Power resistor devices are used in a wide range of applications from simple power supplies to complex military systems such as RADAR and SONAR. Although, in principle, selection of a power resistor for a particular application should be a simple procedure in practice there are many factors to consider. This article covers the key points to evaluate.
One of the most basic laws of electronics (Ohms Law) states the current through a resistor is directly proportional to the voltage across it and inversely proportional to the resistance. The current through a resistor produces heat and it is consideration of heat dissipation that is a major element affecting resistor selection.
Any power resistor chosen for a particular application must be able to withstand the heat generated without any physical damage or short or long term deterioration in performance. The current through the resistor produces a rise in temperature of the resistor above the ambient temperature and this heat must be dissipated in some way and/or the resistor must be able to physically withstand the rise in temperature.
The ability of a resistor to both withstand and dissipate heat is directly related to both its construction (materials) and its physical size therefore key elements to consider when choosing a power resistor include:
1. The required resistance value
2. The Watts to be dissipated by the resistor
3. The required physical size given volts, Watts dissipation (2), maximum temperatures, environmental and mounting conditions.
4. The most appropriate resistor construction.
The resistance value will be determined by the application, the circuit and the correct application of Ohms Law. The power or Wattage can then be calculated using P=I2R. As the power equation is a square law it is important to remember that only a small change in current or voltage will produce a proportionally much higher change in the Wattage so maximum voltage and the actual current should be used.
The required physical size and the resistor construction are then linked. For power resistors thick film resistor and wirewound resistors are the most common constructions. The choice of the most appropriate construction and the required size and mounting method (including heat sinking) is a complex decision involving a number of factors. Some of these factors may not be known and may only be estimated or measured.
Power resistor selection is therefore far from as simple a task as it may first appear and experience and technical knowledge play a major part in correct device selection.