Application / Design Notes on Selection of Wirewound Resistor

Basics

Simply put, a resistor is an electronic component connected into an electrical circuit to insert a specific resistance. Resistance is measured in ohms and as per ohms law, the current through the resistor will be directly proportional to the voltage across it and inversely proportional to the resistance. As the current flows through the resistor, heat is produced which makes the temperature of the resistor to rise above the ambient temperature.

Now whether a particular resistor can be used in a specific electrical circuit is its ability to dissipate the heat generated without physical deterioration and within the temperature limits of that particular circuit.

Resistors are rated to dissipate a given wattage without exceeding the declared “hot spot” temperature. This is largely determined by the size and materials used in the construction of the resistor and this is called “Free Air Watt Rating” or “Full Rating” or “Maximum Power Rating”.

In some cases the conditions actually encountered deviate from the standard conditions and affect the temperature rise which determines whether that particular resistor can be used or not in a particular application.

SELECTION PROCESS – Stepwise Guide

• Decide the resistance value required
  The following formulae derived from Ohms law can be used for this purpose:R = V/I or I = V/R or V = I x R where,(R is resistance in ohms, V is voltage in volts and I is current in Amperes)

• Decide the watts (Power) to be dissipated by the resistor
  W = I² x R or W = V x I or W = V²/R where,(W is Power Rating / Wattage in watts, I is current in Amperes, R is resistance in ohms and V is Voltage in volts.)

Note: Whilst the power rating in watts can be theoretically determined as above, a note of caution is now introduced – It is important that the actual current that will be drawn is used in the determination of the power rating / wattage of the resistor.

Small increases in current or voltage e.g. 20% translate into 44% increase in the power rating / wattage required to dissipate the increased current / voltage within temperature rise limitations. At this point it is also worth mentioning that the designer should also make allowance for the maximum possible line voltage.

• Decide the correct physical size (“watt size”) based on the following parameters: watts, volts, temperature that can be permitted in the particular circuit and mounting consideration.

The wattage rating of a resistor as established under specified standard conditions is defined as “Free Air Rating” (Maximum Power Rating).

The following method is broadly used to determine “Free Air Rating” based on the methods followed by “National Electrical Manufacturers Association” – USA (NEMA), “Underwriters Laboratories Inc.” (UL) and US MIL – R26 – US Military Specification for wirewound resistors.

In US MIL – R26, there are mainly 2 broad characteristics of resistor types – characteristic ‘V’ and characteristic ‘U’.

Characteristic ‘V’ resistors are required not to exceed a maximum operating temperature of 350°C, which corresponds to a maximum temperature rise of 325°C at ambient temperature 25°C.

Characteristic ‘U’ resistors are required not to exceed a maximum operating temperature of 275°C, which corresponds to a maximum temperature rise of 250°C with ambient temperature 25°C.

The temperature is normally measured on the body of the resistor, suspended in free still air space with unrestricted circulation of air. When current passes through a resistor, heat is generated and the temperature stabilizes when the sum of heat loss (by termination conduction, radiation and convection)equals the heat input rate (created by passing current proportional to wattage).

By rule of thumb, the larger the resistor, hence greater the area for heat dissipation, the lower the temperature rise.

Having said this, it must be admitted that certain other factors such as thermal conductivity of the ceramic core, type and gauge of resistance wire selected and the heat-sink effect of the type of mounting all influence the selection of a resistor to be considered having “acceptable service life”.

Further consideration must be given in case the resistor will be operated in elevated ambient temperatures higher than 25°C or 30°C and the power rating must be derated as per the derating curve provided with each HTR series.

For the design engineer’s general guidance, we give below the temperature rise that is generally observed on silicon coated axial resistors (ambient temperature of 30°C) at Maximum Power Rating / Free Air Rating.

Absolute temperature can be arrived at after adding the prevalent ambient temperature at time of test to the temperature rise figures provided.

These figures are given to merely serve as a guide to a design engineer and must be verified in actual practical conditions by the design engineer before selection and use of a particular resistor.

• Decide the actual resistor to be used: based on actual practical considerationsHaving determined the Wattage / Free Air watt rating on theoretical basis, the designer must now take the following factors into account when deciding on the actual resistor to be used in the application, as all these factors will influence the temperature rise: 
  • The influence of Ambient Temperature: All the components of an electronic circuit have their own limitations as to the maximum temperature at which they can reliably function. The temperature that the component rises to in service is the sum of the ambient temperature plus the temperature rise due to heat dissipated by each component during operation. Now some devices can tolerate elevated temperatures whilst others cannot.
    Wire wound resistors can operate fairly reliably at reasonably elevated temperatures, so in order to ensure that the heat generated by the resistor is minimized, the designer may move to a higher power rating from the theoretical calculation to minimize the temperature rise and minimize the effects of heating on other devices which are heat sensitive in the circuit.
  • The design of the Enclosure: The walls of the enclosure form a thermal barrier, preventing heat from escaping and preventing the outside air from entering and providing cooling. Hence, due care must be given to the optimum design / orientation of the ventilation openings of the enclosure.
  • Spacing: In case due to design limitations, if heat generating components are bunched together, they will show a higher temperature rise due to heat received by radiation from each other. Therefore it is prudent if at all possible that the designer tries to prevent bunching of heat generating components and if this is not possible, moves to a higher power rating to minimize temperature rise.
  • Surges: In certain applications for e.g. typically motor controllers, the resistors do encounter surge conditions which if not properly managed and taken into account at the time of designing the resistor, will lead to resistor failure.
    A “Surge” occurs over such a short period of time, in the case of capacitor charge / discharge < 1 msec and in the case of motor start-up < 0.5 sec, that the substrate plays no role in heat dissipation and the energy must be completely absorbed by the resistive element itself. Please refer to the section “Pulse/Surge Capability of Resistors” provided in this catalog.Hence surge conditions, if any must be taken into consideration at this stage to determine the correct resistor for that application.
  • Forced Air Circulation: In cases where the apparatus in which the resistor is mounted is heat sensitive or for certain reasons resistor used is of a lower than optimum wattage for that particular application, forced air circulation removes more heat in a shorter time than natural convection and is advised in the circumstances enumerated above.
  • Derating: It is always advisable that a resistor should be derated and not operated at its actual power rating for long term reliability.
    Please refer to the section “Rating versus Life” in the “Customer Assurance” section of this catalog. Suitable derating also contributes greatly to the minimization of “Drift Underload” phenomena observed in change in resistance value when a resistor is in operation.
  • Higher Resistance Value: In order to achieve higher resistance values, the diameter of the resistance wire wound on the substrate is a very fine gauge, sometimes as little as 0.016mm, hence for maximum reliability it is suggested that the designer opts for a higher power rating if size is not a constraint in order to reduce the temperature rise.
  • High Frequency Circuit: Wire wound resistors may be effectively used in circuits with frequency upto 50KHz when non inductively wound by the ‘Aryton-Perry’ method of winding.
    For further details on this subject, please refer to the section “Wire wounds and their limitations when used in a high frequency circuit” in the “Customer Assurance” section of the catalog.