wheatstone bridge

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zog

Senior Member
Location
Charlotte, NC
does anybody here have a good wiring diagram and instructions on how to build a wheatstone bridge?

Out of curiousity, why? Kelvin bridges are much more accurate and not much harder to build. What are you planning on doing with this?

You do know you can buy these pretty cheap right?, they are disguised under other names. In fact, many people own them and dont even know it.
 

electricalperson

Senior Member
Location
massachusetts
Out of curiousity, why? Kelvin bridges are much more accurate and not much harder to build. What are you planning on doing with this?

You do know you can buy these pretty cheap right?, they are disguised under other names. In fact, many people own them and dont even know it.

i was reading about one in a book and i want one to experiment with. i have no idea what a kelvin bridge is and i dont know what they cost or disguised as
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090107-1938 EST

electricalperson:

Get a 1 turn linear potentiometer, or better yet a 10 turn helipot. 1000 ohms is suitable. A 12 V battery. Put a 220 ohm 1 W resistor in series with the battery to serve as a current limiter if you do something wrong. Get an assortment of resistors from 100 ohms to 10,000 ohms. Possibly 100, 220, 470, 1000, 2200, 4700, and 10,000. Also get one extra 100, 1000, and 10,000 ohm 1 W resistor.

You need a meter that is sensitive to millivolts.

I will continue a little latter. Tell me what you have to work with.

.
 

electricalperson

Senior Member
Location
massachusetts
090107-1938 EST

electricalperson:

Get a 1 turn linear potentiometer, or better yet a 10 turn helipot. 1000 ohms is suitable. A 12 V battery. Put a 220 ohm 1 W resistor in series with the battery to serve as a current limiter if you do something wrong. Get an assortment of resistors from 100 ohms to 10,000 ohms. Possibly 100, 220, 470, 1000, 2200, 4700, and 10,000. Also get one extra 100, 1000, and 10,000 ohm 1 W resistor.

You need a meter that is sensitive to millivolts.

I will continue a little latter. Tell me what you have to work with.

.

one question. can i get that stuff at radio shack? they sell the resistors but i never did look for a linear potentiometer or a helipot
 

IMM_Doctor

Senior Member
Bridge under Pressure

Bridge under Pressure

Wheatstone bridges are also used in industrial pressure transducers, and load cells for weight measurment.

In a pressure transducer, there are four equal resistors cemented to the back of a flat sealed diaphram. A small DC excitation voltage is applied to 2 opposite corners of the bridge, and instrumentation is connected to the other two opposing corners. If the diaphram is flat (no pressure) there will be no voltage differential at the sensing corners, as all resistance is equal.

When pressure is applied to the diaprham via mechanical loading, pneumatic, or hydraulic pressure, the diaphram is deflected, causing the resistance to imbalance proportionaly with the applie pressure.

There will be typically only a few millivolts measured at the sensing corners, even at full pressure, as the deflection is very small.

In the past most sensors we millivolt output only, and external electronic amplifiers were needed nearby, to amplify the measument to an industry standard of (4-20mA) or (0-10v) analog signal.

Now with the miniturization of electronics, this can all fit in the body of the transducer, (and even include digital readout display).


The flat floor scales that you see in industrial plants have (4) Wheatstone bridges (one under each foot). The four are summed in the instrumentation to indicate the total weight on the platform.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090107-2112 EST

electricalperson:

Do not buy anything from RadioShack unless you need it immediately.

Instead of a battery you might use an AC to DC plugin power supply. Maybe an old one from a previous phone, cellphone, caller ID, or some other small electronics device. Many of these have an output in the range of 6 to 18 V DC. Some are unregulated, but plenty good for this experiment, others will be regulated. Maximum current output might be in the range of 100 to 500 MA which is more than enough for initial experiments with the bridge circuit.

For example from DigiKey you can get a 12 V 500 MA plug-in supply for about $7 plus tax and shipping. This has a connector on the output and you either cut it off or get a mating connector. The DigiKey part number is T983-P5P-ND. This could be used for other electronic experiments such as playing with the 555 timer chip.

What you want to learn or intend to use the bridge for will determine the miscellaneous components you need.

The book and professor that I studied bridges under was:
"Basic Electrical Measurements", by Melville B. Stout, 1950, Prentice-Hall. See page 64 and 65. The balanced condition exists when X = (A/B)*R equation (15) where X is the unknown, A/B is a known ratio, and R is a known reference resistance. You adjust the ratio A/B to obtain bridge balance. Balance is when the meter in the bridge circuit reads zero volts.

See Wikipedia:
http://en.wikipedia.org/wiki/Wheatstone_bridge

You need to indicate your depth of interest, currently available equipment (like a Fluke 27 or whatever), and other factors because this greatly affects the cost of the experiment.

.
 
Last edited:

steve66

Senior Member
Location
Illinois
Occupation
Engineer
You can get everything from Radioshack, although it is a little more expensive than mail order.

