Power Resistors

What is a power resistor?
Power resistors belong to a special class of electrical resistors. Power resistors are electrically passive components that limit the flow of current through an electrical circuit. The main special feature of power resistors is their suitability for use at high powers, i.e., at very high loads and resulting temperatures. Special attention must be paid to the maximum power consumption of the components due to power dissipation, which is caused by heat generation during operation.

Power limits - the power derating curve
Electrical power is formed by multiplying current and voltage or by the ratio of the squared voltage to resistance value or by multiplying the square of current by resistance:

P = I*U = I²*R = U²/R

Therefore, electrical power follows a square function of current or voltage, i.e., an increase in current or voltage of 20%, for example, results in an increase in power of 44%. This law must be considered when selecting the component. In addition, the thermal transfer of heat loss must be enabled by the high-power consumption. The heat dissipation is supported by the materials used for the resistor housings. These are often aluminum housings with additional heat-dissipating cooling fins or housings made of heat-dissipating ceramics. In addition, appropriately dimensioned heat sinks, usually in the form of aluminum plates, are used on which the resistor is mounted, if necessary, together with a heat-conducting paste. In the data sheets of power resistors, there is usually a curve that shows the relationship between the maximum permissible power and the ambient temperature, the power derating curve.

Pulsed operation
Power resistors are used to protect electronics when high currents must be drawn for a short time to protect other components from overload. This can happen, for example, when large loads are switched on and off or in the event of a lightning strike. Power resistors are particularly suitable for these applications with high pulse loads. One example is the use in defibrillators. They emit a high amount of energy in a short time, which can be absorbed by power resistors specified for pulsed operation for this pulse time without any problems. For suitable models, specifications for pulsed operation can be found in the data sheet. Depending on the values for resistor, pulse repetition rate and operating temperature, these values change, so graphs on this should be read with caution.

Material technologies of power resistors
As technologies for power resistors, metal wire or metal film technology are particularly widespread. The two technologies differ fundamentally in the structure of the resistor element and the materials used, as well as the resulting properties.

Wire-wound resistors
Wire wound resistors are manufactured by winding a wire around a carrier core. Precision wire-wound resistors are characterized by particularly high ESD resistance, low noise, and a low temperature coefficient. However, due to the winding, these components have non-negligible values for inductance and capacitance, which can lead to problems in high-frequency applications. There are various methods of winding these components, which has different advantages and disadvantages depending on the method. However, wire-wound resistors achieve better long-term and temperature stability than metal film resistors and can be supplied with certain custom resistance values by customizing the winding.

Metal film / thin film resistors
Metal film resistors have an advantage especially in high frequency applications because they have very low values for inductance and capacitance. Numerous processes are used to produce a metal film resistor. There is a thin deposited metal layer (thin film) on a ceramic substrate. This layer has a high resistance on a small area. Depending on the nominal value of the resistance, the thickness of the layer is varied. Basically, thin film resistors have a pronounced aging behavior because the thin film is susceptible to oxidation and self-etching. For precision components, the layer is therefore artificially pre-aged.

Kelvin connection - four-wire measurement
In addition to the two standard connections, some models of power resistors offer additional outputs as an option, which enable 4-pole Kelvin measurement. If the resistance value of the power resistor is particularly small, the connection resistors (and/or other resistors in the line) may be similar in size to the power resistor itself. Then the falling voltage at the power resistor, and thus the power consumed by the power resistor itself, can no longer be directly monitored across the entire voltage drop across the supply and discharge lines. The two additional connections enable the voltage drop across the resistor to be measured directly using an additional measuring device. This allows the current to be calculated because the resistance is known and, in addition, the power at the power resistor can be calculated. The system can then be monitored during operation.