Let us now see how we can measure temperature using an unbalanced Wheatstone Bridge. The transducer which we are going to use here is called a Thermistor, which is a temperature dependent resistor.
Depending on the temperature co-efficient of the thermistor, changes in temperature will either increase or decrease the resistance of the thermistor.
This means that the output voltage V OUT is proportional to the temperature. By calibrating the voltmeter, we can display the temperature in terms of the output voltage. One of the most commonly used applications of Wheatstone Bridge is in the Strain Measurement. Strain Gauge is a device whose electrical resistance varies in proportion to the mechanical factors like Pressure, Force or Strain. For a given strain, the resistance change may be only a fraction of the full range.
Therefore, to accurately measure the fractional changes of resistance, a Wheatstone Bridge configuration is used. The circuit below shows a Wheatstone bridge where the unknown resistor is replaced with a strain gauge. Due to the external force, the resistance of the strain gauge changes and as a result, the bridge becomes unbalanced. The output voltage can be calibrated to display the changes in strain.
In this, the Strain Gauges are carefully mounted as a single unit called as Load Cells, which is a transducer which converts mechanical force to electrical signal. Usually, weight scales consist of four load cells, where two strain gauges expand or stretch tension type when external force is acting and two strain gauges compress compression type when load is placed. If the strain gauge is either tensed or compressed, then the resistance can increase or decrease.
Therefore, this causes unbalancing of the bridge. This produces a voltage indication on voltmeter corresponds to the strain change. If the strain applied on a strain gauge is more, then the voltage difference across the meter terminals is more.
If the strain is zero, then the bridge balances and the meter shows zero reading. This is about the resistance measurement using a Wheatstone bridge for precise measurement. Due to the fractional measurement of resistance, Wheatstone bridges are mostly used in strain gauge and thermometer measurements. You learned What is a Wheatstone Bridge Circuit, what is the meaning of a Balanced Bridge, how to calculate an unknown resistance using Wheatstone Bridge and also how an Unbalanced Wheatstone Bridge can be used to measure different physical quantities like Temperature and Strain.
Your email address will not be published. Now potential of point B with respect to point C is the voltage drop across the Q transistor, then the equation is. Potential of point D with respect to C is the voltage drop across the resistor S, then the equation is.
The electrical resistances of Wheatstone bridge such as P and Q are made of definite ratio, they are ; or known as ratio arms and the rheostat arm S is made always variable from , ohms or from , ohms. The application of Wheatstone bridge is light detector using Wheatstone bridge circuit. Balanced bridge circuits are used in many electronic applications to measure changes in intensity of light, strain or pressure. Wheatstone bridge applications are used to sense electrical and mechanical quantities.
But, the simple Wheatstone bridge application is light measurement using photoresistive device. In the Wheatstone bridge circuit, a light dependent resistor is placed in the place of one of the resistors. An LDR is a passive resistive sensor, that is used to convert the visible light levels into a change in resistance and later a voltage. LDR can be used to measure and monitor the light intensity level. If the gauge factor is GF, the strain measurement is related to the change in Rg as follows:.
The number of active strain gauges that should be connected to the bridge depends on the application. For example, it may be useful to connect gauges that are on opposite sides of a beam, one in compression and the other in tension.
In this arrangement, one can effectively double the bridge output for the same strain. In installations where all of the arms are connected to strain sensors, strain gauges temperature compensation is automatic, as resistance change due to temperature variations will be the same for all arms of the Wheatstone bridge.
In a four-element Wheatstone bridge, usually two gauges are wired in compression and two in tension. For example, if R1 and R3 are in tension positive and R2 and R4 are in compression negative , then the output will be proportional to the sum of all the strains measured separately. For gauges located on adjacent legs, the bridge becomes unbalanced in proportion to the difference in strain. For gauges on opposite legs, the bridge balances in proportion to the sum of the strains.
Whether bending strain, axial strain, shear strain, or torsional strain is being measured, the strain gauge arrangement will determine the relationship between the output and the type of strain being measured. As shown in Figure , if a positive tensile strain occurs on gauges R2 and R3, and a negative strain is experienced by gauges R1 and R4, the total output, VOUT, would be four times the resistance of a single gauge.
In this configuration the stain gauge tempeature changes are compensated. The Chevron bridge is illustrated in Figure It is a multiple channel arrangement that serves to compensate for the changes in bridge-arm resistances by periodically switching them. Here, the four channel positions are used to switch the digital voltmeter DVM between G-bridge one active gauge and H-bridge two active gauges configurations.
The DVM measurement device always shares the power supply and an internal H-bridge. This arrangement is most popular for strain measurements on rotating machines, where it can reduce the number of slip rings required. Although the Wheatstone bridge circuit is one of the most popular methods of measuring electrical resistance, other methods can also be used. The main advantage of a four-wire ohm circuit is that the lead wires do not affect the measurement because the voltage is detected directly across the strain gauge element.
A four-wire ohm circuit installation might consist of a voltmeter, a current source, and four lead resistors, R1, in series with a gauge resistor, Rg Figure The voltmeter is connected to the ohms sense terminals of the DVM, and the current source is connected to the ohms source terminals of the DVM.
To measure the value of strain, a low current flow typically one milliampere is supplied to the circuit. While the voltmeter measures the voltage drop across Rg, the absolute resistance value is computed by the multimeter from the values of current and voltage. The measurement is usually done by first measuring the value of gauge resistance in an unstrained condition and then making a second measurement with strain applied.
The difference in the measured gauge resistances divided by the unstrained resistance gives a fractional value of the strain. This value is used with the gauge factor GF to calculate strain. The four-wire circuit is also suitable for automatic voltage offset compensation. The voltage is first measured when there is no current flow.
This measured value is then subtracted from the voltage reading when current is flowing. The resulting voltage difference is then used to compute the gauge resistance. Because of their sensitivity, four-wire strain gauges are typically used to measure low frequency dynamic strains. When measuring higher frequency strains, the bridge output needs to be amplified. The same circuit also can be used with a semiconductor strain-gauge sensor and high speed digital voltmeter.
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