EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Richardcavell on November 30, 2024, 07:01:02 am
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1. I have been told that an RC circuit can be used as a low-pass or high-pass filter. The way I’ve been trying to understand it is this: Every AC signal is made up of sine waves superimposed on each other. Each sine wave has a frequency. Conceptually, I consider each wave separately. The capacitor has high reactance to low frequencies, so a load in parallel with the capacitor will have a higher voltage wave if the frequency is lower. Does that sound right?
2. In relation to using an RC circuit to separate AC and DC, is it right for me to think of the AC as going through the capacitor?
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The capacitor has high reactance to low frequencies, so a load in parallel with the capacitor will have a higher voltage wave if the frequency is lower. Does that sound right?
Sound right.
But as always ... it depends.
If source is sinus current with the same amplitude for each tested frequency then you will get "higher voltage wave if the frequency is lower".
But it is not easy to have such current source.
In real you will rather have voltage sinus source with some internal resistance (R). And this resistance will work with capacitor being paralel to load as low pass filter.
From low frequencies the voltage at capacitor will be practically the same (capacitor high reactance makes with internal source resistance a kind of voltage divider). If reactance is 1000 times higher than R or is only 100 times higher you will get practically the same result.
When they became equal you can observe source voltage being divided by 2. Starting from that point it behaves like you expect - with rising frequency voltage at capacitor is lover and lover.
If you make little wrong assumption that at the point were they are equal voltage is still not reduced but equal to the source than from that point you can assume - n-times higher frequency - n-times smaller voltage.
Search for "Bode plot", "Bode characteristics".
There is something called "Laplace transform" what is used to analyse electronic circuits. Laplace transform is difficult to be understand and used directly, but allows to transform complicated in time domain equations into equations in s-domain allowing then to calculate everything as simple linear equations (electronics like to simplify things). Bode plot is the effect of using these equations. You need not to understand Laplace transform to learn to use its results practically. It is like you can effectively use calculator without knowing how it works.
I wonder how Laplace could have come up with this many years before electronics actually appeared.
is it right for me to think of the AC as going through the capacitor?
As previously - almost right.
Depending on other elements (like R to GND after this capacitor) low frequencies will be seen as being too close to DC and practically will not be going through it and starting from some frequency AC will pass through capacitor.
Such serial C with then R to GND Bode characteristic will tell you what frequencies will pass and what are to close to DC to pass. Bode plot will be just rising from the left (lo frequencies) until some point (frequency) and then will be horizontal line - all higher frequencies pass 1:1 to the output.