from a series may be stored at a higher temperature but they can withstand relatively lower temperature in a working circuit.
Temperature Coefficient of Resistance – Apart from operating temperature range, the temperature coefficient of the resistor series is another important factor that must be considered in applications where limited change in resistance is permissible. The actual resistance of the resistor can change due to variations in ambient temperature, heat dissipation of the resistor itself, humidity, and mechanical stress. The change in resistance due to variation in temperature is indicated by the temperature coefficient of the resistor series. The temperature coefficient can be positive or negative. A positive temperature coefficient means that the actual resistance of the resistor increases from the nominal resistance (specified at room temperature, 25°C) with the increase in temperature. The negative temperature coefficient means that the actual resistance of the resistor decreases with the increase in temperature. The temperature coefficient is expressed in parts per million (PPM) and typically range from 1 PPM to 6700 PPM for different resistor series from various manufacturers. For example, in the above screenshot, a 1/4 W Metal Resistor series has 200 PPM temperature coefficient. It means its resistance deviates from nominal resistance by 200 PPM for each degree Celsius. At 35°C, a 1000Ω resistor then must have the following resistance:
Change in Resistance in PPM = 200 x (35-25) = 2000 PPM
Actual Resistance at 35°C = 1000 x (1 + 2000/1000,000) = 1002Ω
Generally, the change in resistance against temperature variation is not linear. So, the graph from the manufacturer’s resistor series datasheet must be checked for deriving the actual change in resistance for possible temperature variation in case of using the resistor in a critical circuit or application (like a temperature compensation circuit).
Frequency Response – A resistor is supposed to offer resistance sans any capacitance or inductance. But, due to design limitations and faults, resistors usually have some induced inductance or capacitance. This adds some impedance in the circuit due to the resistor. The higher the operating frequency of the circuit, the greater is the impedance added by the resistor. The manufacturers specify the frequency response of the resistors as a graph between frequency and unwanted impedance shown in the percentage of the resistor value. Typically, resistors can be used for the highest frequency at which the impedance equals the tolerance value of the resistor. Generally, low wattage resistors have better frequency response (meaning low impedance against frequency) than high wattage resistors due to their compact size and design.
Noise – The resistors have some ac fluctuations in response to applied DC voltage. Though these noise signals do not affect the resistance of the resistor, they must be taken into account in circuits designed for low-level signals or circuits processing digital signals. There can be Johnson noise, current noise, shot noise, or contact noise in a resistor. Johnson noise is the temperature-dependent thermal noise which depends on the resistance of the resistor, temperature, and bandwidth of the noise signal. It remains the same for all the frequencies and is a constant noise equal for any type of resistor of the same resistance. That is also called white noise.
Current noise varies inversely with the frequency of the applied signal. It depends on the current flowing through the resistor and its resistance. Contact noise is the noise in the resistor due to construction material and size of the resistor. The higher the power rating of the resistors, the less is the contact noise in them. Shot noise is the noise due to DC current in the resistor. It is difficult to measure noise levels in resistors. The resistors having low tolerance have the least noise levels. So, in circuits that have to deal with low-level signals or digital signals, precision resistors or resistors with least tolerance must be preferred.
Pulse Stability – The pulses deliver much higher voltage to a resistor in a short time than a constant load. The pulse stability of resistor series is often indicated by peak pulse voltage that the resistor can withstand without damaging itself. The pulse stability of a resistor can affect its long-term stability in case of frequent exposure to voltage surges.
Size – The physical size of the resistors can be a considerable factor in size-constraint circuits. Generally, low power resistors are small in size. The higher the power rating of a resistor, the greater are its physical dimensions.
Long-Term Stability – The repeatability of the nominal resistance of the resistor against various operating conditions like temperature variation, humidity, mechanical stress, voltage changes, pulses, and power dissipation is indicated as long term stability of the resistor. It is determined by the manufacturers for resistor series by conducting various short-term and long-term tests like damp heat test, load life test etc. It is indicated by stability class which is directly related to the percentage change in resistance over short-term and long-term tests. We will check out the table of standard stability classes in the Resistor Selection Cheat Sheet later.
Reliability – The reliability of resistors is indicated as Mean Time Between Failures (MTBF) or rate of failure per 1000 hours of operation. Reliability may not be a considerable factor in a ‘common’ circuit, but must be taken into account for critical circuits like circuits to be installed for military use.
In the next article, we will examine various types of resistors and will evaluate them by above-mentioned indicators in an attempt to find their suitability for various applications.
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