Wire Wound Terminology & Glossary

Ayrton-Perry Winding:

Winding of two wires in parallel but opposite directions to give better cancellation of magnetic fields than is obtained with a single winding.

Wirewound technology has long been known as a leading technology for power device needs. The most critical drawback with this technology is that it is inherently inductive. This is logical given that a wirewound inductor and a wirewound resistor are made with essentially the same materials and processes. This fact limits the use of wirewounds for applications with high switching speeds, which require low inductance.

Now the same standard wirewounds can be used for these applications by using a non-inductively wound version. This manufacturing method greatly reduces the inductance of any given resistor size and value combination, however it does not completely eliminate the inductance. A non-inductively wound wirewound has one winding in one direction and one in the other direction; known as Ayrton Perry winding. This non-inductive winding is available in all Direct standard wire wound series.

Dynamic Braking Resistors:

Dynamic braking resistors are used on VFDs (AC variable frequency drives) to dissipate energy that is produced in the motor as the drive provides braking torque to stop the motor. The resistors are either used alone for decelerating or in conjunction with compressed air brakes for stopping. The excitation voltage of the traction motors generally comes from a static converter powered by the overhead catenary and operates as a DC voltage transformer.

Direct's BOX, and RNW resistors are used for most dynamic braking applications. These are conveniently provided on L-shaped mounting brackets or in standard enclosures with louvered or screened covers. For heavy duty applications that require even higher wattage, we recommend Direct's Oval Edge-Wound DOE, and Power-Wound DST resistor elements.

Neutral Grounding Resistor:

A suitably rated industrial resistor that is connected between the neutral of a transformer (or generator) and the system ground. It serves to limit fault currents and prevent damage to the equipment.

Rated Continuous Current:

The current expressed in amperes (RMS), that the device can carry continuously under specified service conditions without exceeding the allowable temperature rise.

What is a Ground Fault:

A Ground Fault is an unwanted connection between the system conductors and ground. Ground faults often go unnoticed and can cause problems with plant production processes. They can also shut down power and damage equipment, which disrupts the flow of production leading to hours or even days of lost productivity.

Undetected ground faults pose potential health and safety risks to personnel. Ground faults can lead to safety hazards such as equipment malfunctions, fire and electric shock. Ground faults cause serious damage to equipment and to your processes. This damage can seriously affect your bottom line.

Power Dissipation:

This is a measure of the amount of power that a resistor can dissipate without causing it to overheat. Resistors are manufactured in standard power ratings and mostly these are in fractions of 1 Watt with some larger carbon and metal resistors available in 1 Watt to about 5 Watts. Higher power ratings are available. Wirewound units are normally available in power ratings of up to about 50 W. However, industrial wirewound types are made by component manufacturers in much higher power ratings to the specification of the customer.

Industrial Wirewound Variables & Adjustable Resistor - Resistive Controls:

Controls that produce a varying voltage using resistance are called either adjustable or variable resistors (potentiometers). Although both types of control may be functionally the same, it is the way they are connected that differentiates between the two types.

A common construction is for the control to have three connections. One connected to a sliding contact called the wiper and the other two to either end of a fixed resistor called the track. The wiper is able to be moved along the track either by use of a linear sliding control or a rotary "wiper" contact. Both linear and rotary controls have the same basic operation.

High Power Wirewound Industrial Resistors:

Industrial resistors are high temperature, high power, wirewound types, and non-inductive, they generally coated with a vitreous or glass epoxy enamel for use in resistance banks or DC motor/servo control and dynamic braking applications. The resistance wire is wound around a ceramic or porcelain tube covered with mica to prevent the alloy wires from moving when hot. Industrial resistors are available in a variety of resistance and power ratings. With one main use of high power industrial resistor is in the electrical heating elements of an electric fire which converts the electrical current flowing through it into heat with each element dissipating up to 20000W, (20kW) of energy.

Because the wire is wound into a coil, it acts like an inductor causing them to have inductance as well as resistance and this affects the way the resistor behaves in AC circuits by producing a phase shift at high frequencies especially in the larger size resistors. The length of the actual resistance path in the resistor and the leads contributes inductance in series with the "apparent" DC resistance resulting in an overall impedance path Z. Impedance (Z) is the combined effect of resistance (R) and inductance (X), measured in ohms and for a series AC circuit is given as, Equation Z2 = R2 + X2.

