Resistors are playing important roles in regulating the supply output and protecting the supply from faults. Besides linear regulators and switched-mode power supplies, resistors also has other usage in AC-DC and DC-DC supplies.
Inrush limiting resistors limit the amount of current that can surge when AC-DC supplies are initially switched on and the storage capacitor is charging. These resistors are usually of a very low value and are designed in series with the AC power line. For higher-power supplies, negative temperature coefficient (NTC) resistors are often used for this purpose. These resistors have a resistance that drops as they self-heat. One downside to using this type of resistor is that during operation the temperature must be kept regular to ensure the low resistance is maintained. A third type of solution involves using pulse-resistant resistors, which are normally energy-rated in Joules. This gives a better idea of their capabilities than the normal continuous power rating that wattage designates.
Bleed resistors are mainly used to discharge capacitors in the circuit. Microhm Electronics' NUAL series, NLR140U and FLR200 are suitable as bleeder resistors. They are situated in parallel with the load and are used in AC-DC and DC-DC converters for discharging the smoothing capacitors and reservoir capacitors respectively. The capacitors maintain their charge after the supply is switched off and can be hazardous to users. There are two main points to balance when choosing resistors for this task: they should be a high enough resistance to consume little power when the circuit is operational, while being low enough value to discharge the capacitors quickly.
Balancing resistors modulate a load current when using more than a single supply. Often, using more than one DC-DC converter in a parallel arrangement can be cheaper, while also being more energy efficient and compact than using a single high power supply. When designing this type of circuit, it is not possible to just tie the outputs together; there must be a way to ensure the load is shared equally.
This method of load sharing is also used in other types of power-supply designs, especially those using power transistors. Multiple transistors in parallel supply the load, and load-sharing resistors are used in series.
High voltage dividers are used to provide the feedback to the regulation circuit. These resistors can often have a secondary purpose, such as monitoring the high voltage supply in defibrillators, charging the storage capacitor and switching off the supply at the desired charge level.
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. Microhm Electronics provides resistors with resistance value from micro-ohm to T ohm for engineers' options.
