Re: Understanding the (point of) the Wheatstone Bridge

On Thu, 20 Sep 2007 21:42:36 -0500, Dave wrote:

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

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.

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.

.