When used in AC circuits this inductance value changes with frequency (inductive reactance, XL = 2πƒL) and therefore, the overall value of the resistor changes. Inductive reactance increases with frequency but is zero at DC (zero frequency). Then, resistors must not be designed into AC or amplifier type circuits where the frequency across the resistor changes. However, Direct offers special wirewound non-inductive resistors winding Ayrton-Perry Method are also available for alternative choice.

Adjustable Resistor Application Notes

Determination of End Resistance Value of FVR, DQS, DSRA, DSRB, BSR, BSQ::

  1. Resistance Range means you can choose one maximum resistance value (End resistance value)
    at one of FVR, DQS, DSRA, DSRB, BSR, BSQ VR (Adjustable Resistor) type.

  2. After End Resistance Value confirmed, the minimum resistance (start resistance value) will be determined
    by depending on resistance of wire and wirewound type.

Power Rating of Adjustable Resistor::

The part Nunber formation of FVR, DQS, DSRA, DSRB, BSR, BSQ:

Product type - Rated Wattage - Resistance Value (Ω) - Resistance Tolerance

  • Product type means one of FVR, DQS, DSRA, DSRB, BSR, BSQ.
  • Rated Wattage means power rating at End Resistance Value.
  • Resistance Value (Ω) means maximum resistance value (End Resistance Value).
  • Resistance Tolerance means precision range of End Resistance Value.
  1. Power Rating of VR (Variable Resistor) is determined by the maximum resistance value (End Resistance Value).
  2. Resistance and Power Rating should be decreased while you are adjusting the screw.


  • Adjustability is 10% to 90% of full resistance value.
  • Wattage is proportional to this adjusted resistance value.

Power Rating::

  • Based on 25°C free air rating. The stated wattage rating applies only when the entire resistance is in the circuit.
  • Setting the lug at an intermediate point reduces the wattage rating by approximately the same proportion.
  • Example: If the lug is set at half resistance, the wattage is reduced by approximately one-half.

If you need current constant type or special specifications, please feel free to cntact us.

Wire-Wound Resistor General Information

Benefits & Features:

Providing design engineers with an economical resistor with high quality performance, Direct Electronics offers industry grade power wirewound devices.

Direct provide terminal blocks, thermal switches, fusing, fans, junction boxes, screened or solid bottom plates, conduit knockouts, and customer specified requirements. For large applications a welded frame construction is utilized to provide a robust design for power resistor mounting in both indoor and outdoor environments.

Products range from large capacity metal clad, nonflammable fixed and adjustable, wave ribbon wire-wound, slide, starter, box type, to nonflammable flat type. Direct extends a complete line for both military and commercial applications.

Utilization Notes:

  1. Smoke emitted from non-flammable resistors on initial use in powered circuits is a normal phenomenon and the component can be safely utilized.
  2. All resistors manufactured by Direct Electronics Industry Corporation comply with the U.S. UL-94 non- flammability test, Class V-0, a continuous combustion period of zero seconds.
  3. Never use organic solvents to clean non-flammable resistors.
  4. Non-flammable resistors cannot be utilized in oil.
  5. Non-flammable resistors cannot be used in high frequency machinery because of the inductance produced by the windings. A suitable type of resistor must be selected. Contact us for details.
  6. In applications where resistors are subject to intermittent current surges and spikes, be sure in advance that the components selected are capable of withstanding brief durations of increased load.
  7. Do not exceed the recommended usable load. Resistors must used within the rated voltage range to prevent the shortening of service life and/or failure of the wound resistance elements.
  8. Minimum load. Resistors must be utilized at 1/10 or more of the rated voltage to prevent poor conductance due to oxidation build-up.
  9. Although the hardness exceeds that of a 3H pencil lead, do not nick the resistor coating with screw drivers or other pointed objects.
  10. Avoid touching non-flammable resistors in operation; the surface temperature ranges from approximately 350°C ~ 400°C when utilized at the full rated value. Maintaining a surface temperature of 200°C or less will extend resistor service life.
  11. Keep temperature from rising by choosing a resistor with a higher rated capacity; do not use a component having the exact load value required. For considerations of safety in extended period applications, the resistor rating should be more than four times higher than the actual wattage involved, but never use a resistor at less than 25% of its rated power.
  12. Application and Placement: Wire-wound resistors use different gauges of wire as resistance elements. Sometimes the gauge is extremely thin (finer than a strand of human hair) and very susceptible to breakage in environments containing salts, ash, dust and corrosives. Avoid utilization in such environments. Do not install in dusty areas because the accumulation will cause shorts and poor conductance.

PDF Power Wirewound Resistor Design Notes and Catalogue Download: