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In Reply to: RE: "Each cap only sees a 60 Hz charging current." posted by FenderLover on October 15, 2016 at 18:21:55
Well, it's shown in the SPICE analysis. The blue line indicates the presence of 120 Hz ripple frequency at the top of C2 in the schematic. This is the output of the power supply. What this seems to imply regarding the C1/C2 string - that the caps are filtering 120 Hz and conducting AC currents at that frequency - isn't true. The connection between the two diodes and the caps is such that each cap individually sees 60 Hz, not 120 Hz. Here's how it works:
When the top of secondary L2 goes positive, D1 turns off and D2 turns on. Conduction through D2 charges C2 for a half cycle. When the transformer reverses, D1 turns on and D2 turns off. Now C2 is no longer being charged. Instead, current through D1 and the transformer winding charges C1. This means C1 and C2 each receive charging current over half the 60 Hz waveform. At the top of the stack, one pulse of the 120 Hz wave is created when C2 gets its charge. The next pulse occurs because the charge on the anode of C1 that occurs a half-cycle later causes a voltage rise that's applied to the cathode of C2. That rise causes the anode of C2 to also rise, due to the low impedance of C2 at 60 Hz. So, even though each cap is charging at a 60 Hz rate, the charge currents occur on each cap on alternate half-cycles, and they add in the series stack to create the 120 Hz wave at the top.
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Follow Ups:
Got it, thanks. So, the residual 60Hz you are seeing is due to the frequency each cap is charging?
I'm not sure what you mean by "residual." Ripple output from the supply is 120 Hz. Yes, it's created by the caps being charged at 60 Hz on alternating half-cycles.
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Buy Chinese. Bury freedom.
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