I am planning to build a small power supply, probably (for now) in the range of 18 to 32 VDC after filtering. But I'll be using a transformer secondary with no center tap and a bridge rectifier. So I was planning to use two filter caps in series, grounding the center point.
+)-------+-----+-----> (+) + | | C1 ----- R1 ----- | | | +-----+----+ + | | | ----- | Gnd C2 ----- R2 | |-)-------+-----+-----> (-)
This got me to thinking about using parallel resistors to keep the voltages across each cap "balanced". In a higher voltage PS, I'd also want to avoid exceeding the cap's rated max at any point during its operation (like at "turn on"). This last aspect is academic for the current project but I'd still like to know more about the design process.
How does one normally choose R?
At turn on, you have a high current (low impedance) and low voltage for both caps, so initially voltage imbalance shouldn't be an issue (assuming both are discharged initially).
At full (or nearly full) charge, each C has low/no current (high impedance) and high voltages.
Starting at some point in between (midpoint? 60%?) you want neither cap not to over charge past it's maximum voltage, while the "weaker" cap is catching up.
Given the tolerance for electrolytics is something like +/-
30%, I would guess you need to look at C1 (electrolytic) as being 30% over, and C2 as -30% in value. Then look at the voltage curves as they charge to see where the unbalanced risk is. Then compute a suitable R around that with some sort of a safety margin (10%?).I can do the above but I wonder if there is a simpler procedure to arrive at a suitable value - a method that can be used on the back of a envelope over lunch?
Warren.