# Pi LC FILTER IN POWER SUPPLY

#### dnaeem

Joined Jul 16, 2007
1
hi,

I am designing a additional board for my circuit for this i am using a same power supply but problem is that i am having high frequency noises 400 or 500hz i need to design a pi lc filter in which i use 2 capacitor and 1 inductor could u please tell me what should be the cuttoff frequency and how to calculate values how i should i get this flat dc voltage i am confused.please help me as soon as possible

#### mrmeval

Joined Jun 30, 2006
833

#### oz7acs

Joined Jul 21, 2007
3
Hi Dnaeem,
The cut-off frequency for a simple LC lowpass filter (Lseries + Cparallel) can be calculated as: fcut = 1 / (2pi*sqrt (LC))
If you want to minimize 400 Hz noise (and maybe some residual 100Hz from a double bridge rectifier in the PSU) I suggest that you select a cut-off frequency of 25Hz. Since an LC filter attenuates asymptotic with 12dB/octave and 6dB at fcut, it means that this way 100Hz noise would be attenuated with at least 24dB and 400Hz noise with as much as 48dB!
Us f.x. a 10mH series coil and a 3900uF parallel capacitor...(fcut=25.5Hz)
Considering the strong attenuation of especially 400Hz noise from this filter you probably don't need a pi filter...