Understanding precision resistors: temperature, power & voltage

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The term “precision resistor” generally refers to a resistor whose value is correct to 1 percent or better. This value isn’t set in stone; different manufacturers may set different points, such as 0.5 percent or even lower. In any case, the percentage will be clearly stated in an unambiguous manner. However, this is only the starting point, as the actual resistance of a 1 percent, 500-ohm resistor is guaranteed to be between 495 ohms and 505 ohms only under ideal conditions. But real-world designs must function under real-world conditions.

Temperature and Power Resistor Behavior

Electronic devices often have to operate in high-temperature environments, so it is vital to know how the resistor you choose will behave with variations in temperature. The Temperature Coefficient of Resistance (TCR) is specified as the change in resistance, in parts per million (ppm), for a change in temperature of one degree. The value is usually specific for degrees Celsius, as opposed to the Fahrenheit. Closely related to the TCR is the Power Coefficient of Resistance (PCR), because the temperature changes in a resistor are closely related to the amount of power dissipated.

Changes due to TCR are reversible; when power is turned off and the resistor cools back down to room temperature, the resistor’s value returns to normal. But, over the longer term, the temperature under which the resistor operates can also cause permanent changes in a resistor’s value. The effect varies with resistor technology, and it can usually be predicted in advance based on information supplied by the manufacturer.

Voltage

The Voltage Coefficient of Resistance (VCR) is the change in resistance with applied voltage. This is entirely different and in addition to the effects of self-heating when power is applied. A resistor with a VCR of 100 ppm/V will change 0.1 percent over a 10-Volt change and 1 percent over a 100-Volt change.

Especially Precision Resistors

There are many factors precision resistors share with all resistors that are, perhaps, more important for the former simply because they don’t often factor in with applications that can tolerate 10 or 20 percent variation, but can be devastating in an application requiring higher fidelity.

Every resistor is rated by the maximum power it can dissipate, but the stated amount of safe power dissipation is derated by how far the temperature of the resistor deviates from ambient room temperature. A good spec sheet for any precision resistor will include a graph specifying the fall-off.

Resistor noise can be thought of as tiny, random electrical signals emanating from a resistor. This can be intolerable to designs that measure low-level analog signals. Also, the value of resistors can change over time, and in response to many sorts of environmental stress, with power dissipation and heat being the usual malefactors.

Different Types of Resistors

There are many different types of resistors on the market today and some types are better suited than others to specific challenges when high precision, and its handmaiden, high stability, are required. Generally speaking, precision thin-film resistors perform well on this basis, and foil resistors are even better. Vishay’s Bulk Metal® Z-Foil resistors exhibit TCRs as low as 1 ppm over the entire –50 °C to +125 °C temperature range.

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Figure 1: Vishay Bulk Metal® Z-Foil Resistors (Source: Vishay Precision Group)

Vishay’s Z201-1K0000 family of high-precision resistors is available at Arrow Electronics. The datasheet reveals a family of through-hole foil resistors with TCRs as low as ± 0.05 ppm/°C over a range of 0 °C to +60 °C.

Specifying a precision resistor can be a daunting task, and they are not inexpensive. The designer needs to completely understand the environmental stresses the final product will need to survive, and carefully study the datasheets offered by reputable manufacturers. He or she must avoid not only under-specifying, but over-specifying, too.

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