I need to build a low pass filter, to filter frequencies that are over 550Hz. By the cutoff frequency formula fc = 1/2piRC, RC has to be around 1/3500 so that fc is approximately 550Hz. I have a lot of resistors, from 220 ohms to 10k ohms, but my capacitors are ceramic ones that range from 2pF to 100nF. Even with the 100nF capacitor, my resistance would have to be huge in order for the calculations to match approximately 1/3500 (this sentence could be wrong, I'm new to electrical engineering). Is there a way to combine the resistors and capacitors that I have, in order to build a low pass filter that filters frequencies over 550Hz? Do I have to buy bigger capacitors?


2 Answers 2


550 Hz = about 3500 rad/s, with a period of 288 us.

With a 100 nF capacitor, you only need 2880 Ω to get that time constant.

10 kΩ and 27 nF gets you there as well.

  • \$\begingroup\$ OP ----How much power do you need to pass??? Consider that 10k will be in SERIES with whatever you are driving. If the input impedance of whatever that is isn't >100k or so, you're gonna have problems. If this is a small audio signal, it's "probably" OK. If you're driving a speaker, this 100% won't work. \$\endgroup\$
    – Kyle B
    Commented Jan 5, 2021 at 23:57

Adding capacitors in parallel adds their capacitance together: Cp=C1+C2.

Adding resistors in series adds their resistance together: Rs=R1+R2.

This is by far not the ideal way of doing things, but if you have very limited resources it is one way to do things. Keep in mind the ESR of the capacitance also lowers when adding them in parallel which may have unintended consequences for your application.


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