Understanding the (point of) the Wheatstone Bridge

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

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

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

You need to know the value of R2.

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.

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.

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

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

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