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Linear power supply advice
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RogerThat:
Really good information  :-+

I played with the calculator and was struck by the large amount of ripple. For efficiency, would it be more beneficial to use many small capacitors to lower the ESR or will this give some issues? I'm thinking something like 11x 1000uF alu polymer would be better than my electrolyts.

The ideal diod solution might be interesting as the improvement is quite large, I will look into it.


not1xor1:
I noticed Rod Elliot disagrees with what I've written and confirms what mariush wrote ???

I resurrected an old LT spice simulation and according to that (but the transformer model was quite rough) there is only about 10% increase in RMS current for an ten-fold increase in capacity...
so I was probably wrong  :scared:
mariush:

--- Quote from: RogerThat on January 24, 2020, 03:16:03 pm ---Really good information  :-+

I played with the calculator and was struck by the large amount of ripple. For efficiency, would it be more beneficial to use many small capacitors to lower the ESR or will this give some issues? I'm thinking something like 11x 1000uF alu polymer would be better than my electrolyts.


--- End quote ---

Modern electrolytic capacitors already have very low ESR, and very high lifetime.
For example, look at Panasonic FR datasheet: https://industrial.panasonic.com/cdbs/www-data/pdf/RDF0000/ABA0000C1259.pdf

A 3300uF 25v Panasonic FR capacitor has a 14 mOhm ESR value, 3.8A current ripple and it's rated for 10k hours

You're working with classic transformers and very low frequencies (60/120Hz) so polymer capacitors are overkill.
Paralleling so many capacitors is waste of money, you're better off with 2-3 electrolytic capacitors, or a single big one (but generally one goes for 2 or 3 capacitors with smaller volume.


--- Quote ---The ideal diode solution might be interesting as the improvement is quite large, I will look into it.

--- End quote ---

Yeah, but look at it like this ... pay 2-4$ for the ideal diode controller and 4 mosfets (in volume), or you could pay 50 cents more for a bigger transformer (ex instead of 9v 50VA, go with 12v AC 50VA or 75VA) and that may also allow you to use smaller bulk capacitors (and save money there as well)

You get a cheaper product but you're losing efficiency (and more heat produced maybe) and weight (because transformer would have more copper) and the transformer may be a bit bigger (which may affect the case you use for the psu box)
David Hess:
I would not recommend any power supply circuit which does not include short circuit current limiting.

As a practical matter, low noise designs will include a differential pair in the error amplifier and remote sense.  Something like the design shown below but with an LM338 replacing the LM109 is very high performance with low complexity.  The JFET can be replaced with a PNP transistor.  I made a tuned version with a 7805 and LT1007 that had a load regulation of better than 0.0001% which is beyond what common multimeters can measure.
RogerThat:
Thanks again for interesting reads.

I checked price for bigger transformer and price for copper is high: double the current capability and price is up roughly 30% (10€).

Correct me if I am wrong but isn't ESR close to 1 ohm in the 50hz frequency? The stated ESR is usually at 100khz and as such much lower, in the mohm range.
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