PSUD Capacitor Value and Ripple Tuning

[Hearinspace]

I have been noodling around in PSUD a little with a supply for a phono stage using parts I already have on hand. Using the default power transformer, bridge and then a couple of 45 Henry / 300 Ohm plate chokes for LC filtering I got it down to 2.7uV ripple after 5 sec. with C1=470uF, C2 = 600uF.

If I inch C1 either up or down in value (even 469 or 471uF), the ripple immediately starts to increase. The rise starts small of course but moves to millivolts fairly soon. It's interesting that it has this lowest point with the 470uF cap. Using other values (C1=150uF, C2=450uF) with a Full Wave rectifier nets a similar sort of thing.

Does this behaviour have something to do with resonant relationships in the power supply or is it simply a simulation software artifact?

Thanks !

[DSP_Geek]

Originally Posted by Hearinspace

If I inch C1 either up or down in value (even 469 or 471uF), the ripple immediately starts to increase. The rise starts small of course but moves to millivolts fairly soon. It's interesting that it has this lowest point with the 470uF cap. Using other values (C1=150uF, C2=450uF) with a Full Wave rectifier nets a similar sort of thing.

Does this behaviour have something to do with resonant relationships in the power supply or is it simply a simulation software artifact?

PSUD's been pretty solid for me, so it might be a resonant condition in the power supply - although the resonance of a 45H/470uF circuit is closer to 1 Hz than 120 Hz. Try making the capacitor series resistance 1 ohm instead of 20 milliohms to see whether the notch is broader and less deep.

[Hearinspace]

Thanks D. Here are some figures. You're right , the notch is shallower at 1 Ohm.

ESR = .02 Ohm / notch centre @ 470uF

460uF / 2.7947uV
465uF / 2.7513uV
470uF / 2.7111uV
475uF / 2.7952uV
480uF / 2.9468uV

Spread around centre = 0.1521uV
---------------------------------------

ESR = 1 Ohm / Notch centre @ 461uF

451uF / 3.1864uV
456uF / 3.1427uV
461uF / 3.1257uV
466uF / 3.1441uV
471uF / 3.2047uV

Spread around centre = 0.0183uV

[DSP_Geek]

Originally Posted by Hearinspace

Thanks D. Here are some figures. You're right , the notch is shallower at 1 Ohm.

I ran your circuit on PSUD2, and changing C1 up and down from 470 uF had the expected result: smaller meant more ripple, larger showed less. I did wait for 8 seconds to let it stabliize, however. Zoom in way tight on VR1 to look at the ripple, and you'll see the voltage is still rising from zero and wiggling around a bit from the startup transient at 5 seconds. On the one hand C1 will attenuate 120 Hz ripple the bigger it is, but on the other hand it will also slow down the DC rise time. That's likely why you had such a contradictory result.

[Hearinspace]

I actually went out as far as 37 seconds and found the wiggle out there too . . . . at just over 0.1 Hz.
Thanks again for motivating me to look !