[sdiy] Power Supply Start-up Issues

Liam Wall liam.wall at gmail.com
Wed Aug 26 14:18:55 CEST 2020

Sorry, yes, I was taking an expansive view of the circuit, I.e. including
whatever you have the supply currently plugged into.

But if you don't have (some equivilent of) C9 somewhere, I dont have any
other ideas I'm afraid.

On Wed, 26 Aug 2020, 12:52 ackolonges fds, <ackolonges at hotmail.com> wrote:

> 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> 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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