Understanding the (point of) the Wheatstone Bridge

Discussion in 'Electronic Basics' started by Jamie Jackson, Sep 21, 2007.

1. Jamie JacksonGuest

My son and I are working through some circuits from a Forrest M.
Mims / Radio Shack learning lab. We got to a Wheatstone Bridge
circuit, but I'm trying to understand the usefulness of it.

Let's use this diagram, for reference: http://en.wikipedia.org/wiki/Wheatstone_bridge

If you need to adjust R2 in order to get the value of Rx, how do you
even know what R2 is anymore (since you've adjusted it)? Is R2 some
fancy high-accuracy *graduated* variable resistor that I've never
heard of?

After seeing the diagram, I'm thinking: Well, if I have to measure my
R2 with my multimeter, I might as well measure Rx while I'm at it,
which blows the point.

Is the point just that they're perfectly balanced, and the point is
not what the actual values are?

Thanks,
Jamie

2. DaveGuest

Hey Jamie,

Well, so far as usefulness of this circuit is concerned, I used to work on
high-speed printers (mainframe environment like banks and oil companies)
that used the Wheatstone Bridge (they called the actual 'box' that housed
the circuit an H-switch) to drive the servo motor that moved the paper at
incredible speed and stop on an instantaneous command. The servo was so
strong it would literally rip your arm off if you happened to be holding the
shaft in your hand when it took off. You couldn't let go fast enough... I
know it has applications in testing unknown components, but that was what
that company used it for. For unknown components, a built in readout can be
used to tell you what is plugged into it, without running the readout
current through the device under test. Can make a useful piece of test
equipment...

All I can think of.

Dave

3. Jamie JacksonGuest

Thanks Dave,

I think I'm getting it now, actually. I reread the wiki article. It
seem that a variable resistor at R2 is just a nifty way to test for
balance (and not necessarily for finding the value of Rx).

Rather, to find the resistance of Rx, you work with three resistors of
known value, and use Kirchhoff's rules based on those values and the
readout of the meter.

Sometimes it just helps to post, and then the mind clears on its
own. ;-)

Thanks,
Jamie

4. NobodyGuest

Yes, it's typically a variable resistor with a graduated knob on it.

Or it might be a multi-way switch and a ladder of fixed resistors. Or
several of them (e.g. one switch with 10 1kOhm resistors and one with
10 100Ohm resistors, giving 0-9900 Ohms in 100-Ohm steps).
You need to know the value of R2.

5. NobodyGuest

That's an H-bridge, not a Wheatstone bridge.

A bridge is any circuit with that "shape", i.e. two "legs" joined at the
top and bottom, an input applied between the top and bottom, and an output
between the centres.

A Wheatstone bridge has resistors, a bridge rectifier has diodes, an
H-bridge typically has MOSFETs (although they could be BJTs or even
relays).

An H-bridge allows you to vary the magnitude and direction of a current
through a load, typically a motor. The name arises from the fact that the
circuit is normally drawn with the legs vertical, rather than the diamond
shape which is commonly used for the Wheatstone bridge or a bridge
rectifier.

6. Don BoweyGuest

In expensive bridges, such as the ones the telcos use to measure cable
faults, the R2 in the diagram is not a variable resistor. It is a large
group of fixed, precision, resistors on rotary switches than can be switched
in, in a sequence, to balance the bridge. The Rx resistance is then read
from the positions of the switches.

To be balanced R2 must equal Rx. The ones I used in the Toll Testroom could
measure to within .01 Ohm.

7. Rich GriseGuest

I saw an app where there were, in effect, three fixed, calibrated
resistors and the fourth was a magnetron filament. When a magnetron
is in use, some of the electrons fall back to the cathode (back-
bombardment), and help to heat it. The bridge was used to monitor the
resistance of the filament, and the loop was calibrated to the tempco of
whatever the filament was- probably thoriated tungsten. Anyway, the
filament current went through the bridge, and a diff amp picked up the
balance point; the niftiest part was that the whole thing was biased at
about -1500V. Because of the bridge arrangement, there were two advantages -
one, that it could float, like I said, but that it doesn't really care
how much current is flowing in the load, since it's comparing resistance,
to make a temperature controller.

Hope This Helps!
Rich

8. Rich GriseGuest

Actually, R1/R2 has to be equal to R3/Rx. But both sides don't necessarily
have to be the same. (i.e. 100/20 == 5/1, for example.)

Cheers!
Rich

9. Don BoweyGuest

That, of course, was a given, and was likely also explained in the Wiki
blog.

10. JamieGuest

It then must have a Kelvin set up?