W
Warren
- Jan 1, 1970
- 0
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.
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.