Wire-wound resistors

The resolution of potentiometric transducers is dependent upon the construction of the resistance element. In the case of a wire-wound resistance, in order to obtain a high resistance in a small space, the resistance wire is wound on to a mandrel or card which is straight or formed into a circle or helix depending upon the motion of the contact. This limits the resolution of the transducer as the wiper moves from one wire to the next on the mandrel. The best resolution that can be obtained is about 0.01 per cent (see Section 6.10.1). Typical wire-wound potentiometers have strokes of between 0.0025 m and 0.5 m and rotational versions from about 10° of arc to 50 turns. An alternative often employed is the conductive plastic film element. This provides a continuous resistance element and thus, a zero resolution, but such elements suffer from a higher temperature coefficient of resistance. A more recent development is a combination of earlier types in which a conductive plastic coating is sprayed on to a wire-wound resistor. 

The first use of ceramics in the electrical industry took advantage of their stability when exposed to extremes of weather and to their high electrical resistivity, a feature of many siliceous materials. The methods developed over several millennia for domestic pottery were refined for the production of the insulating bodies needed to carry and isolate electrical conductors in applications ranging from power lines to the cores bearing wire-wound resistors and electrical fire elements. 

Electrical heating circuit for solution calorimeter. The standard resistor should be a wire-wound resistor with a low temperature coefficient and rated for 2 W. If a heating coil of higher resistance (say, 60 2 is to be used, the current can be reduced to 0.5 A and a 1-H standard resistor can be used. 

Precision wire-wound resistors are used for critical applications in which low noise and resistance stability are important, but since they are coils, they have an inductive reactance that must be taken into account for ac circuits. By folding the insulated wire in the middle and carefully forming parallel windings, the inductive component can be reduced by about a factor of 100. Precision standard resistors with an accuracy of about 50 ppm in the range of 1 fl to 10 Mfl are available from Electro Science Industries and other companies. These are often combined in the form of decade resistance boxes to provide a wide range of fixed values. 

Fig. 4-6. Detector circuits for vapor-phase chromatography, (a) Thermistor detector Di, D=, Victory Eng. Corp. 32A12 thermistors Ri, Ri, 1,0000 wire wound resistors Rz, 1,0000 Helipot Rt, 10,0000 1% carbon film resistor Rs, 5,0000 1 % carbon film resistor Rt, Rj, 2,5000 1 % carbon film resistor Sw, single-pole four-position switch, (b) Hot-wire detector Rz, filament current control, to adjust filament current between 150—300 ma ( 20 ohm 5w) Ri, R4, reference detectors Rs, R, sample detectors Re, zero control 20 Re, 600 1 % carbon film resistor R, 300 1 /, carbon film resistor R, 150 1% carbon film resistor Rse, 7.5Q 1 % carbon film resistor Rs, 7.SCI 1 % carbon film resistor M, 300 ma meter,- S, single-pole six-position sv/itch.

Historically ceramics were exploited for their electric insulation properties, which together with their chemical and thermal stability rendered them ideal insulating materials in applications ranging from power lines to cores bearing wire-wound resistors. Today their use is much more ubiquitous — in addition to their traditional role as insulators, they are used as electrodes, catalysts, fuel cells, photoelectrodes, varistors, sensors, and substrates, among many other applications. 

Low-permittivity ceramics are widely used for their insulative properties. The major requirements are good mechanical, thermal, and chemical stability good thermal shock resistance low-cost raw materials and low fabrication costs. These include the clay- and talc-based ceramics also known as electrical porcelains. A large-volume use of these materials is as insulators to support high tension cables that distribute electric power. Other applications include lead-feedthroughs and substrates for some types of circuits, terminal connecting blocks, supports for high-power fuse holders, and wire-wound resistors. 

During the current noise measurements a constant current was sent through the sample via a wire-wound resistor whose resistance was at least one order of magnitude higher than that of the sample. This wire wound resistor did not produce any additional current noise, and it prevented the noise signal from being loaded by the current supply. 

The measuring cell was made of PTFE and had a four-electrode system connected to its outer cylinder. One pair of electrodes served as terminals for the measuring amplifier, and the outer pair was connected to the current supply through a 10-MO wire-wound resistor (the sample resistance was much less than 10 MQ). The measuring cell was carefully shielded from electric and magnetic fields and from mechanical vibrations. 

As the name indicates, for wire wound resistors the resistive element consists of a wire. To get sufficient length this wire is wound around a ceramic rod or tube. The ends of the wire are welded or soldered to caps or rings, which are attached to the rod. In turn, these caps or rings are attached to the leads, which may be wires or tags. 

As a result of the high allowed surface temperature the maximum load of wire wound resistors is relatively high. Their behavior under pulse loads is also good, and they have a low current noise. Because of the windings they tend to have a high inductance. Special resistors are produced in which two layers of wire are wound in opposite directions to minimize the inductance. 

In wire wound variable resistors the resistive element is a wire that is wound around a support. The contact slides over one side of the wires. The wire metals are the same as used for fixed resistors, but care should be taken that no thick oxide layers are present as this may cause a high contact resistance. Wire wound resistors can be made with good tolerance they also have excellent stability and can have high rated power. They are, therefore, nearly always used when a high-power rating is needed. Their disadvantages include rather bad resolution and high price. 

The good pulse behavior of wire wound resistors is primarily based on the relatively large heat capacity of the wires. For pulse behavior, a large wire diameter should be used. As for a given resistance value, a thinner wire also implies a shorter wire the maximum pulse rating for different wire wound resistors may be quite different. 

The noise energy depends on the n umber of charge carriers in the resistive element. Resistors with many charge carriers have a low current noise level. Therefore, wire wound resistors are extremely good in this respect. For film resistors, metal films are better than carbon films and metal glaze. Carbon composition resistors have an extremely high noise level. Within one type, low values are better than high ones and big... 

Both the capacitance and the inductance tend to increase with larger dimensions of resistors. The series inductance is high for resistors with a resistive element such as a cod. This is found in most cylindrical film resistors and especially in wire wound resistors. Special types are available with two coils wound in opposite direction to minimize the inductance. Very low inductances and capacitances are found for small rectangular SMD resistors. One should remember that here interactions with the rest of the circuit may be much more important than the impedance of the resistor itself. 

In commercial potentiometers, R /f is generally a wire-wound resistor formed in a helical coil. A movable contact, called a wiper, can be positioned anywhere between one end of the helix and the other, allowing to be varied continuously from zero to the input voltage... 

The first band is the one closest to one end of the resistor. A first band wider than the others indicates a wire-wound resistor. 

When a resistor of a larger power rating is required a wire-wound resistor should be chosen. This consists of a resistance wire of known value wound on a small ceramic cylinder which is encapsulated in a vitreous enamel coating, as shown in Fig. 3.70(b). Wire-wound resistors are designed to run hot and have a power... 

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