the voltage divided between the left and right resistance.
We would use this mode of operation when we want to
manually set a voltage that is used in some other part of
an electronic system. For instance, we might have the pot
hooked up to a voltage controlled audio amplifier. We
would then adjust the audio volume by moving the knob
to the left and right while listening to the sound volume
change until it was where we want it.
We might also want to communicate a magnitude to
a computer. We could then have the wiper pin connected
to an ADC that would measure the voltage and tell the
computer what the voltage is. Then, the software would
make decisions based on that voltage. An example of this
would be to set the starting drill position in a CNC
(Computerized Numeric Control) machine. You’d have
one pot to set the drill’s X position and a second pot to
set the Y position. When the drill is right where you want
it, you could press a button telling the computer to start
the drilling. The computer would read the X and Y
voltages and ‘know’ the location of the zero points for
both the X and Y directions in the work surface.
You can also use a pot to control an electric current
as shown in Figure 4. If you only use the wiper pin and
one of the other pins, you now have a variable resistor —
not a resistor divider. If you apply the input voltage to the
wiper pin, then as you move the wiper that input voltage
is across a varying resistance (from 10,000 Ω when
turned all the way to the left, and 0 when turned all the
way to the right). Since we know by Ohm’s law that the
current is set by the voltage and resistance, we know
that the current varies with the resistance which is set by
the position of the wiper. This configuration is called a
rheostat and is used to control current.
Figure 4 shows how this works. In the
case of the wiper being all the way to the
right (notice how it doesn’t work ...
boom!), you see a physical illustration of
the old ‘divide by zero’ error. You can’t
divide by zero nor can you apply a voltage
across zero ohms. You can try, but the
system will attempt to generate an infinite
current — which, of course, it can’t.
Figure 5 shows this concept with a
water current metaphor and a soon-to-be
burned out LED. To prevent this situation,
you will want to add a resistor in series
with the side of the pot being used in the
Figure 6 shows a 100 Ω resistor used
as a current-limiting resistor. Never
forget to add this resistor since as we’ve
already seen, applying any voltage across
zero resistance is a sure path to disaster.
Let’s get our hands dirty in some
■ FIGURE 6:
Pot with current-limiting resistor.
■ FIGURE 5:
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