[sdiy] Power Supply Start-up Issues

[ackolonges fds]

Thanks a lot for this in-depth reply - much appreciated!

In my testing case all the modules that are plugged into the supply are analogue ones with roughly balanced draw on both rails, and each module has 10uF decoupling on each rail. I'll order some 1N5819s, try them out, and see if I have any luck. Thanks for the tip!

________________________________
From: Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of Tony Allgood <oakleysound at gmail.com>
Sent: Thursday, August 27, 2020 3:29 AM
To: Synth DIY <synth-diy at synth-diy.org>
Subject: Re: [sdiy] Power Supply Start-up Issues

A common cause of start up problems is often too much capacitance on the
input of the modules which causes a significant power on surge coupled
with an imbalance between the positive and negative steady state loads.
By the latter I mean, there's 500mA on the +12V and 125mA on the -12V.
One rail powers up faster than the other and latch up occurs in the
other. I have had some success with using 1N5819 diodes in place of the
1N400X diodes that are reversed biased across the rails (D2 and D4 in
Doog's schematic), but it doesn't always work.

Another problem is caused by the power on surge coupled with an large
input voltage applied to the regulators. The current protection
mechanism of these three terminal regulators appears to be affected by
the difference between the Vin and Vout. The very large difference in
voltage, which occurs at start up since Vout is 0V, coupled with a large
start up current causes the foldback current limiting to kick in and
device latches up.

The cure for each is the same. Reduce start up current. And the easiest
way to do that is reduce the decoupling capacitance on modules. I've
seen modules with a couple of 220uF connected to their power headers.
Perhaps it's best not to use any more than 4u7 per rail per module,
unless series resistors are used.

Tony

www.oakleysound.com<http://www.oakleysound.com>
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