However, I don't see why all the parts would cost more than $10 even from RS.

And you should be able to get a cheap meter for $10-$20 bucks, if you already don't have one. Although, if you are interested in electricity, I would suggest spending enough to get a good meter. And for bridge experiments, you really need a meter that measures both positive and negaitve currents or voltages.

Also, buy a photoresistor, and try using that as one of the resistors in the bridge as you vary the light levels. You can detect really small changes in light level like this.
 

electricalperson

Senior Member
Location
massachusetts
You can get everything from Radioshack, although it is a little more expensive than mail order.

However, I don't see why all the parts would cost more than $10 even from RS.

And you should be able to get a cheap meter for $10-$20 bucks, if you already don't have one. Although, if you are interested in electricity, I would suggest spending enough to get a good meter. And for bridge experiments, you really need a meter that measures both positive and negaitve currents or voltages.

Also, buy a photoresistor, and try using that as one of the resistors in the bridge as you vary the light levels. You can detect really small changes in light level like this.

i have a fluke 117 as a meter. does it have to be analog?
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
That should work fine. Although IMO, wheatstone bridges are a little more fun if you have a panel type meter (the analog meters where the needle rests in the middle of the scale, and can swing in either direction.)
 

electricalperson

Senior Member
Location
massachusetts
That should work fine. Although IMO, wheatstone bridges are a little more fun if you have a panel type meter (the analog meters where the needle rests in the middle of the scale, and can swing in either direction.)

ill search online for that stuff. i know radioshack might sell circuit boards i could use but i dont even know what im doing. i just need to research it and learn more about the tool before i build one. i would like to build one to use in the real world if possible and not just to play in my living room with
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090108-1725 EST

electricalperson:

My guess is that you will not use a bridge circuit for routine measurement of resistance.

With your meter, resolution of 0.1 MV, 10 V across the bridge, and all resistors approximately the same value, then if my calculations are correct you should be able to see a change in the resistance under test of 0.002% or 2 parts in 10^5.

To measure absolute resistance this accurately your reference resistor would need to be more accurate than this, and the ratio of the ratio resistors would need to be greater than this accuracy.

However, this degree of resolution and operating in an unbalanced mode would allow you to see very small changes in resistance, or do comparison of resistors under test.

I have a 45 year old Helipot, 500 ohms, with a 10 turn capability and knob. It is listed as 0.25% linearity. That means relative to full scale. A check of it against my Fluke 27 in resistance mode appears to confirm that it is within spec.

I just setup a bridge using my 500 ohm Helipot and two 1 K 1 % metal film resistors. At a dial setting of 500.0 out of 1000 the bridge unbalance was -0.005 V, at 500.5 it was +0.001 V, and at 501.0 it was +0.004 V. Total change for 1 division 0.009 V. The calculated change is 0.01 V. Good correlation. The granularity of the Helipot wires was about 0.002 V. This particular pair of 1 K resistors were very close to each other in resistance.

If you go thru the equations for the bridge unbalanced output voltage vs change of resistance of the resistor under test you will find that the output is non-linear. Although for small resistance change it is close to linear.

Since you just want to play with a bridge circuit I suggest you start with a single turn linear potentiometer for your variable resistor.

My objection to Radio Shack is that almost everything is from China and much is of low quality in comparison to quality manufacturers. In addition there is very little selection.

.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090108-1825 EST

http://books.google.com/books?id=qLMXAAAAIAAJ&pg=PA324&dq=wheatstone+bridge+before+1900#PPA265,M1
The section on Electricty starts at p 265. 324 (Wheatstone), thru 326.
This is a physics book circa 1900. The whole book is availabe on Google Books. However, it is slow. Quite interesting to observe history of that time.

Look at the crude means for measuring current. But much more advanced than 1879.

http://www.cliftonlaboratories.com/1n400x_diode_family_forward_voltage.htm

Dinner time.

.
 

ohmhead

Senior Member
Location
ORLANDO FLA
Galvanometer

Galvanometer

Well i like to add just a little you need a( galvanometer) which are hard to find today , and if your going to take the time and build a nice project like a WB than please use a voltage regulated dc power supply the galvano miiliamp meter must read posistive and negative so you can null the meter center scale when you find your resistance of what is not known . Then you can measure off that adjustable pot with your ohmmeter . You want the best pure fullwave dc output for your project and then you can use it for other projects .best to ya
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090110-1445 EST

ohmhead:

Why do you need a well regulated excitation voltage for a Wheatstone Bridge used in a null balance mode?

One of the outstanding features of the Wheatstone Bridge is very high accuracy of resistance mesurement in the null balance mode relatively independent of the excitation voltage.

In those early days and until after the middle of the last century there were no extremely precise means of defining the volt and ampere. However, resistances could be related to each other with extreme accuracy even in 1850.

Some historical references of interest:
http://chem.ch.huji.ac.il/history/wheatstone.html
http://www.smeter.net/daily-facts/6/fact25.php

.
 
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