Fixed resistors

The sensor usually consists of a coil of wire made from the material that is wound on a former and the whole sealed to prevent oxidization, although a film of the metal deposited on a ceramic substrate can also be used. The resistor is connected in a Wheatstone bridge network (Figure 17.17), using fixed resistors in the other three arms. The instrument connected across the bridge is calibrated directly in terms of temperature. The range is limited by the linearity of the device and the upper temperature, which can be measured, must be well below the melting point of the material. 

Above Electrical test system. The four fixed resistors... 

Most battery tests are based simply on cell discharge through fixed resistors (with or without interspersed recovery periods). From the times required to reach predetermined cut-off voltages, estimates of the practical capacity, QP, and of the practical energy, [Pg.61]

The balanced bridge circuit (Figure 10) uses a galvanometer to compare the RTD resistance with that of a fixed resistor. The galvanometer uses a pointer that deflects on either side of zero when the resistance of the arms is not equal. The resistance of the slide wire is adjusted until the galvanometer indicates zero. The value of the slide resistance is then used to determine the temperature of the system being monitored. 

There are two types of conductometric procedures commonly used. Firstly, a Wheatstone Bridge circuit can be set up, whereby the ratio of the resistance of unknown seawater to standard seawater balances the ratio of a fixed resistor to a variable resistor. The system uses alternating current to minimise electrode fouling. Alternatively, the conductivity can be measured by magnetic induction, in which case the sensor consists of a plastic tube containing sample seawater that links two transformers. An oscillator establishes a current in one transformer that induces current flow within the tube, the magnitude of which depends upon the salinity of the sample. This in turn induces a current in the second transformer, which can then be measured. This design has been exploited for in situ conductivity measurements. 

Resistors whose values can be varied are termed potentiometers. They are made of the same materials as fixed resistors but have a movable wiper to contact a coil of resistance wire or a strip of resistive film at any point along the resistor. Potentiometers used only occasionally to adjust a circuit are called trimmers, while those employed for high-wattage applications such as control of heating mantles and ovens are called rheostats. Precision potentiometers have played an important role in the measurement of electrical quantities and are considered in detail in a later section. 

Fixed resistors are used when you need to reduce the current by a certain amount. They are easily identified by their size and shape (see Figure 1.3). Their resistance level is indicated by means of colored bands painted on the resistor. 

Fixed resistors are color coded to identify their resistance values. They are color coded rather than having their values printed right on their bodies for two reasons. First, the resistors are only % inch long. Second, they re cylindrical. In other words. It is rather difficult to print on them (and the print would be difficult to read at that size anyway). 

Resistance Measurement. The common methods of resistance measurement in resistance thermometry are the bridge method and the potentiometric method. Basically, the bridge method uses the resistance sensor together with a variable resistor and two fixed resistors to form the four legs of a conventional Wheatstone bridge circuit. On the other hand, the potentiometric method, also called a half bridge, connects the resistance sensor in series with a known resistor. 

For most applications, an alternative is employed. Recall that, in measuring the resistance of a thermistor, a fixed resistor is normally connected in series with the sensor. If a constant-voltage source ( s) is used, the circuit current is inversely proportional to the total resistance. Then the relationship between the measured voltage drop across the fixed resistor and the thermistor temperature can be almost linear over a range of temperature. The linear part of this curve can be shifted along the temperature scale by changing the value of the fixed resistor. 

These curves can be almost linear over a range of 40 to 60°C at temperatures above 0°C and over a range of about 30°C for temperatures below 0°C. The choice of the value for the fixed resistor Rfis such that the operating temperature range of the thermistor would coincide with the linear portion of F(t), thus simplifying the data reduction process Actual F(t) curves for different thermistor materials are given in Ref. 18. 

The equipment is basically a Wheatstone bridge and conductivity cell, as illustrated in Fig. 15.21. Resistance A is made up of the cell containing the sample B is a variable resistance resistances D and E are fixed. Resistor B and variable capacitor C may be adjusted so that the balance point can be reached. At this point... 

The present paper furthers the discussion of these kinSs of circuit models in two major respects. Hie first has to do with the point-to-point variations which may be expected to occur. This has been discussed to some extent by Choudhury and Patterson (19, 24). They provide a very useful parametric method which automatically adjusts the local resistances but it was not presented in a form that is easy to visualize. Hence, a reformulation of their approach will be outlined here which ties it in more closely with the more traditionally accepted "fixed resistor" analogs. The second major point has to do with extending the equivalent circuit models in such a way as to allow for additional mobile ionic species in the electrolyte. ... 

Electrical Symbols AAVv— Fixed resistor Photo resistor — Variable r istor +1 -. .-N> h- j, l ode Ci acitor r 7 vm K <1 Meter Speaker Solar cell Single-phase motor... 

The most commonly used type of resistor is the fixed resistor the opposition to the flow of electrons provided by the resistor is fixed and cannot be adjusted. These fixed resistors are typicality made of carbon (Figure 15-9). The carbon resists the flow of electrons and converts the electrical energy... 

Component Electric Hoist Motors and Controls. Both hoists have enclosed, nonventilated, 220/440-V, three-phase, 60-Hz, 1200-rpm (rev/min) motors. The control used on hoist motions is referred to as the static stepless magnetorque control on the manufacturer s drawings. With this control, drive motor torque is controlled by means of fixed resistors and saturable reactors in the... 

Electric hoist motors Fixed resistors Saturable reactors Motor secondary circuits Induction master Magnetic amplifiers Motor shaft extension... 

A fixed resistor R especially for low frequencies f suffers several limitations. Therefore, the resistor R is replaced by an amplifier with a variable gain according to Fig. 12.7. This results in a variable impedance Zx(ffl) which can be adjusted to the impedance of the sample Zs(m). For the sample impedance, then... 

In the non-isothermal method the catalytically active sensing element and a similar but catalytically inert element form two arms of a Wheatstone s bridge. Power is supplied to the circuit to heat the elements to their operating temperature, and the values of the fixed resistors selected such that the bridge balances in air. When flammable gas is passed over the elements, reaction takes place on the sensing element, imparting power to the element and thus... 

The isothermal method employs a similar Wheatstone s bridge circuit to the earlier method. However, the compensator is replaced by a fixed resistor, and the voltmeter is replaced by a feedback circuit (17) which governs the electrical power supplied to the catalytic element. In this way the heat liberated during reaction is sensed by the feedback circuit and the electrical power is reduced to maintain the element at a constant temperature. Thus, if P is the electrical power required to maintain the element at... 

Fixed resistors, which in first approximation should have a constant resistance, independent of temperature, voltage, etc. 

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