Data Sheet
-83-
154. RC Low Pass Filter Circuit
Build the circuit shown on the left and turn on the switch (62). Press the
press switch (61) and you will see the bi-directional LED (71) light red.
Release the press switch (61) and the bi-directional LED (71) will stay
bright briey and then turn dim. One important application of RC circuits
is that they can be used as lters to pass certain frequencies and reject
certain frequencies. Consider the Input/Output of the RC circuit shown
below. It has been discussed that capacitors can store charge from a
voltage source. However, up to now we have only been considering a
constant voltage source (called a Direct Current or DC voltage source).
Imagine turning the input signal to the circuit below on and off quickly. If
you turn the input signal on and off fast enough, then there will not be
enough time for the 100mF capacitor (73) to charge and the output will
just be 0V. However, if the input signal is turned on and off slow enough,
then the 100mF capacitor (73) will have time to charge and the output
will look like the input. We have just described a low pass lter (high
frequencies do not get through while low frequencies do).
155. RC Low Pass Filter Cutoff Frequency
Replace the 1kW resistor (42) in project #154 with the 5.1kW resistor
(43) and turn on the switch (62). Press the press switch (61) and
you will see the bi-directional LED (71) light red. Release the press
switch (61) and the bi-directional LED (71) will stay bright for a little
while and then turn dim. As discussed in the last project, an RC circuit
can act like a low pass lter. The easiest method for determining the
cutoff frequency of an RC lter circuit uses frequency domain analysis
techniques like Fourier Transform theory which is beyond the scope
of this manual. But it turns out that the cutoff frequency of a single
pole (single resistor and single capacitor) circuit is f
cutoff
= 1/(2*p*R*C)
where p (pronounced as “pie”) is a constant dened as approximately
3.1416. If the 5.1kW resistor (43) and 100mF capacitor (73) in this
project were used in an RC low pass lter, then the cutoff frequency
would be 1/(2*3.14*5100*100e
-6
) = 0.31Hz. Only very low
frequencies (near DC) would pass through such a circuit.