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Passive RC low pass filter tutorial!

Passive RC low pass filter tutorial!


In this tutorial I’m going to talk about
passive low-pass RC filter circuits. So at a basic level… what are
filters? Filters are used to change the frequency
content of signals. You can experiment with this in software
by playing with the graphical equalizer settings in your MP3 player. Here is a normal unfiltered sound clip… Now if you decrease the low frequencies
and allow the higher frequencies to pass through unchanged, you create a high pass filter. It sounds like this… You can hear that there is much less
bass and only the higher frequencies remain untouched. Now if you decrease the high frequencies
and allow the low frequencies to pass through unchanged, you get a low pass
filter. And that sounds like this… You can hear all the bass but the high
frequency sounds are much quieter. Now I’m going to show you how to do this
in hardware and you’ll be able to do this with any signal, whether it be audio,
video signals, radio-frequency signals or whatever. Let’s start with RC low pass filters
because they are easy. An RC filter is just a filter made
out of a resistor and a capacitor. The original signal goes in and the
filtered signal comes out. The reason why this works is that the
voltage on a capacitor cannot instantaneously change. When you have a resistor that slows the
charging of the capacitor, your output voltage might not be able to follow the
sudden changes in the input voltage. As a result higher frequencies get
filtered out. I’m going to build a low pass filter
with a 10 nanofarad capacitor and a 10 kilohm resistor and show you how it affects different
frequency signals. There’s no particular reason why I’m
using these values it’s just for the sake of an example and
I’ll do more examples later. So this is very important equation. This is the equation that is used to
determine the cut-off frequency of the filter. If I plug in my chosen values of
10 kilohms and 10 nanofarads I get a cut-off frequency of
1592Hz. And I’m going to round that up to
1600Hz. Okay so this low pass filter has a
cut-off frequency of 1600Hz, but what does that mean? This means that at frequencies below
1600Hz the signal passes through unchanged. If I feed in sine waves
of different frequencies, you can see that from 45Hz to 200Hz the output is the same as the input: ten volts peak to peak. Now at frequencies approaching 1600Hz, the amplitude is slowly dropping on the output. By the time we reach the 1600Hz
cut-off frequency we’ve got a reduction of about thirty percent. The cut-off frequency is the point where
the filtering effect really starts being effective. And another thing you’ll notice is that
the two sine waves no longer line up. There has been a shift in phase. That’s because the filter is introducing
a small delay in the signal. Most the time you won’t care but it is
something you should be aware of. Now let’s increase the frequency way
beyond the cut-off point. At 15kHz there is a
massive reduction in amplitude. And as we get into higher frequencies like
50kHz and beyond, there’s almost nothing left of the
signal. Now the oscilloscope example was just a
visual introduction to the concept of a low pass filter and how it affects voltages in your
circuits. Normally what engineers do is describe
filter characteristics with a graph called a bode plot. Earlier I mentioned that the cut-off
frequency is the point at which the filter starts being effective. Well you should know that in the real
world you will never get a filter like this where as soon as you hit the cut-off
frequency all the higher frequencies get reduced to zero. Here is a more realistic bode plot. In realistic filters what happens is
that for low frequencies you get this flat response where the amplitude doesn’t
change. Then as you slowly approach the cut-off
frequency, things start to decay a little. At the exact cut-off frequency the
amplitude is reduced by three decibels. (Or even simpler than that, at the cut-off frequency the amplitude
is reduced by twenty nine percent.) Now a little bit after you’ve hit the
cut-off point, the filter keeps reducing the amplitude by twenty decibels
per decade. That means that for every time the
frequency increases by a factor of ten, the amplitude decreases by a factor of ten. Bode plots like this are a great way to
predict how your filter will perform at a wide range of frequencies. In another video I will show you how
to make them. (Search for “LTSpice tutorial”) For now let’s get some practice creating
some more low pass filters. Let’s say I want a low pass filter that
goes before subwoofer’s amplifier to make sure that the woofer is only
outputting really low bass frequencies. Let’s say I want a cut off frequency of
about 150Hz. Here’s the equation again to calculate
the cut-off frequency. Now I can play around with random values
of R and C to get the cutoff value of 150Hz…
But from experience that I know that I should choose R, and then
let the equation determine the value of C. I’m going to choose a 1 kilohm resistor
because I don’t want to overload the audio source. In this case it’s
an MP3 player. So here I’ve rearranged the equation to
give me a way of calculating the capacitance needed for the filter based on a chosen resistance value. And it turns out that the value I need is
1.06 microfarads. Let’s round that down to 1 microfarad. So here’s the 150Hz low-pass filter
as a schematic, and here it is on the breadboard. Now I’m going to play two sound clips. The first one has no filtering. The
second one has been filtered by our low pass filter and amplified a little. You can hear that the bass is clearly
there, but most of the rest of the music is missing. Okay that was an audio example but I
want to show you how low-pass filters can be used with much more than just
sine waves. Let’s say you’ve got a microcontroller
that is outputting a pulse width modulated square wave. Now let’s say you want to convert that
to a smooth continuous voltage that is an average of the square wave’s voltage. Essentially we want to filter out all
high frequencies and be left with D.C. So let’s take our handy cutoff equation
and make the cut-off a very low value of 1Hz. I’m going to choose a resistor value of
100 ohms so that this filter can still power something useful like
an LED. You should always think about what
resistor value will work best with your source signal and your chosen load. So with R=100 ohms, and
reusing the same equation, we get a capacitance of 1592 microfarads. Some extra capacitance won’t hurt in
this case because we want as much filtering as
possible so let’s round that up to 2000 microfarads. Now let’s see how this heavy filtering
affects a square wave. Okay here I have a 54Hz PWM
signal and average voltage is 4.24 volts. At the output of the filter it looks
like we’ve got almost pure D.C. with an average voltage of 4.16 volts.
That’s pretty close! Now I’ve lowered the duty cycle of the
signal and on the input the average voltage is
1.68 volts. After the filter it looks like we’re
very close to clean D.C. with an average voltage of
1.44 volts. Again that’s pretty close and as you have
probably figured out, real world filters aren’t 100% efficient so
you’ll always lose a little energy as heat. Now you might be wondering… can you put
filters back to back in series for an even heavier filtering effect? The answer is yes! However since each filter puts a load on
the previous filter, the math behind calculating the cut-off frequencies gets
really complicated. That’s the point where I bust out a
circuit simulator like LTSpice and let it do the hard work for me. I’ll make a tutorial on filter
simulation with LTSpice in another video. (Search “LTSpice tutorial”) In the meantime check out my video on
passive RC high-pass filters. The good news is they are almost exactly the
same as low-pass filters!

