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The Wheatstone Bridge: Still Alive and Kicking

Posted May 04, 2016 12:00 AM by Engineering360 eNewsletter

Invented in 1833, the Wheatstone bridge is still used today. The fundamentals are unchanged. A resistance is measured by applying a voltage across two parallel voltage dividers. Of the four resistors, three have known values and the fourth is the one to be measured. One of the resistors is varied until zero current flows between the midpoints of the dividers. The two ratios are therefore equal and the unknown value can be calculated. The method is useful for measuring sensors, such as strain gauges, in which resistance changes represent a physical quantity. Modern interface ICs enable the Wheatstone bridge to be easily integrated into a factory automation network.


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#1

Re: The Wheatstone Bridge: Still Alive and Kicking

05/04/2016 9:25 AM

Thank you for putting that up, it made my whole day.

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#2

Re: The Wheatstone Bridge: Still Alive and Kicking

05/04/2016 5:45 PM

Or you could just measure the applied voltage and resulting current flow through the resistor and figure it out from there.

Personally, I never found the Wheatstone Bridge to be all that useful. It just seemed like a more complicated way of doing something simple. Extra components and extra math and whatnot.

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#3
In reply to #2

Re: The Wheatstone Bridge: Still Alive and Kicking

05/04/2016 6:47 PM

I used a Leeds & Northrup bridge long ago to measure cable loop resistance. The advantage of a bridge is that to make precision measurement only requires precision resistors. A sensitive galvanometer does not need calibration to indicate the null voltage of a balanced bridge.

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#4
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Re: The Wheatstone Bridge: Still Alive and Kicking

05/04/2016 7:50 PM

And now we have hand held devices that are plug and play that will tell you things about your cables electrical properties that Wheatstone couldn't have even imagined back then.

As much as I like playing with my assorted collection antique electrical and mechanical thingies that were very clever for their time very few of them serve any real day to day purpose now.

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#5
In reply to #4

Re: The Wheatstone Bridge: Still Alive and Kicking

05/04/2016 8:08 PM

Agreed. The device was an antique when I used it!

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#8
In reply to #4

Re: The Wheatstone Bridge: Still Alive and Kicking

05/05/2016 8:48 AM

..."hand held devices that are plug and play"...

so plug and play means it is never out of calibration specification, and that theory of measurement is always sound? I sort of doubt that, but for the average geek, yes, plug and play will get you through a work day in most places.

In spite of what you might think, there are places that still require the utmost in precision and accuracy in measurements, especially when the signal one is looking for will be a slight change over a period of time.

Granted that to produce high levels of precision in electronic measurement requires a calibration of all components from the ground up. The Redundant Department of Redundancy Department will make repeated measurements to ensure that insurability is ensured. This level of scrutiny requires an inscrutable attention to detail with scruts tightened to specified torque settings. So don't be torqued off.

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#10
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Re: The Wheatstone Bridge: Still Alive and Kicking

05/07/2016 3:50 PM

Yes, I am aware that in special highly circumstantial applications special care needs to be taken but in the vast majority of day to day applications that level of precision and accuracy is largely irrelevant.

Do you care if you vehicle speedometer is off by a +- 1 - 2 MPH @ 65 MPH or if the Odometer is off by a few 10's of feet in a mile?

How about home electrical power? does it need to be at exactly 120 VAC or can it vary by +- 10% and not cause problems?

How about mowing your grass? Accuracy to +- .01" or just use arbitrary level notch 2 or 3 on the nonspecific height adjustment and call it good.

How about all those components in your home electronics devices? +- 10% on those resistors and transistors or +-.1?

A guy has to know where one stands on accuracy and precision! You can't just go through life using 'good enough to work properly' as a standard, can you?

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#11
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Re: The Wheatstone Bridge: Still Alive and Kicking

05/09/2016 8:35 AM

The one or two mph error on a speedometer I can live with, and probably the odometer too. The 120 Volt power is supposed to be controlled to high levels of precision in power plant control rooms (they add or subtract VARS to balance the reactive load with resistive load, etc.) If they let the voltage droop - then it is bad news for any motors powered on that voltage, they will burn out. Light bulbs will not last as long, and refrigerator compressors will overheat. Other electronics powered directly off this will experience issues.

Some parts of circuits require precision (as in a precise voltage reference, or a current to voltage conversion that needs to be accurate and precise), while others do not (pull-up resistors on logic state inputs).

The thing is when you buy something, unless data is provided in the specifications (and adhered to by the OEM), one never knows how good a piece of equipment is, or whether it is even relevant to measuring the data at hand.

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#12
In reply to #10

Re: The Wheatstone Bridge: Still Alive and Kicking

05/13/2016 1:53 PM

"Do you care if you vehicle speedometer is off by a +- 1 - 2 MPH @ 65 MPH or if the Odometer is off by a few 10's of feet in a mile?"

Oh, you'll care about ┬▒1-2MPH if you claim you were doing 64MPH in a 60MPH zone (not a 'pull over and ticket' offense) and the cop claims you were doing 66MPH (ticketable).

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#13
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Re: The Wheatstone Bridge: Still Alive and Kicking

05/13/2016 2:26 PM

It does matter either if one had the most precise of speedometers available if the calibration was off and it produced bad results (tickets).

NO, your honor, I was really, really going an indicated speed of 64.87543 mph, when the nice officer pulled me over and handed me a ticket for 75 mph in a 60. Moot.

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#7
In reply to #2

Re: The Wheatstone Bridge: Still Alive and Kicking

05/05/2016 8:41 AM

For quick and dirty reads, a simple voltage divider will tell you the X resistor value, and it works to precision when (1) no component heats up over testing period, (2) no change in applied voltage takes place (i.e.-extreme stability of reference voltage), proper choice of up leg resistance (basically source resistance) is made that allows a good range of values to be presented as Vx at the data logger, readout, Arduino input, etc.

For high accuracy work, known values of precision resistors are required for either the voltage divider or for the Wheatstone bridge circuit. There are ways to introduce electronics into a bridge configuration when null balancing is required with a moving reference also, perhaps for servo motor application.

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#6

Re: The Wheatstone Bridge: Still Alive and Kicking

05/04/2016 10:04 PM

The utility of the Wheatstone bridge is in the strain gauge application, and specifically, the ability of this circuit to cancel out temperature drift. All the resistive elements are incorporated on the same substrate with the one element in the position to measure the displacement. Any change in temperature is seen by all the elements and hence they track. The article mentions this, but just glosses over it. The other thing is that the elements will also 'age' similarly. Sure, to just measure a resistor, there are plenty of 4-wire meters that will do that very well.

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#9
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Re: The Wheatstone Bridge: Still Alive and Kicking

05/05/2016 9:01 AM

Another 2 cents to go with your nickel:

http://www.allaboutcircuits.com/textbook/direct-current/chpt-8/kelvin-resistance-measurement/

This is very clear on measurement using a voltmeter + ammeter as compounding error of measurement more so than simply using a precision resistor as a current shunt, and how to apply 4-wire test measurement, very good points taken. Yes, precision resistor + voltmeter propagates less error of measurement.

Has anyone considered how to use Wheatstone bridge for position feedback in machinery, or other mechanical device?

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