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PS Filter C in Series, w Parallel R

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.
 
W

Warren

Jan 1, 1970
0
cassiope expounded in @i6g2000vbe.googlegroups.com:
....

If you have loads that aren't constant, you won't be able to achieve
balance without a lot of power loss or more complexity.

They will be balanced mostly, since this is just powering a small 8W rms
class-B ss amp. But after reading your post and the next one, I think
what I was planning is just a bad idea.
Since you're
still in the design phase, why not use a voltage doubler topology:

---+---|>|-------+-----+-----> (+)
| + | |
| C1 ----- R1
| ----- |
| | |
-----------------+-----+----+
| + | | |
| ----- | Gnd
| C2 ----- R2
| | |
+---|<|-------+-----+-----> (-)
You can still use the bridge rectifier though you won't need
everything in it. You'll need roughly half the transformer voltage,
twice the current, and bigger capacitors, of course. But it's immune
to +/- load imbalance.

Diodes are no problem, but I think I will spend a little time seeing if I
can get a surplus split secondary transformer instead.

Some things are probably just not worth messing around with.

Warren
 
W

Warren

Jan 1, 1970
0
NT expounded in @q9g2000yqe.googlegroups.com:
...
I've used this approach before in an odd situation, and it does work,
but has its issues.

Tolerance is the biggest issue. If you're using +100%-50% caps, the 2
can have a capacitance ratio of 4:1, and voltage balance doesnt even
begin to happen. And as caps age, things only get worse. So you really
need to use zeners across the caps. That of course puts a pair of
zeners across the whole psu, so you check that with max possible line
voltage, worst case transformer regulation and no load, you dont end
up with both zeners conducting.

The next issue is that as the load i changes on one rail, it alters
the V in the _other_ capacitor too, since C charging conditions are
changed for the other cap. So you get noise/signal fed from one side
to the other. Fine in some circuits, not ok in some.

Check those points and its fine.
NT

You've talked me out of it.

While it will be just powering a small 8W rms audio amp (class-B), it's
not the kind of thing I want to come back to and fix later. I might as
well look for a surplus xfrmer with the split secondary and be done with
it, once and for all. I hate doing things twice.

Warren.
 
L

legg

Jan 1, 1970
0
cassiope expounded in @i6g2000vbe.googlegroups.com:


They will be balanced mostly, since this is just powering a small 8W rms
class-B ss amp. But after reading your post and the next one, I think
what I was planning is just a bad idea.



Diodes are no problem, but I think I will spend a little time seeing if I
can get a surplus split secondary transformer instead.

Some things are probably just not worth messing around with.

Warren

It's the asking of the question that makes the most sense.

Another alternative is to make a single supply rail, then capacitively
couple the load.

RL
 
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