# Wheatstone Bridge - Standard vs. Unbalanced

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I had never heard of the unbalanced wheatstone bridge until just recently. I have used and designed and implemented plenty of the 4 resistor standard wheatstone bridges. I have never worked with the unbalanced wheatstone bridge.

Why would you use the unbalanced wheatstone bridge over the standard wheaststone bridge?

When is the unbalanced wheatstone bridge used, instead of the standard wheatstone bridge?

#### MikeMl

##### Well-Known Member
A Wheatstone bridge operates as covered here. Its differential output is a continuous function that goes through zero as one element of the bridge is varied. If connecting a detector to the bridge, you can choose to detect any level you want, be it zero or something else. If the latter, the bridge would be "unbalanced" when that non-zero level is reached. however, that would mean that you lose the huge advantage of detecting zero; namely that the bridge balance point is insensitive to the excitation voltage!

Look at this example. When the unknown element is 10KΩ, the bridge balances, and the differential output voltage is zero, regardless of the excitation voltage V(ex).

If you tried to use this bridge in the unbalanced mode to detect say 8KΩ, then the output voltage is either 0.3V, 0.6V or 0.8V, depending on the magnitude of the excitation...

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##### Member
I knew most of it and followed you up until...

If connecting a detector to the bridge, you can choose to detect any level you want, be it zero or something else. If the latter, the bridge would be "unbalanced" when that non-zero level is reached. however, that would mean that you lose the huge advantage of detecting zero; namely that the bridge balance point is insensitive to the excitation voltage!
I have only ever used a wheatstone bridge as outlined in you link, with some high impedance voltage measuring device to measure points C, D, or C-D. The unbalanced wheatstone bridge includes a resistor or load (depending on the resource) between points C and D, but can't grasp why.

#### Ian Rogers

##### User Extraordinaire
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I have always assumed unbalanced bridge to have 2 sensors rather than 4... The opposing resistors (fixed) will clearly not match the variable resistors ( sensors ).. I might be wrong. ( Usually I am )..

#### MikeMl

##### Well-Known Member
I added an example to post #2.

If you load the bridge with a low impedance detector, it still works, but with reduced output. It is still best used to detect balance as indicated that the output is zero.

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#### ericgibbs

##### Well-Known Member
I have always assumed unbalanced bridge to have 2 sensors rather than 4... The opposing resistors (fixed) will clearly not match the variable resistors ( sensors ).. I might be wrong. ( Usually I am )..
hi,
You are not wrong.
A half bridge can used with two matched opposing resistors, usually the same value as the half bridge or higher.
Two half bridges are sometimes used to make up a full bridge.

An unbalanced bridge is as Mike describes.
E

#### MikeMl

##### Well-Known Member
...A half bridge can used with two matched opposing resistors, usually the same value as the half bridge or higher...
Is a half-bridge a Wheatstone bridge? Please show a diagram.

#### dr pepper

##### Well-Known Member
Like Ian I might be wrong too.
A load cell weighing scale is a example of an unbalanced bridge, when there is no force on the cell the bridge is balanced, however when you put your measuring jug on there and press tare the bridge is no longer balanced, and when you weigh something the bridge goes out of balance even more, but the display reads correct and everything works, in practical use the bridge doesnt balance.
In just having a quick check to make sure what I just said is sane I found a reference to a quarter bridge, I think its just referring to the element in the bridge that changes value.

##### Member
Like Ian I might be wrong too.
A load cell weighing scale is a example of an unbalanced bridge, when there is no force on the cell the bridge is balanced, however when you put your measuring jug on there and press tare the bridge is no longer balanced, and when you weigh something the bridge goes out of balance even more, but the display reads correct and everything works, in practical use the bridge doesnt balance.
In just having a quick check to make sure what I just said is sane I found a reference to a quarter bridge, I think its just referring to the element in the bridge that changes value.
That is incorrect. A 4 resistor bridge with one of the resistors as varying value (such one resistor being a load cell) is a standard wheatstone bridge. An unbalanced wheatstone bridge has 5 resistors. In post #2, Mike has a link to a standard wheatstone bridge, and a circuit simulation of an unbalanced bridge. As many are fond of saying, check google.

#### ericgibbs

##### Well-Known Member
Is a half-bridge a Wheatstone bridge? Please show a diagram.
hi,
I know this is a 'come on' , but I will play along.
This link covers the diagram of the well used half Wheatstone bridge method.
To explain, it is named as such, when the bridge has only two active elements, they are available from many suppliers.

https://www.transducertechniques.com/wheatstone-bridge.aspx

Eric
Also quarter bridges are common.

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##### Member
Right now I'm only trying to understand the usefulness of the 5-resistor unbalanced wheatstone bridge. I know and have used the 4-resistor bridge. Until recently I didn't even know about the 5-resistor version, and still don't see the usefulness of it.

#### MikeMl

##### Well-Known Member
The "quarter", "half", and "full bridge" terminology stems from the strain gauge world, and specifies how many of the four bridge elements vary as the load cell is strained.

When Wheatstone came up with the "bridge" idea, he was trying to compare an unknown impedance to a known one, so only the balance aspect was important.

When you build a weighing scale using a "quarter" bridge, then you have the complexity of linearizing the output curve (see the simulation results, post #2 and #5), as well as having to regulate the excitation voltage to prevent that from effecting the slope of the output function.

A better way of building a weighing scale is the use a "half bridge load cell" (where two elements vary, usually one increases resistance while the other decreases), making the function more linear, and partially cancelling thermal effects.

An even better way of building a weighing scale is to use a "full bridge load cell" (where all four elements vary, two increasing while the other two decrease), making the function still more linear, and better cancelling thermal effects.

When interfacing something like an LDR or thermistor to the AD input of a uProcessor, folks refer to that as a "half bridge", but IMHO that is not a bridge at all; just a resistive voltage divider made out of a fixed resistor and a variable resistor. (Has nothing to do with Wheatstone or bridge).

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#### ericgibbs

##### Well-Known Member
Mike,
I have been designing commercial and industrial bridges using strain gauges and since the late 1950's, they have always been referred to as I described.
A practising engineer knows exactly what is meant by quarter, half or full bridge.

I agree with your comments regarding the well known linearity and thermal effects encountered when using quarter/half bridges.
It is upto the engineer who is designing the project, to use a bridge type that meets the clients/customers specification

I fail to see why you are having a problem and an issue with terminology that is in general use.?

E

BTW: Wheatstone did not come up with the bridge idea, it was demonstrated 10 years earlier.

The Wheatstone bridge was invented by Samuel Hunter Christie in 1833 and improved and popularized by Sir Charles Wheatstone in 1843. One of the Wheatstone bridge's initial uses was for the purpose of soils analysis and comparison.[2]

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#### dr pepper

##### Well-Known Member
ADW yes it seems as though terminology is my downfall.

#### saif111

##### New Member
I am also curious to know the application of unbalanced wheatstone bridge. As we know that in balanced condition both arms voltages remain same:

Source: https://circuitdigest.com/article/what-is-wheatstone-bridge

Now as I think that we can use this circuit as temperature controlled light in unbalanced condition if we connect relay in between C and D point with a bulb. And remove one resistor and replace it with thermistor and set like that if thermistor resistance changes the bulb should turn on. means when temperature goes high it should turn on.