Traditionally, a technician would lift one leg of a resistor to get an accurate resistance measurement. This can be difficult today since
surface-mount components are often used.
There is a technique taken from automatic
test equipment called guarding that uses a
controlled voltage source, ammeter, and
strategically placed grounds to measure the
current going through a single component.
With a known source voltage and a current
reading, the exact value of an individual
resistor can be calculated.
Figure 2 shows how the resistance of the
center resistor can be determined using a 1V
source and an ammeter. First, place the
voltage source on one side of the resistor and
the ammeter going to ground on the other
It is often impossible to
directly measure the
resistance of resistors
because of the presence
of parallel current paths.
Let’s see what we can do
In the resistor bridge
circuit in Figure 1, you
can see that the center
4. 7 kΩ resistor reads just
1 kΩ because of the
parallel current paths.
56 July 2016
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