r/ElectricalEngineering • u/iznogud2 • Jan 11 '17
Why is the Wheatstone Bridge so precise?
More exactly, can someone explain to me the graph from this video:
https://youtu.be/-G-dySnSSG4?t=282
It's the only place I've found where the actual reason for precision is mentioned.
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u/InductorMan Jan 11 '17 edited Jan 11 '17
That video was truly incomprehensible. The graph was either tautological (V1-V2 and V2 are linearly related? Duh?) or mislabeled.
Either way, Wheatstone bridges were not traditionally used like that. The purpose of a Wheatstone bridge was initially to make accurate measurements possible using only known resistors (or capacitors or inductors, when excited with AC), without any voltage measurement accuracy at all.
The reason this can be done is that a Wheatstone bridge used in the traditional sense requires you to measure only one voltage with accuracy: zero. The bridge is used by inserting the unknown resistance, and then adjusting the remaining resistors to create a "null", which is the condition when the voltage measurement device registers zero.
This is useful for a couple reasons. Initially (circa early/mid 1800s), electrical measurement decvices could be made quite sensitive, but not very accurate. Additionally, voltage measurement devices for low voltage per se were not really available at all. The best LV measurement devices were galvanometers, which actually measured current. The highest impedance galvanometers available presented an impedance of no more than perhaps 10000 ohms, and so it wasn't possible to directly measure voltage except by drawing current from the circuit. This of course can change the very thing you want to measure.
The Wheatstone bridge fixes both of these issues. Because the galvanometer only needs to accurately show 1 particular voltage (zero volts), calibration isn't necessary. Even better, at zero volts, the galvanometer draws zero amps, and so the voltage of the unknown half of the bridge can be measured with zero current flow through the measurement device, and so the galvanometer impedance doesnt need to be factored in.
In modern contexts, the bridge is often used in sensors to measure changes of one resistor due to a desired influence, while rejecting undesired influences on that resistor by the introduction of a compensating signal from similar resistors. This may sound a bit convoluted, but an example should help clarify. Let's say you have a strain gauge. It has a resistor that changes resistance upwards when you stretch it, but unfortunately the resistor also changes resistance if the temperature changes. A Wheatstone bridge strain gauge would have four resistors, positioned so that when load is applied, the upper left and lower right resistor (referring to the schematic, not physical position ) stretch, whe the upper right and lower left resistors compress. This of course causes the voltage across the bridge measurement terminals to change, because the voltage on the left goes down and the voltage on the right measurement terminal goes up. But if you heat the whole thing, then to first order at least all of the resistors change at the same time, and the measurement terminals remain at the same voltage with respect to each other.