100 comments on “Passive RC low pass filter tutorial!

  1. When the capacitor is discharging, wouldn't its discharge interfere with the low frequency signal through Vout? thanks

  2. Cool man, thanks.
    But how do you make a filter that works on just 300Hz to 3,000Hz (3kHz)?
    In other words, it filters out anything <300Hz and >3kHz.
    Such a filter might be useful for listening Human Speech, older recordings & such.

  3. BEST MOST SIMPLE "EXPLANATION" OF CROSSOVERS AND CROSSOVER "CAPACITORS" OUT THERE  !

    It is amazing that no one else can just explain this so simply ???

    Most every question answered !

    GOOD JOB !

    Rick – Southern California

  4. Hi, what does it mean to not overload the audio source? How does having a 1k resistor achieve that?

  5. I'm building a sub box for my home stereo, the satellite speakers start to decrease in volume at 60hz and are completely quiet at 35hz, what cutoff frequency should I use assuming the subwoofer is near flat response.

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  7. Man, your videos are QUALITY! Honestly, from all my experience from universities, books and other tutorials, your videos make the BEST intuitive sense. Excellent content, you can communicate the content in an EXCELLENT manner and very funny too! And most importantly, the mix of theory and practical stuff. Amazing channel, will await for more!!!

  8. I spent freaking two quarters to find out what the hell the different filters do and learned nothing form the class. but this video explained me what I need to know. Thank you so much Sir Afrotechmods. Keep up the good work!!!!

  9. I wired it exactly as in the video, but the potentiometer only acts as kind of a volume knob – it does NOT control the cutoff frequency… If I use a 10k pot and a 100nf ceramic capacitor, the capacitor also does absolutely nothing. When using a 100uf electrolytic capacitor, the high frequencies are cut but the knob just controls the volume and not the cutoff freq?!
    Can anybody help me, I do not find the problem…

  10. so what i get from reading comments and such is that due to the low impedance, its make the higher frequencies go through the cap to GND, but my question is why doesnt the low frequencies just go to ground as well?

  11. you make an informative video and for that i applaud you

    however calling people out for loud car audio is pretty pathetic man…
    makes me cool ? , no, not generally but the appreciation i get from some, the smiles, the video's, the trophy's ive won and the handshakes ive gotten makes my day at the time.
    have you never heard a system capable of 145+DB that also is a class winner in SQ ? … i think its time you experienced it rather than being so closed minded.

    with that being said with power comes responsibility, going full tilt through the suburbs especially at night is an unwritten rule if your actually loud.

  12. So using this method, could you convert 24v AC power from an HVAC thermostat into logic level DC for an Arduino? How would you determine the resistor value in this scenario?

