Balanced Bridge Circuit

Ions are formed by an electron beam offset at 1/2 r because ions cannot be excited at the center of the cell. This is one also to allow the largest possible cyclotron orbit between the z-axis and the ring electrode. The cyclotron mode of the ions is excited across the ring and end caps, and image currents are detected by using a balanced bridge circuit (5.2). 

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. 

If the RTD becomes open in either the unbalanced and balanced bridge circuits, the resistance will be infinite, and the meter will indicate a very high temperature. If it becomes shorted, resistance will be zero, and the meter will indicate a very low temperature. 

Resistance variation can be detected by either null-balance or deflection-balance bridge circuits. In a null-balance bridge the sensor resistance change is balanced (zero output) by a variable resistance in a bridge adjacent arm. The calibrated null adjustment is an indication of the change in sensor resistance. The deflection-balance method, on the other hand, makes use of the amount of bridge unbalance in order to determine the change in sensor resistance. 

Fig.4.74a-c. Calorimeter for measuring the output power of cw lasers or the output energy of pulsed lasers (a) experimental design (b) calorimeter with active irradiated thermistor and nonirradiated reference thermistor (c) balanced bridge circuit... 

A common known method to get eddy-current informations about material flaws is the measurement of real- and imaginary part of the complex impedance of a coil in absolute circuit. The measurement, shown in this paper, are done with an impedance analyzer (HP4192A). The device measures the serial inductance L, and the serial resistance Rs of the complex impedance with an auto-balance bridge measurement circuit [5]. 

As long as no samples, only carrier gas, goes over both detectors, the bridge circuit is balanced. There s no signal to the recorder, and the pen does not move. 

While most laboratory conductivity bridges are manually balanced, the Wheatstone bridge circuit also finds use in a variety of conductivity monitors, controllers, and recorders where it is mechanically rebalanced by a servomechanism operated by the detector. Generally in these devices, advantage is taken of the phase shift, which occurs in the detected signal as the bridge is driven through balance by the servo motor. 

DESCRIBE the bridge circuit conditions that create a balanced bridge. 

When balance exists, R3 will be equal to the unknown resistance, even if the voltage source is unstable or is not accurately known. A typical Wheatstone bridge has several dials used to vary the resistance. Once the bridge is balanced, the dials can be read to find the value of R3. Bridge circuits can be used to measure resistance to tenths or even hundredths of a percent accuracy. When used to measure temperature, some Wheatstone bridges with precision resistors are accurate to about + 0.1°F. 

Two types of bridge circuits (unbalanced and balanced) are utilized in resistance thermometer temperature detection circuits. The unbalanced bridge circuit (Figure 9) uses a millivoltmeter that is calibrated in units of temperature that correspond to the RTD resistance. 

A slidewire resistor is used to balance the arms of the bridge. The circuit will be in balance whenever the value of the slidewire resistance is such that no current flows through the galvanometer. For each temperature change, there is a new value therefore, the slider must be moved to a new position to balance the circuit. 

Figure 11 is a block diagram of a typical temperature detection circuit. This represents a balanced bridge temperature detection circuit that has been modified to eliminate the galvanometer. 

An electronic instrument has been developed in which the DC voltage of the potentiometer, or the bridge, is converted to an AC voltage. The AC voltage is then amplified to a higher (usable) voltage that is used to drive a bi-directional motor. The bi-directional motor positions the slider on the slidewire to balance the circuit resistance. 

The bridge circuit is considered balanced when the sensing ammeter reads zero current. 

In both these bridge circuits, the upper arms will be assumed to be pure resistance arms of equal value, but the calculations can be readily extended to cover the case where they are not equal.101 The balance conditions for the two bridge circuits of Figure 6.24b,c (summarized in Table 6.7) show that when... 

TABLE 6.7 Impedance Balance Conditions for the Two Bridge Circuits of figure 6.24... 

Figure 22.10. Schering bridge circuit. The capacitance being measured (the test capacitance) is represented by Cl and in series. R3 is a fixed resistance. Balance is obtained by adjustment of C3 and either Cl or R2. D is the detector.

Further lumped circuit options include circuits that allow accurate measurement of the current in, and voltage across, the unknown impedance (thus allowing calculation of that impedance), and the auto balancing bridge (Agilent Technologies, 2003). 

Fig. 9b. A Wheatstone bridge circuit as used in power plants for C02. It operates by balancing the output voltage of two circuits, one in a reference gas concentration, and the other in the gas to be measured.

Figure 11 Description of dual microcoil probe. (A) Two coils wrapped around a polyimide sleeve (B) Dual-coil probe mounted on top of the probe head. (C) Schematic of balanced tank circuit used for each microcoil. Coil (L), series capacitors (Cs) 3.3 pF, tuning capacitors (CT) 0.6-4.5 pF, matching capacitor (CM) 0.6-4.5 pF, bridge capacitors (CB) 24 pF. (Reproduced with permission from Ref. 41. Copyright 2002 American Chemical Society.)...

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. 

---