# Low pass filter

Discussion in 'The Projects Forum' started by noingwhat, Sep 9, 2010.

1. ### noingwhat Thread Starter Member

Jul 23, 2010
61
0
I want to create 3 different low pass filters and I want to know how to calculate the values of the capacitor and resistor I need to use. I want to build a passive filter for each, and I want to make one 100Hz, one 500Hz, and one 1KHz. Here is the kind of circuit I would like to try to use:

What kind of formula can I use to calculate the capacitor and resistor's values?

2. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
201
http://tinyurl.com/37t2g5g

3. ### noingwhat Thread Starter Member

Jul 23, 2010
61
0
lol, that's kinda cool, but I had already tried searching for low pass filter calculator, and I didn't find anything useful. I'll try this though.

btw, what does the RC stand for?

4. ### noingwhat Thread Starter Member

Jul 23, 2010
61
0
Well now, that is embarrassing. Thank you very much Marshall, that helped quite a bit. Once last question, how do I know what kind of wattage I need for the resistor?

5. ### hgmjr Moderator

Jan 28, 2005
9,030
214
R is for resistor and C is for capacitor.

hgmjr

6. ### noingwhat Thread Starter Member

Jul 23, 2010
61
0
wow, i am such a noob... RC.. duh!

Thank you.

7. ### noingwhat Thread Starter Member

Jul 23, 2010
61
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One last question:

Where do I connect the ground to? From what I understand, the capacitor is supposed to act like a variable resistor based on the frequency, but if it is just attached to ground, it seems like it would be doing no good, as it is an open circuit, so is there something I am missing somewhere, or do I just need to connect the capacitor to ground and it should all be fine?

8. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,658
885
The capacitor acts like an open line for straight DC voltage levels. They pass AC voltage, and depending on the value of the cap will exhibit a higher or lower resistance to an AC voltage of any specific frequency.

The 'ground' for AC signals is more of a 'common' line for the components. In DC circuits ground is usually, but not always, the negative side of the voltage supply. Most AC circuits will have a DC component somewhere in amongst the various components, so ground in that case will be the power supply negative(not always though)

9. ### noingwhat Thread Starter Member

Jul 23, 2010
61
0
Ahhh... so it is setup how I originally thought, the positive passes through the resistor into the speaker, and the negative comes out, while the capacitor is a bridge between the two. Great, thank you very much everyone!

10. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
201
Like anything in Google (or any other search engine) it's a matter of figuring out the correct seach words. I've learned a lot of things I probably didn't need to know by using the wrong terms or spelling throughout the years and I mean some really bizarre stuff.

My rule of thumb is - unless I'm looking for something that's rather complex in nature - if I don't get quite a few decent "on topic" hits within the first page or two I change my search terms.

11. ### jhili New Member

Sep 19, 2010
2
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Dear all

I am doing an exercise but I am stuck on this, maybe someone can help

I have designed a constant K filter with a pi network where the load is 600ohms and the cut off freq is 15kHz, now i need some help to design the circuit response showing it as a Butterworth response.

Found already the L =12.7mH and the C = 17.68nF then need some help.

Thanks.

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12. ### Wendy Moderator

Mar 24, 2008
20,735
2,498
This is referred to as hijacking, and is strongly discouraged here at AAC. I noticed you started your own thread, which is the correct procedure. A thread, once started, belongs to the person who started it.

13. ### Wendy Moderator

Mar 24, 2008
20,735
2,498
I didn't see anyone else say it, though it may have been in the URL. Where Xc = R, that is the filter frequency.

14. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
201
Let us not forget, there are about 10 common lab projects going on right now and different people asking the exact same questions.

Too bad we can't conslidate them into "Professor Orr's class project of lifting a door latch" or something like that.

I think most of the info you seek is under electroic filter in wikipedia.

15. ### noingwhat Thread Starter Member

Jul 23, 2010
61
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I will admit that about 3 of those are mine, but I am trying to teach myself, knowing nothing when I had started, and I don't want to just build circuits, I want to understand them, that is why I go into so much detail, so far that some people may consider overkill, but I want to be sure I am clear on everything.