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Apart from the basic resistance there are several other parameters that need to be considered when looking at a resistor specification.
The resistance is obviously the key resistor specification. The value of the resistance is required by the calculations for the particular application in which it is to be used.
It is always best to use preferred values as these are easier to obtain. There are several series of resistor values that are used. These are referred to as the E-series. E3 has three values in a decade, i.e. 1.0, 2.2 and 4.7. Values of 10Ω 22Ω 47Ω are available in the tens of Ohms decade, 100Ω 202Ω 470Ω are available in the hundreds of Ohms decade and so forth.
It is always preferable to use as few values in a circuit design as possible as this reduces the number of different types required for any one design. Other series are also available, E6 with six values in each decade: 1.0, 1.5, 2.2, 3.3, 4.7, 6.8. There are also E12, E24, E48 and E96 etc values available, although their costs can increase marginally and they mean that many more component types are needed in a given design.
Power dissipation specification
Although the resistance is the key parameter for any type of resistor, another important parameter in the resistor specification is the amount of power it can dissipate.
When current passes through a resistor power is dissipated and this manifests itself in the form of heat. In turn this cases the temperature of the resistor to rise, and if too much current passes through the resistor, the temperature rise can be too great and it can cause the resistance to change, or in extreme cases it can cause damage to the resistor.
The power dissipated in a resistor is easy to calculate. The basic equation for power is:
W = power in watts
V = voltage in volts
I = current in amps
It is often easier to combine this equation with Ohm's Law to create a more useful equation which calculates the power dissipated from a knowledge of the resistance and the voltage across it:
R = resistance in ohms.
All resistors have a power dissipation rating specification. This is the maximum power that they are designed to dissipate. The resistor type should be chosen so that this power level is never exceeded in operation. In fact good design practice dictates that the maximum power dissipation should be well inside this. Many electronics design companies operate a practice where they state that the maximum actual dissipation should never exceed around 60% of the rating of the particular type of resistor. By doing this, the reliability of the circuit is improved.
Power de-rating specification
The resistor specification for power de-rating can be important when components may be expected to run at higher temperatures.
Under these circumstances the resistor will be running hot and it is necessary to ensure that its capability is not exceeded.
Typically the same power dissipation will be quoted up to a given temperature, after which the derating is applied. Typically this is a linear curve above the given temperature.
Temperature coefficient specification
In certain circumstances the resistor specification for temperature coefficient is of importance.
The temperature coefficient specification is the parameter that indicates the change in resistance with changing temperature. The resistor specification for the temperature coefficient will be very dependent upon the type of resistor, and it may also vary from one manufacturer to another. IT is therefore important to check the resistor specification for the temperature coefficient to ensure the particular resistor is suitable for the given application.
The temperature coefficient is the change in value of the resistance over a given temperature change. Normally it is expressed in term of parts per million, ppm, per degree Celcius, i.e. ppm/°C.In other words a 100kΩ resistor with temperature coefficient specification of 1000ppm/°C for a 10 °C temperature rise would change would change by 1000 /1 000000 * 100 * 100 000 Ω = &10Ω. This could be quite significant in some circumstances.
Maximum temperature specification
The resistor specification for temperature needs to be adhered to. Above certain temperatures the resistor may function outside its specified operating parameters. Also under extreme conditions damage could result and the overall circuit may cease to function.
If resistors are operated well above their rated temperatures for extended periods, the value of the resistance can permanently increase, and this could cause the overall circuit to malfunction.
A further reason for operating below the rated temperature is overall reliability. Resistors, and all other components are more likely to fail if operated outside their specified ranges. Often components are operated inside their specification with a good margin to ensure that the reliability is maximised.
Resistor specification for maximum voltage
Resistors are designed to operate up to a certain voltage. Above this voltage there is the possibility of breakdown as a result of the electrical stress applied to the component.
As a result of this resistor datasheets will contain a resistor specification for the maximum voltage that should be applied.
The actual value will depend on a variety of factors including the physical size of the resistor, its structure, the technology used, and a variety of other factors.
Typically it is not good practice to run a resistor close to its rated voltage specification. Often design standards recommend running a resistor at a maximum of 60% or even les of the maximum rated voltage to ensure reliability is maintained.
These resistor specifications are some of the more commonly seen resistor specifications and parameters. Other exist and manufacturer datasheets should be consulted before settling on a given type.
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