01-27-2014, 03:43 PM
Morzh, what you are in effect describing is a resonant circuit. For the inductive current to equal the capacitive current and cancel, their reactances have to be equal but out of phase.
This is exactly the situation in a parallel resonant circuit where the L and C reactances cancel and the impedance at the resonant freq ideally approaches infinity. Since this parallel resonant circuit is in series with the DC supply, a trap is formed and the 120 Hz component is not passed.
It really works pretty well. The 118 power supply uses a separate choke and resonating cap to trap the ripple freq. I actually connected a scope to both sides to see the ripple reduction. On the input side there was about 60 V P-P of ripple while at the output it was reduced to a fraction of a volt. Changing the cap value affected the null, so with the exact specified value what was left was only 60 Hz and higher harmonics, the 120 Hz component was totally gone!
This is exactly the situation in a parallel resonant circuit where the L and C reactances cancel and the impedance at the resonant freq ideally approaches infinity. Since this parallel resonant circuit is in series with the DC supply, a trap is formed and the 120 Hz component is not passed.
It really works pretty well. The 118 power supply uses a separate choke and resonating cap to trap the ripple freq. I actually connected a scope to both sides to see the ripple reduction. On the input side there was about 60 V P-P of ripple while at the output it was reduced to a fraction of a volt. Changing the cap value affected the null, so with the exact specified value what was left was only 60 Hz and higher harmonics, the 120 Hz component was totally gone!
