Wirewound resistor
A wirewound resistor is a type of electrical passive component that limits current by using a conductive wire wound around a non-conductive core. It is one of the oldest technologies for manufacturing resistors and is still widely used today for high-power and high-precision applications.
These resistors are known for their ability to handle large amounts of power and dissipate heat effectively, though their coil-like structure introduces inductance, which can be undesirable in high-frequency circuits.
Construction
The construction of a wirewound resistor is mechanically robust.
- Core: A resistive wire is wound around a central core made of ceramic, plastic, or fiberglass. Ceramic is the most common choice due to its high thermal conductivity.
- Winding: The resistance wire is typically made of alloys such as Nichrome (Nickel-Chromium), Manganin, or Constantan. The length, thickness, and material of the wire determine the resistance value.
- Coating: The wound assembly is coated with an insulating material to protect the wire and aid in heat dissipation. Common coatings include vitreous enamel, silicone, or a ceramic case (often referred to as "cement resistors").
Characteristics
Wirewound resistors possess distinct characteristics that dictate their specific use cases.
- High Power Rating: They are capable of dissipating significant amounts of heat, with power ratings ranging from 0.5 watts to over 1000 watts.
- Precision: Depending on the alloy used, they can have very tight tolerances (down to 0.005%) and excellent temperature stability.
- Inductance and Capacitance: Because the wire is wound in a coil, the resistor acts as an inductor. This creates reactance, making standard wirewound resistors unsuitable for frequencies above 50 kHz.
- Non-inductive winding: Special winding techniques, such as Bifilar winding, can be used to cancel out the electromagnetic fields, minimizing inductance for audio or precision applications.
- Noise: They generate the lowest current noise of all resistor types, making them excellent for DC circuits where noise is a concern.
Applications
Wirewound resistors are primarily chosen for power handling and precision.
- Power Supplies: Used as load resistors to test power supplies or as bleeders to discharge capacitors.
- Audio Equipment: Commonly used in the output stage of power amplifiers (e.g., emitter resistors) and in speaker crossover networks due to their ability to handle high currents.
- Current Sensing: Precision wirewound resistors are used as shunts to measure current with high accuracy.
- Circuit Breakers: Used as fusible resistors that open the circuit under overload conditions.
Comparison with Film Resistors
| Feature | Wirewound | Metal/Carbon Film |
|---|---|---|
| Power Handling | Very High (Up to Kilowatts) | Low to Medium |
| High Frequency | Poor (Inductive) | Excellent |
| Noise | Very Low | Low to Moderate |
| Physical Size | Large | Small |
| Cost | Moderate to High | Very Low |