Mains voltages usually power AC-DC supplies, while a DC-DC supply could be powered from a battery or any other DC power source. These DC-DC converters use switched mode technology to change the input voltage to a higher or lower output voltage.
 
Off-the-shelf power supplies are available for many markets and for general use, but in some cases a custom design is needed. Resistor manufacturers and suppliers such as Microhm have many years of experience in helping customers choose the correct component for every application. The resistor values are chosen to give the required ratio, so the most important consideration is accuracy. If the comparator circuit features high gain and high input impedance, the worst case value can be easily calculated using the equation above.

 
High current sensing is used for measuring the supply current. The measurement is made using shunt ammeter principles, where a low value resistor is placed in series and the voltage drop is measured to calculate the current. A designer of this type of circuit must choose between a low value resistor to minimize heat generated and the loss of power, and a high resistance for easier measurement.
 
Almost every application for resistors in power supply design has different specification priorities and performance requirements. These include resistors that need to be able to handle high voltage, current and power, as well as ones that require low tolerances. Often specialized attributes are needed, like surge capability or negative TCR.