Doesn't the linear regulator eliminate the voltage spikes caused by U1? Redwire said it did.
Not that I've seen, maybe reduced it a bit.
His version without Q1 suffers from a ramp in voltage at the output once power is cut until the filter caps drain, a runaway spike if you will. I removed the the paralleled 4K resistors on his board across C1 and put a resistor across each cap (3 4700's in parallel with a 15k each) and it reduced the bleed time but the spike remained higher than 30V no matter where the voltage pot was set.
With Q1, the spike is limited to the ranges I posted. My version without Q1 behaves in the same way as his version with Q1 with a subtle difference, my version holds the voltage at P1 until the caps drain, no runaway. With Q1, the caps drain quicker on his version. My version holds voltage for ~10 seconds or so. I haven't removed my boards to put a Q1 in yet but will soon. The way I've got mine hooked up makes it a pain to remove the boards once everything is hooked up, live and learn. I get no voltage spikes on power off, just a slow bleed. Power on gives me the same results I posted, identical in fact.
Does the linear regulator produce less noise than the zener diode/U1 circuit?
I never put the scope accross the output of U1 or the regulator, just the output of the supply. His version is much cleaner on the output than mine, even at very, very low V/div so I think it's probably an avenue worthy of exploration though I'll say it's having an odd effect on the results so far. People keep talking about inversion and that if a TL081 were there and Q1 were hooked to pin 3 it would induce that. I don't know, I get lost at that point.
When the mains power is turned off then the 47uF filter capacitor C3 for the negative supply takes time to discharge. The speed of any little NPN transistor is much faster.
Then the slow discharge time of C3 delays activation of Q1. Then the voltage spikes are not completely squashed.
When a powered inductor is suddenly unpowered then it produces a voltage spike.
Then does a powered transformer that is suddenly unpowered when its mains current is at a peak also produce a voltage spike?
I know that an inductor produces a voltage spike and that inrush current is also an issue with any type of inductor. Inductance is still kind of a foreign issue to me though. Bear with me, I'm learning. How long can an inductor hold the charge that it releases though? My thinking on this is that while an inductors spike can be an issue, what I'm seeing with voltage over time is related to the filter caps stored energy and U2 doing something I don't understand. As I said, my version isn't suffering this problem on power off. With Q1, this is limited in scope to a couple of volts depending on P1. Without Q1, at least on redwire's version, the spike rises to 36V regardless of where the voltage pot is set and then slowly drains. I'll try and get one of my boards modified with a Q1 within a day or two and post the results from that vs. the other board I have where I'll leave it off for now.