![]() ![]() ![]() The short circuit current becomes a very high voltage during the 1/2 the cycle (and high current during the other 1/2). When I built some really high power output filters for a 180kW AC power supply I realized that at resonance, the filter was shorting the power supply (since Xl and Xc cancelled and all that was left was the tiny R of the inductor). You put 1V in and get 100V out (from a circuit with a Q of 100). In school I thought that LC circuits had some mysterious physics behind them that I couldn't understand. Sure looks like a 2-pole lowpass filter to me.ĭale- I had an epiphany about resonance after many years. Where does the 25mH (or other value) inductor come from? take apart an old power transformer from an old TV (not LCD TV) and start winding turns on it until you get the L you want.īandpass? Huh? Dale's figure is flat below the knee and drops off at -40dB/decade above it. How about something in the middle? How about 39mH and 100uF? or 50 mH and 80uF? or 25mH and 160uF? ![]() Let's pick the L as 396uH instead (1/10000 the size of the the 3.96H we first calculated), now the C is 10000uF. The answer to the second question is: (2*pi*80Hz)squared = 1/LC or 252,000 = 1/LC in this example. I answered the question - "What frequency should I choose for a resonant frequency for the filter?" If the question was: "I need to build an LC filter with a resonant frequency of 80 Hz for example. I may have not answered your question properly. You can make the output stable, but your waveform won't be something you want to show your kids. You can make the waveform cleaner with a lower frequency filter but it will make your output unstable. This is a clear case of what one of my friends calls the "Conservation of Misery Principle". Now here's the really nasty part of your LC filter: It's so close to the fundamental frequency (less than an octave), that it will be boosting the fundamental (peaking) instead of having a flat characteristics. A double-pole filter like the LC reduces the amplitude by a factor of 4 for every octave, so your filter has to have a corner frequency of slightly less than 1.5*fundamental frequency. To reduce the third harmonic to 3% of the fundamental (for a THD of something like 5%, which is about as ugly a waveform as most sensitive circuits can stand) you need a filter that reduces the third harmonic by a factor of 5 (3%/15%). In the Crowley & Lueng paper hypertexted above, figure 9 shows a third harmonic equal to about 15% of the fundamental (160V for the fundamental, 25V for the third harmonic). People normally only look at the first one-to-three large, low order harmonics since usually the high order harmonics are smaller amplitude and also easier to filter. The purpose of the filter is to reduce the amplitude of the harmonics to a specified maximum level. However, for a first pass, I found an example where someone calculated the spectra for a 100% modulation sinewave. This is a very complex task to provide a complete and accurate simulation for various modulation indices, switching frequencies, and load impedances. Useful when you look at the data sheets for SAW filters that don’t provide this info up front.OK, here goes. Filter Bandwidth – compute the bandwidth and center frequency from the start and stop frequencies.Power supply pi filters – for both equal and unequal impedance.Pi Filter – includes four different calculators for both low pass and hi pass filters.CLC Filter – provides component values for a desired frequency response.A first step in understanding the design intent. Then either of the two calculators can be used to determine the cutoff frequency. In this case, the configuration can easily be understood to be either Low or High pass depending on the inductor and capacitor configuration. These filters are commonly used around power supplies, Phase-locked Loops (PLLs), etc. Many times analog designers have to understand a schematic with pi filters. Typically the attenuation for a well-designed filter is less than 2 dB. Frequencies lower than this are attenuated by a smaller amount. In this case if the cutoff frequency is 100 kHz for example, then this is the point where the signal power is attenuated by 3 dB. ![]()
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