Correct implementation of R-C-R-C filtering in PSU - diyAudio
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Old 20th December 2012, 02:00 AM   #1
Gopher is offline Gopher  United Kingdom
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Default Correct implementation of R-C-R-C filtering in PSU

Of the many examples of R-C-R-C filtered PSUs I've looked at, there are 2 variants:

(1) Bridge - C1 - R - C2 - R - C3

(2) Bridge - R - C1 - R - C2

Q1: Is there any advantage in introducing an R between the bridge and the first C, as in (2), or is it better to have a C directly after the bridge, as in (1), noting that (1) does have a higher total capacitance than (2)?

Q2: What are the suggestions for the optimum scaling of C1 and C2/C3? I've seen some examples of (1) where the value of C1 is half that of C2/C3 to reduce charging currents, but others where C1 is much higher than C2/C3 to reduce ripple. Does that logic still apply in (2) with a resistor before C1?

Q3: In most examples of the genre, the Rs are only found in the positive and negative supply lines. However, some examples have matching Rs before and between the capacitors in both the supply lines and the 0V lines. I always understood the 0V lines should be kept as low an impedance as possible, so why are the Rs there in those few cases?

I should add that I'm using a transformer with two separate secondary windings, not centre-tapped.

Last edited by Gopher; 20th December 2012 at 02:40 AM.
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Old 20th December 2012, 02:18 AM   #2
Lo_Tse is offline Lo_Tse  Canada
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Quote:
Originally Posted by Gopher View Post
Of the many examples of R-C-R-C filtered PSUs I've looked at, there are 2 variants:

(1) Bridge - C1 - R - C2 - R - C3

(2) Bridge - R - C1 - R - C2

Q1: Is there any advantage in introducing an R between the bridge and the first C, as in (2), or is it better to have a C directly after the bridge, as in (1), noting that (1) does have a higher total capacitance than (2)?

Q2: What are the suggestions for the optimum scaling of C1 and C2/C3? I've seen some examples of (1) where the value of C1 is half that of C2/C3 to reduce charging currents, but does that logic still apply in (2) with a resistor before C1?

Q3: In most examples of the genre, the Rs are only found in the positive and negative supply lines. However, some examples have matching Rs before and between the capacitors in both the supply lines and the 0V lines. I always understood the 0V lines should be kept as low an impedance as possible, so why are the Rs there in those few cases?

I should add that I'm using a transformer with two separate secondary windings, not centre-tapped.
The case (1) that you show is a very common CRCRC (pi) filter and is well understood.

Case (2) looks curious - putting a resistor before C1 will drop the maximum voltage that C1 can be charged upto (how much depending on the value of the R), why? One can always choose a different transformer if the secondary voltage is too high? Wait a minute, is the R before C1 in series or in parallel?

Regards,

Lo_Tse
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Old 20th December 2012, 02:22 AM   #3
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Gopher,
I am wondering in the second example if that resistance is not just the secondary winding resistance that is being shown? Could this just be an example of someone trying to show all the parasitic resistance values in the circuit? Are they showing the resistance values or is it just a generic schematic?

Steven
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Old 20th December 2012, 02:37 AM   #4
Gopher is offline Gopher  United Kingdom
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Steven, Lo-Tse

To answer your two responses together, the R between the bridge and first capacitor is a real resistor and I think the main argument for including it is to reduce charging currents into C1 for less radiated RFI.
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Old 20th December 2012, 04:15 AM   #5
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Secondary resistance can be significant especially HV coils. Even in low voltage coils they might be significant like say 0.2 ohms or more which you see in series in some psu circuits.
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Old 20th December 2012, 04:30 AM   #6
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Old 20th December 2012, 05:51 AM   #7
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Hi Gopher, I used R's in the 0V lines on my regulated supply. I had not seen it done before, but it simmed well, and it made a difference when built as well. It pretty much halved the ripple magnitude for a given load.

I can't remember if it made a difference to the final voltage or not. Possibly the same ripple reduction would be obtained by only using resistors (of double the size) in the + - lines.

OK Just simmed that, overall ripple magnitude is identical with either 3r3 in both + - and returns, or double that at 6r6 in just the +- The actual simmed result is identical picture attached.

points monitored are after CRCRC wich is 4700u - 3r3 - 4700u - 3r3 - 1000u

one has 3r3 in both positive and zero return and the other has 6r6 in only the +ve

load was ~250mA after the reg circuit. measurements taken on input to reg circuit. Input voltage approx 18V

So if you are concerned about resistance in the return lines, the exact same ripple reduction should be possible by doubling the resistance in the +ve and -ve lines and leaving the return lines resistance free

Whether there are any differences that will show up in real life as opposed to a sim I don't know, as it wasn't something I tested with my scope when I was doing the real world tests.

Tony.
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Old 20th December 2012, 07:14 AM   #8
Gopher is offline Gopher  United Kingdom
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Tony

It was your YARPS PSU I was thinking of actually, but I didn't want to name names.

Something about putting resistance in the 0V return lines just goes against everything I've read and seen previously so guess I'll stick with double the value in the supply lines instead.

Thanks for the input.
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Old 20th December 2012, 08:21 AM   #9
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Hi Gopher, no probs!! I was being unconventional in most respects with that PSU The idea of putting the resistors in the returns was that it would add further decoupling (just a thought and not based on any solid engineering). I'm not sure that it does and I would like to have tested it as I just simmed above when I was doing all of the testing, unfortunately I just didn't think about it!

My electronics knowledge is enough to be dangerous I wasn't sure whether the transition from simulation to real world was going to work, but it did, so I was happy

There may well be a very good reason not to put the resistors in the return lines. It is certainly something I've not seen in any other PS. Will cop the flack if it was a dumb idea!!

For me it just seemed the logical thing to do, especially since the YARPS uses a positive reg for both the positive and negative rails. It would have been rather more unsymetrical if I'd left out the resistors in the return lines and the point of using the two positive regs was to try and make the -ve and +ve lines as close to the same as possible

Of course whether it makes any difference at all I can't say, but it was an interesting excercise!! Just a shame I still haven't built (except a breadboard prototype) the crossover it is supposed to be powering

Tony.
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Old 20th December 2012, 10:38 AM   #10
DF96 is offline DF96  England
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Putting resistors in the 'ground' line could have the advantage of forcing the designer to think about correct PSU grounding, which many people get wrong. If you can pretend that every ground connection is a resistance (which it actually is!) then you can design good grounding. Then you don't need to actually include the ground resistors.
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