Resistor for Capacitors: Fairfild Solutions for Charging and Discharging

Capacitors are energy-storage devices capable of storing energy within an electric field through conductive plates separated by a dielectric material.

Their main purpose is to add capacitance to an electrical circuit. During charging and discharging operations, however, dedicated resistors are required to protect both the capacitor and the other components within the circuit.

Why Is a Resistor Necessary in a Capacitor Circuit?

The resistor performs a different function depending on whether it is used during capacitor charging or discharging.

During the charging phase, it limits the current flowing into the component and helps control overvoltage. During the discharging phase, it prevents the stored energy from being released almost instantaneously.

The resistor therefore helps protect the capacitor and the other circuit components from the effects of high currents or sudden electrical discharges.

Resistor for Capacitor Charging

When a capacitor bank is charged, a high inrush current may be generated. This current can seriously damage the capacitor’s internal dielectric material and the other components within the circuit.

Installing a resistor limits the current flowing into the capacitor and controls overvoltage, which could otherwise cause the component to fail and result in serious damage to the circuit.

A resistor for capacitor charging therefore helps to:

  • limit inrush current;
  • protect the dielectric material;
  • protect the other components within the circuit;
  • control overvoltage during charging.

Resistor for Capacitor Discharging

Resistors are also necessary during capacitor discharging.

Without a resistor, the component could discharge almost instantaneously. At high voltages, this behaviour can be dangerous and may result in sparks or fires.

Using a resistor prevents the stored energy from being released immediately and protects the circuit from the possible consequences of an uncontrolled discharge.

Where Are These Resistors Used?

Capacitors are used in a wide range of electronic devices and electrical circuits. Their dimensions and voltage ratings depend on the characteristics of the system in which they are installed.

Typical applications for Fairfild resistors include traction converters and auxiliary converters. The resistor is generally installed in the pre-charge circuit and connected to the contactor.

Which Fairfild Resistors Can Be Used?

Fairfild offers several resistor series designed for capacitor charging and discharging.

The available solutions differ in terms of construction technology, electrical characteristics and configuration. Each of these elements must be evaluated according to the specific requirements of the application.

RC Series Resistors

The RC series includes wire-wound resistors manufactured on a tubular ceramic support coated with inorganic phosphate cement.

The coating provides mechanical protection for the wire, promotes an even distribution of heat and limits the thermal expansion of the windings.

The series is also available in:

  • low-inductance versions;
  • versions with an adjustable slider;
  • versions with a fixed intermediate tap.

One example is the RC TC 220, which Fairfild also recommends for use in RC circuits as a capacitor charging and discharging resistor.

RCA Series Resistors

RCA resistors are manufactured using wire wound onto a tubular ceramic support coated with inorganic phosphate cement. They also feature collars positioned underneath the coating.

The series includes:

  • the low-inductance RCAN version;
  • the RCAR version with an adjustable slider;
  • the RCAP version with a fixed intermediate tap.

The RCA 100 model can be used in RC circuits as a capacitor charging and discharging resistor.

RFD Series Resistors

The RFD series uses a different construction technology.

The wire is wound onto a cement-coated mica plate, embedded in quartzite sand and installed inside an anodised aluminium enclosure.

Sealing with silicone elastomer allows the enclosure to achieve an IP54 protection rating.

The following configurations are also available:

  • versions equipped with a thermal cut-out;
  • enclosures cut at a 45-degree angle for different mounting arrangements.

The RFD 550 model is recommended by Fairfild for dynamic braking applications and for use as a capacitor charging and discharging resistor in RC circuits.

Flat Metal-Cased Resistors and Pre-Charge Applications

In addition to solutions specifically intended for capacitor charging and discharging, Fairfild offers flat metal-cased resistors that can also be used in pre-charge applications.

These resistors provide a practical solution for:

  • braking;
  • pre-charging;
  • capacitor discharging.

The range includes numerous series and models, with different options relating to protection ratings, the presence or absence of a thermostat and cable length.

Fairfild: Different Technologies for Capacitor Charging and Discharging

The selection of a resistor for a capacitor must take into account the function the component will perform within the circuit and the characteristics required by the specific application.

Through its RC, RCA and RFD series, Fairfild offers resistors designed for capacitor charging and discharging in RC circuits.

For capacitor pre-charge and discharge applications, the Fairfild catalogue also includes several series of flat metal-cased resistors.

The datasheets for the individual models, available in the Fairfild catalogue, provide detailed information about:

  • construction and electrical characteristics;
  • available configurations;
  • installation requirements.

This information makes it possible to identify the most suitable solution for the needs of each specific application.