If you need help calculating the power dissipation, check out our power
tutorial. The multimeter tutorial describes how to measure the current drawn
by a circuit.
As a Measurement Tool
Decade boxes can be very useful on an electronics test bench. One
traditional application of a decade box is to measure an unknown
resistance.
Wheatstone Bridge
The Wheatstone Bridge is a DC circuit that uses three resistors of known
value to measure a fourth resistor (or even other materials, such as soils or
liquids). The Wheatstone bridge is commonly drawn as a diamond, as
shown below.
Notice that there is a battery (or other DC source) on the left, powering the
top and bottom of the diamond. At the center of the diamond, between
points A and B, is a voltmeter.
The diamond drawing obscures what’s really happening in the circuit. If we
rearrange and redraw the circuit, we recognize that it’s a pair of resistive
voltage dividers.
The voltage at point A is determined by the ratio of R and the variable
resistance R , while the voltage at point B is determined by the ratio of R
and the unknown R . If the two ratios are the same, the A and B will have
the same voltage, and we can infer the value of R .
R /R = R /R
Which we can solve for R :
R (R /R ) = R
And if R is equal to R , it simplifies to
R = R
If R is a decade box, we can simply read it’s value, and we’ll know the
value of the unknown resistor!
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