[sdiy] Power Supply Start-up Issues

Tony Allgood oakleysound at gmail.com
Wed Aug 26 17:29:34 CEST 2020

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.



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