GUARDED WHEATSTONE BRIDGE PDF

A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit , one leg of which includes the unknown component. The primary benefit of the circuit is its ability to provide extremely accurate measurements in contrast with something like a simple voltage divider. The Wheatstone bridge was invented by Samuel Hunter Christie sometimes spelled "Christy" in and improved and popularized by Sir Charles Wheatstone in One of the Wheatstone bridge's initial uses was for soils analysis and comparison. At this point, the voltage between the two midpoints B and D will be zero. Detecting zero current with a galvanometer can be done to extremely high precision.

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This system provides a simple yet effective way to activate both the high and low guard circuits. Adjusting the source outputs to set the high impedance side of the bridge circuit to zero volts reduces errors caused by meter circuit loading.

A DVM measures the voltage across the bridge and a low thermal scanner is used to switch the resistors in the test. The standard low thermal scanner has leakages of about 10 12 W. In a normal arrangement the leakage currents would cause errors of about 1 ppm at the 1 M W and ppm at M W. By using a fully guarded scanner, leakages can be significantly reduced. Because the sources have low impedances, the high guards can be connected directly to the source outputs.

The low guard can be connected directly to ground because the sources are always adjusted so that the DMV reads zero. Keeping both sides of the bridge at zero volts reduces leakage errors. This is done by using two voltage sources for two arms of the bridge as shown in the diagram below. Voltage source 1 is adjusted so that the DVM always reads zero, which sets the center point of the two resistors being compared to zero volts.

The tare standard is always in the circuit, and the low thermal scanner is used to switch the standard and test resistors into the circuit one at a time. This provides a convenient means to set up an make high resistance measurements. OhmRef will allow up to 8 resistors to be compared at a time. Different values of resistors can be compared over a wide range with the uncertainty is primarily dependant upon the scaling accuracy of the voltage source used.

Page By using a guarded scanner and two sources to form a guarded resistance bridge, measurements from K W to 10G W can be made with excellent accuracy.

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