What circuits use high-power resistors?

High power resistors (usually referring to resistors that dissipate power from a few watts to several kilowatts or even higher) are mainly used in circuits that require the dissipation of a large amount of energy, withstand high currents or voltages. Here are some common application scenarios:

1. Power circuit:
Surge limiting/starting resistor: used at the input end of a switching power supply or large linear power supply to limit the surge current during initial power on, protect the rectifier bridge, filtering capacitor, and switching transistor. The initial surge current is large, and the resistor needs to withstand high power pulses.
Discharge resistor: parallel connected at both ends of a high-voltage filtering capacitor (such as the output capacitor of a switching power supply or the discharge of an X capacitor), it safely releases the dangerous charges stored in the capacitor after power failure to prevent electric shock. It needs to withstand steady-state voltage and discharge power.
Pseudo load: In linear regulators or certain switching power supplies, a minimum load resistance may be required to consume a certain amount of power to ensure stability under no-load or light load conditions.
Voltage regulation/voltage division: In the voltage division network of a high-voltage power supply (such as several hundred volts), even if the current is not large, the power loss on the resistor may be significant due to the high voltage.

2. Motor control and drive:
Dynamic braking resistor: This is one of the most common applications. When motors (especially DC motors, motors driven by AC frequency converters, servo motors) need to slow down or stop quickly, the motor is in a generator state, generating regenerated electrical energy. If this energy is fed back to the DC bus, it will cause a sharp increase in bus voltage and damage the equipment. The dynamic braking resistor is connected in parallel on the DC bus and controlled by the braking unit to convert the regenerated electrical energy into thermal energy for consumption. The instantaneous braking power is extremely high.
Starting/current limiting resistor: used in star delta starting or soft starter of large AC motors (such as induction motors) to limit the starting current. After the startup is completed, it is usually short circuited, but during the startup process, the current is high and the resistance power is high.

3. Electronic loads and testing equipment:
Power testing/aging: The core function of electronic loads is to simulate various load conditions and consume the power output of the tested power supply. High power electronic loads use high-power resistor arrays (or combinations of transistors and resistors) to absorb and consume large amounts of electrical energy for testing, validation, and burn-in processes of power supplies.
RF load: In RF power amplifier testing, a pure resistive load (such as 50 ohms or 75 ohms) that can withstand high power is required to absorb the transmitted power and prevent signal reflection from damaging the amplifier.

4. Load of power amplifier:
Fake load: When testing audio power amplifiers and RF power amplifiers, a resistive load (such as 8 ohms, 4 ohms, or 50 ohms) that can withstand the maximum output power of the amplifier is required for debugging, measuring power, and preventing damage to the amplifier under no-load conditions.

5. Voltage divider and attenuator:
High voltage divider: used to measure high voltage (such as in power systems, X-ray machines, radar pulse modulators). In order to withstand high voltage and ensure the stability and safety of the voltage divider ratio, the voltage divider resistor must have a sufficiently high rated power and voltage.
Power attenuator: In situations where it is necessary to reduce the power level of RF or microwave signals, a resistor network capable of withstanding input power is used inside the power attenuator.

6. Discharge circuit:
Discharge of energy storage components: In energy storage devices such as supercapacitor packs, large capacity battery packs, and high-voltage pulse capacitor packs, in order to safely maintain or quickly release energy, high-power discharge resistors need to be connected in parallel to convert stored energy into thermal energy for release. The discharge current may be very high.
Electromagnetic coil/relay demagnetization: When disconnecting the electromagnetic coil or large relay coil, a high back electromotive force will be generated. Parallel connection of a resistor (freewheeling resistor) can consume this energy and protect switch devices (such as transistors and relay contacts). This resistor needs to withstand peak voltage and energy.

7. Current detection (special application):
High current sampling: Although current detection usually uses low resistance resistors in the milliohm range to reduce power consumption, in situations where high currents of hundreds or even thousands of amperes need to be detected (such as welding machines, high-power inverters), even if the resistance is very small (such as 0.1 milliohm), power loss may reach tens of watts or higher, requiring the use of specialized high-power, low resistance, low temperature coefficient shunts (Shun Resistors).

8. Heating application:
Resistance heater: Although not strictly a "circuit function", resistance heating itself is a basic characteristic of high-power resistors. Industrial heating equipment, constant temperature baths, defrosting devices, etc. directly use high-power resistors as heat sources.

Key considerations for choosing high-power resistors:

Rated power: It must be greater than or equal to the average power actually consumed in the circuit, taking into account peak power and pulse tolerance. Usually, derating is used (such as using only 50% -70% of rated power) to ensure lifespan and reliability.
Rated voltage: must be greater than the maximum voltage that the two ends of the resistor can withstand.
Resistance value: meets the functional requirements of the circuit.
Heat dissipation method: High power resistors usually require forced heat dissipation (such as adding heat sinks or fans) or installation on metal shells/heat dissipation plates. The packaging form of the resistor itself (such as aluminum shell resistors with heat sinks, wire wound resistors, non inductive resistors, etc.) is also closely related to its heat dissipation capability.
Environmental temperature: High temperatures significantly reduce the actual power carrying capacity of resistors.
Pulse power tolerance: For instantaneous high-power applications such as surges and braking, the rated pulse power of the resistor is crucial.
Inductance/capacitance: For high-frequency applications (such as RF loads), it is necessary to choose non inductive or low inductive resistors.

In summary, any application that requires the safe and effective conversion of excess electrical energy into thermal energy, or the stable operation of resistors in high voltage and high current environments, may require the use of high-power resistors. They play a crucial role in ensuring system security, stable operation, and conducting high-power testing.