Ceramic Encased Wirewound Resistors
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Non-Inductive Film Resistors
Very High Power Resistors
The unit of resistance is the Ohm, (symbol W 'Omega' - last letter of the Greek alphabet) named after the German Physicist Georg Simon Ohm (1787 - 1854).
'R' is the normal algebraic representation of Resistance
Many types are available, including Carbon Film, Metal Film and Wirewound.
Metal Film resistors, suitable for low current applications, are readily available in ratings up to 0.5 Watt and to close tolerance, commonly ±1%.
Carbon Film types are generally manufactured to a wider tolerance ± 5% to ± 20% and may be rated up to 2 Watts.
Wirewound resistors are available ceramic encased in ratings from 2W to 30W at ± 5% tolerance.
Non-Inductive resistors are now available in a film construction rated from 0.25W to 5W with tolerances of ±1%, ±2% and ±5%. These are specifically designed to minimise inductance to provide extremely low distortion signal transmission.
For loudspeaker applications, where high currents may be encountered, wirewound resistors are normally specified. Non-inductive resistors may offer significant performance advantages in critical circuit configurations or specialist 'audiophile' applications. All four types are available from Expotus Components.
May be calculated as
P = Power (Watts ) I = Current (Amps)
V = Voltage (Volts) R = Resistance (Ohms)
The effective resistance of parallel resistors may be calculated:
Power rating will be increased, but to take maximum advantage of this aspect, the power ratings of the individual resistors must be balanced so that their dissipations are approximately equal to their respective power ratings which may be calculated as above.
For example, a 10W resistor might be paired with a 5W resistor to obtain an effective resistance of 3.3W . For 10V applied voltage, the 10W resistor will dissipate 10W and the 5W resistor 20W, so the power rating of the 5W resistor would need to be twice that of the 10W to obtain maximum power dissipation from the combination, and the combined equivalent power rating would be 30W. If, however, the power ratings were not balanced in this way, the maximum permissible power dissipation would be limited by the lower specified of the two resistors.
In practice, it is most common to combine two equal resistors, in which case the effective resistance is halved and the power rating doubled.
The equivalent resistance of series resistors is simply the sum of their individual resistances. Power dissipation may be calculated in a similar manner as for parallel resistors, and again balancing the power ratings will maximise this aspect.
In the simple, most common case of two identical resistors in series, the effective resistance and the power dissipation would both be doubled.
These resistors are primarily intended for use in loudspeaker crossovers, but their close tolerance and rugged characteristics lend themselves to many other applications. They are available in power ratings from 2W to 30W, and resistance values from fractions of an ohm to thousands of ohms.
They are fabricated from a coil of resistance wire, wound about a former, and welded at each end to a cap, to which in turn the tinned copper lead is fixed. The unit is sealed with a ceramic mix into a preformed ceramic case bearing the component description and ratings.
The materials and construction employed result in an extremely rugged component and ensure excellent flame and moisture resistance as well as self extinguishing capability. These resistors will withstand the most rigorous loading test.
As a single coil of wire these resistors are inductive; although this is unlikely to have any significant effect at low resistance values. Non-inductive types are available if necessary, to special order.
The peculiarly stringent demands placed by today's loudspeaker designs upon crossover networks may demand resistors with specifications other than those listed on the following pages. If you require non-standard components, Expotus will be pleased to quote for items manufactured to your exact specifications.
The design of any electrical or electronic circuit should take into consideration likely causes of failure and their consequences - particularly with regard to safety.
By their nature these Ceramic Encased Wirewound Resistors will withstand extreme overload conditions for prolonged periods without failure, and without themselves sustaining serious damage. If the overload is allowed to continue unchecked, it can result in considerable overheating and damage to adjacent components.
Unfortunately, loudspeakers are often subjected to overload - moreover failure (usually a drive unit going open-circuit) frequently goes un-noticed and the overload condition is not removed. The associated section of the crossover network will then be subjected to abnormal loads and can, in some circumstances, be required to dissipate excessive power. Sensibly designed overload protection can prevent the initial failure and subsequent serious damage, but it is frequently (and often justifiably) spurned by audio purists as detrimental to sound quality.
