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Old 11th February 2013, 02:16 PM   #1
udailey is offline udailey  United States
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Default PSUD high ESR question

Please help me understand why adding high ESR to the 100uf cap gives such drastically different results.
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Old 11th February 2013, 02:20 PM   #2
udailey is offline udailey  United States
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maybe better pics
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Old 11th February 2013, 02:58 PM   #3
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What happens when you turn the series resistors into some small value instead of zero? Say 10 milliohms?
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Old 11th February 2013, 03:21 PM   #4
DF96 is offline DF96  England
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Drastically different results are to be expected as you are removing the reservoir cap; your results are not as drastically different as I would expect. With 100R I would expect to see a full-wave rectified sine, not a normal ripple waveform.

It may be that you are outside PSUD2's capability. It is not a general circuit simulator like Spice, so it may make assumptions in its models which you are violating with such a large ESR and zero ohm R in the CR sections.
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Old 11th February 2013, 03:25 PM   #5
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Quote:
Originally Posted by DF96 View Post
Drastically different results are to be expected as you are removing the reservoir cap; your results are not as drastically different as I would expect.
That's not my reading- the main reservoir caps (10mF) remain, the 100u cap is nearly nonfunctional. The voltage output when he goes to zero ohm series resistors shows zero ripple and only about 1 volt of output. The fact that he loses it in two stages suggests that the sim is being abused, thus my suggestion of putting in small but finite series resistances.
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Old 11th February 2013, 03:32 PM   #6
DF96 is offline DF96  England
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Sorry, my mistake. I thought the 10mF were 10nF - bypasses!

Yes, try small but non-zero series resistors - which reflect reality anyway unless this is a superconducting supply.
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Old 11th February 2013, 03:42 PM   #7
udailey is offline udailey  United States
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I have tried this "trick" in many circumstances and it always gives crazy results. Here are pics with .1R series resistors.
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File Type: jpg 2-Fullscreen capture 2112013 103834 AM.jpg (232.2 KB, 24 views)
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Old 11th February 2013, 03:45 PM   #8
udailey is offline udailey  United States
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Look. Even with just 1R ESR on 100uf its down a huge amount.
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Old 11th February 2013, 03:49 PM   #9
DF96 is offline DF96  England
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OK. It seems the PSUD2 cap model assumes that ESR is realistic. You could try emailing Duncan to discover what the limit is for this. Some supplies with big chokes might want to add a snubber to damp LF ringing but this won't be possible if PSUD2 can't handle it sensibly.
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Old 11th February 2013, 03:49 PM   #10
SY is offline SY  United States
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No, that actually looks OK- with 100m-ohm series resistances, you're getting the drop you'd expect with the 1R ESR. Perhaps 100R ESR is also out of bounds for the sim?
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