  13. hey.how can i connect the jack to the breadboard?? can u please explain or tell me how can i get that adapter?

  14. Hello! Interesting tutorial!
    What if I want to remove a narrow "notch" in my triangle wave output from VCO? I fear that if I'll just insert 1-pole filter for ~15-17kHz then it will not be enough or will be reducing the sound of the wave on high octaves. Can this thing be done with a single trimmer… so one resistor will be choosing a "starting point" and other RC filters will be a kind of "an offset" to make an efficient (let's say) 12-pole filter…
    I can't remove that notch… it's a simple saw -> waveshaper with a pull-down 1N4148 diode on the positive side of an op.amp that makes the waveshaping. I've tried small caps in the negative feedback (on the same op.amp) but it doesn't helps…
    http://img.picshare.at/1521937883_JP8_SawCore_3-wave.jpg

  15. Don't you think when we put this filter for making smooth DC power, we get almost 90% of the power. But if the resistance is in high power, a Full-bridge rectifier or three phase full bridge rectifier will have to deliver maximum power at the output with with good Quality DC. What should be the scenario in that case.

    Keep in mind that almost every transformer produces some amount of harmonics which make the circuit a little unstable.

  16. i shit u not a guy rolled up to me at the gas station the other day and he was rattling the whole fucking store with his subs

  17. If I'm telling the truth, I wish I didn't fall asleep during this lecture. If I'm telling more of the truth, I wish I fell asleep more often. I get these. I think my professor's draining me of my life essence.

  18. HelloThere!!
    I have an 80 w subwoofer. What do you suggest I set cutoff frequency to? And probably could you tell what R- value to use for that particuler cutoff frequency. Help very much appreciated.

  19. sir yr video on RC LOW PASS FILTERS TUTORIAL
    An excellent video of the BEST quality that i have come across in my life. I can never forget this video because of its outstanding features like CRT viewing, hearing music variation at different frquencies, Practical demonstration of components and calculation, ideal graphs and Practical one, such a clear explanation of theory, what to say and what not to say. Very well made video.
    thank u so MUCH
    s.vatsa India

  20. Most practical & efficient presentation of the subject. Provides conceptual background to dive into more mathematical treatment.

  21. Hi; like and watch your vids. Thank You.

    Q: why don't you use an inductor and a capacitor for another viddy, so that you can explain how a speaker system x-over works? Play with the values and the speakers used to show how they are formulated?

  22. Im a beginner: Why is the Subwoofer connected directly after the resistor? I thougt the filtering occurs due to the capacitor hence behind the capacitor is the filtered signal or am i missing something?

  23. 6:20 I had always thought guys like that (and it's always guys, isn't it?) were jackasses too, but now I wonder if they're scientists who take their lab everywhere.

  24. can you please do one for a high pass filter? specifically for audio frequencies like this one? need to make a dc blocking filter and Im having trouble on my choice of caps as the design of caps in the AF range has a big effect on the sound

  25. Hey dude,
    Great job in doing that video… Ive got one question:
    I wanna filter out the bass frequency of a audio signal and then higher its min. Voltage with a dc offset, to make its voltage completely positive to read it out with a arduino…
    But the problem is:
    How to do that? As soon as i connect the dc offset voltage source in series, it makes the low pass filter not work anymore….

  26. I can't get it to work at 100 hz with 10 milli farad capacitor either did the math wrong (because I am in grade 7 I know nothing about math) or it doesnt work with that high farad capacitor

  27. What if I use 100-ohm resistor with 10micro farad capacitor instead of 1K resistor and 1 micro Farad capacitor
    both give 150Hz frequency in low pass filter.
    which one to choose ?

  28. if my sub ends up rattling my door off the car i'm going to consider that a win. realistically though personally my goal is to get the loudest cleanest bass with little to no rattling.

  29. Again a resistor. I can't understand why there should be a resistor. Is it to tune the output voltage peak or ampere peak or both?

  30. How is input voltage taken into the equation? It seems that more or less peak voltage would effect this circuit changing the cutoff frequency.
    Also, how would I go about filtering a frequency based signal (50% duty) into a variable DC output?

  31. Howdy.
    One may cascade RC filters to get steeper attenuation. The trick is simply that each next branch should have an impedance of 10 times the previous. For most practical uses each branch may be calculated using the basic formula. The inter-loading will be small.
    So if we want a 40 dB attenuation per decade we use a 1 k resistor in the first branch and calculate the corresponding capacitor. In the next branch we use 10 k and a capacitor of 1/10 of the previous.
    However. The following amplfier input impedance needs to be 100 k or higher.
    If some inter-loading can be accepted a smaller factor may be used, say 5 times.

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