Crossover design should also take into account the heat generated by resistors in normal use and their proximity to heat sensitive components which might, if heated, sustain damage or cause changes to the loudspeaker's performance.
The resistor is specified by three main parameters:
1. Resistance Value
2. Power Rating
1. Resistance Value
Measured in ohms (W), ranging from fractions of an ohm to thousands of ohms (KW). The resistance value, be it 0.82 ohms or 82 ohms has no bearing on the size of the resistor, which is determined solely by its power rating.
2. Power Rating
Measured in Watts, this ranges from 2W to 30W. The power rating required depends on the current to be passed by the resistor, which in turn relates to circuit configuration and power rating of the loudspeaker system. The higher the power rating, the larger the size of the resistor.
For hi-fi loudspeakers, power ratings of 5W, 7W or 9W are generally adequate but requirements can vary widely, so it is advisable to examine individual circumstances to obtain the most appropriate rating. Some designers prefer to use a higher power rating than theory dictates in order to minimise heating in use and the effects that may have on adjacent components.
It is also prudent to consider the consequences of component failure elsewhere in the system, since this may result in excessive loads being applied to some resistors. Please see Precautions to Observe.
Wire wound resistors of this type are usually available with resistance value tolerances of ± 5% or ±10%. All Expotus wire wound resistors are specified to ± 5%.
The designer should ensure that the circuit configuration will accept variations of ± 5% in resistor value without undue variations in performance. See also Definition of Tolerance
Most users prefer the axial leadout wire format which is convenient for both printed circuit board connection and hard wiring.
Radial (RAD) types are also available for applications where they would be more suitable.
Not resistors at all, but printed circuit board links which resemble 0.25Watt resistors in size.
Standard Specifications for Expotus Ceramic Encased Wirewound Resistors common to all types:
Temperature Coefficient: ± 300 ppm/șC
Insulation Resistance: > 100 Mohm
The resistance value may be expected to deviate from the original value as follows:
Load Life (1,000 hrs): ± 5% + 0.05 ohm
Short Term Overload: ± 2% + 0.05 ohm
Moisture Resistance: ± 5% + 0.05 ohm
Shock and Vibration: ± 1% + 0.05 ohm
Effect of Soldering: ± 2% + 0.05 ohm
These resistors, manufactured using a patented film type construction, have been designed specifically to minimise the self-inductance inherent to a small extent in all such components. In the most demanding audio applications, they can offer improvements in sound quality by reducing the extremely low-level, but nonetheless audible distortion, which may be generated by the inductance of conventional resistors. With power ratings as high as 5W, their use in loudspeaker crossover assemblies, to improve sound quality, becomes a feasible proposition.
The use of these Non-Inductive Resistors in high quality transistor based audio amplifers has been shown to produce a 'softer', 'sweeter' sound - comparable in some respects with the characteristic sound of valve designs.
They may also be used to advantage in audio instrumentation, and other applications where extremely low levels of distortion are necessary. The reduction of switching spikes in power supplies and controllers by using Non-Inductive Resistors can sometimes allow expensive bipolar power transistors or similarly costly components to be designed out of the circuits offering considerable savings in manufacture.
Caution: The use and specifications of these Non-Inductive Resistors are broadly similar to those discussed for the ceramic encased wirewound variety - although it should be noted that their construction renders them rather less rugged than their ceramic counterparts in the case of severe and/or long-term overload.
This more traditional form of enamelled power resistor construction provides extremely high power rating, compact dimensions, linear coefficient of resistance, excellent stability in long-term use and resilience to short-term overload. Available with or without mounting brackets.
The QH and QL types are wound with conventional circular cross-section resistance wire, while the QR and QRZG types are wound with rectangular cross-section 'ribbon' wire resulting in an even more robust construction where very low resistance values are required to handle extraordinarily high currents.
Caution: Where high power resistors of this nature must be specified, the designer should make due allowance for adequate cooling and heat dissipation to prevent damage to nearby components and/or fire hazards. Although a resistor may have a very high power dissipation capability, the heat it generates in operation must still be carried away, via a heat sink, by natural convection or by forced cooling.