[sdiy] Power Supply Start-up Issues

ackolonges fds ackolonges at hotmail.com
Wed Aug 26 13:52:46 CEST 2020

I believe C7, C8, and C9 in the schematic are meant to represent capacitance on the modules to be powered - ie. not part of the power supply itself. In my own modules I generally just put capacitors to the rails from ground, so at the moment there isn't really a "C9", but since it's for a modular synth, it needs to be able to handle modules that do have it.

From: Liam Wall <liam.wall at gmail.com>
Sent: Wednesday, August 26, 2020 11:39 PM
To: ackolonges fds <ackolonges at hotmail.com>
Subject: Re: [sdiy] Power Supply Start-up Issues

>From the description there the diodes are not the problem. C9 is the problem, with the diodes being a (partial) fix.

Do you have an equivalent of C9 in your circuit?

On Wed, 26 Aug 2020, 12:26 ackolonges fds, <ackolonges at hotmail.com<mailto:ackolonges at hotmail.com>> wrote:
Hi All,

I've been having issues with a very standard +/- 12V power supply design for a modular synth. Every once in a while when powering up, the positive rail doesn't rise, and it needs to be reset to work. After some trial and error, and research, I found mention of this issue in Douglas Self's 'Small Signal Audio Design' book, where he shows a circuit almost identical to my power supply. A screen grab of the related page can be found here: https://pasteboard.co/Jo8PuIu.png

This paragraph from that page describes the issue I'm having perfectly :

"D2, D4 will prevent damage in this case, but leave the power supply vulnerable to start-up problems; if its output is being pulled down by the 15 V regulator, the 15 V regulator may refuse to start. This is actually a very dangerous situation, because it is quite easy to come up with a circuit where start-up will only fail one time in 20 or more, the incidence being apparently completely random, but presumably controlled by the exact point in the AC mains cycle where the supply is switched on, and other variables such as temperature, the residual charge left on the reservoir capacitors, and the phase of the moon. If even one start-up failure event is overlooked or dismissed as unimportant. then there is likely to be serious grief further down the line. Every power supply start-up failure must be taken seriously."

Unfortunately after stressing how important it is to fix such an issue, no solutions are mentioned...  I've tried removing all of the protection diodes, increasing and decreasing the value of the smoothing caps, tried many different makes of voltage regulators, and also tried an NTC thermistor to slow the in-rush current, all to no avail.

I'm starting to run out of ideas, so if anyone on the list has come across a fix it would be great to hear about it.

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