zalzon said:

No matter how much effort I put into understanding these low and high

pass circuits I always come up clueless. I can't seem to visualise

what the electrons are doing at the resistor and capacitor. Its been

slowly driving me insane.

Why is low frequency AC allowed to pass but not high? Can someone

explain this to me in easy to understand terms referring to the

electrons in particular as they move around the circuit.

Please don't give me equations like Xc = 1/2pifC..etc cos that is just

crap. I know that already and its not helping me visualise anything.

I want to know what the electrons do when they reach the resistor in a

low pass circuit with low frequency AC. What happens then when they

encounter the capacitor. Why is low freq allowed to pass but not high

frequency?

Thank you

Do you understand how a series RR voltage divider can reduce the

voltage applied across it to something less across one of the

resistors?

There is a resistor, say R1 between input and output, and a second

resistor, say R2, between output and ground. Lets also say the source

of the input signal has zero impedance (current from this source does

not affect its voltage) and the output signal feeds an infinite

impedance load (the output voltage causes no current to pass through

the load). So the only currents are those through R1 and R2 from

source to ground. The way voltage across a resistor is related to the

current through a resistor is proportionality. The voltage is always

proportional to the current. Ohms are just a short hand way of saying

"volts across per ampere through". Since there are no other paths for

current in this circuit except through R1 and R2, those currents must

be equal. So the voltages across those resistors must be proportional

to their resistances by the same factor (the common current). So the

input voltage is used up partly by R1 and partly by R2. If R1 is 4

times the resistance of R2, then 4/5 of the input signal voltage must

be dropped across R1 and 1/5 must be dropped across R2. The frequency

(or wave shape, which can be thought of as a combination of

frequencies) of the signal voltage does not matter, since this voltage

division is instantaneous. All frequencies are treated equally.

RC high and low pass filters are similar voltage dividers, except that

one of the elements (the capacitor) has different impedances (AC

voltage across per ampere through) at different frequencies.

For example a resistor R is between the input signal and the output

signal, and capacitor C is between the output signal and ground. The

source and load impedances are as in the first example.

The voltage drop across the resistor is instantaneously proportional

to the current through it (its resistance is the proportionality

factor), but knowing the voltage across a capacitor does not tell you

anything about the instantaneous current through it, and vice versa.

The relation between voltage and current for a capacitor is that the

current through the capacitor is proportional to the time rate of

change of the voltage across the capacitor, regardless of what the

voltage happens to be. Capacitor voltage rises as current passes one

way, and falls as current goes the other way.

You might picture a capacitor with a fluid analogy as a rigid tank

that has two pipe connections and an elastic membrane stretched across

the tank between the two pipes, separating the tank into two volumes.

The voltage across the capacitor is equivalent to the difference of

pressures in the two halves of the tank. You might imagine that this

difference has nothing directly to do with how much fluid is passing

into one pipe and out of the other, but has a lot to do with how much

the membrane is stretched to one side or the other at the moment.

flow changes the pressure difference by changing how much the membrane

is stretched. If you think of the resistor as a capillary tube you

have all that you need to visualize the RC low pass filter. The input

end of the capillary is driven by a pressure that varies sinusoidally

at some frequency. The electrical ground (zero volts) is replaced

with a vent to zero (atmospheric) pressure. Now, as the input

pressure swings positive and negative, fluid flows into and out of the

side of the tank that is connected to the capillary tube, and the

membrane is pushed and pulled by various amounts and the pressure in

the half of the tank connected to the capillary varies positive and

negative, but never quite catches up with the input pressure wave.

The pressure in that side of the tank is also the output signal.

The faster the input wave changes, the further the output signal falls

behind and the smaller the relative pressure swing it achieves.

At very low frequencies, the tank pressure almost reaches the applied

pressure before the direction reverses, and there is little difference

in output amplitude compared to input amplitude. Small changes in

either the resistance of the capillary or tank membrane elasticity

(resistance or capacitance) have little effect on the output signal.

This shows frequencies that are in the pass band. But at some

characteristic frequency, the pressure swing in the tank reaches only

70.7 % or the applied pressure in each direction and falls behind the

input wave by 45 degrees. This is the filter corner frequency and

represents the transition between pass band ands stop band. Above

that frequency, the tank pressure swing drops rapidly with increasing

frequency. At much higher frequencies the ratio of the pressure

swings in the capillary side of the tank to the applied pressure

swings fall almost in proportion to the source frequency. Double the

frequency and the output swing falls by almost half.

The characteristic or corner frequency where the the filter response

changes is related to the product of the resistance and capacitance.

When 2*pi*f=R*C, f is at the corner frequency.