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Possible issue with L7812 voltage regulator

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Re: Possible issue with L7812 voltage regulator

PostPosted: Tue Oct 09, 2018 12:54 pm
by ef37a
Reading Self again he mentions +V rail startup problems. Likely due it seems when there is a capacitor "rail to rail" across the whole regulated supply.

Two diodes reverse biased across the output helps but they also protect the regulators from large downrange capacitances. Peanuts, fit them.

You can still have +V rail start up issues and it seems you need some asymmetry in the rectifier side circuitry. I would remove at least one of those 4,700s and have more in the + side than the -ve. Actually that sounds counter-intuitive! You would think you want to delay the -ve side more than +ve? Cut and try is all I can say!

This really needs sorting because one rail not coming up can burn out the very common NE5532 IC.

Dave.

Re: Possible issue with L7812 voltage regulator

PostPosted: Tue Oct 09, 2018 3:25 pm
by dwebb
Re your earlier questions the mains enters the case via a switched and fused IEC socket. I've put the transformer in a sealed box separate from the rest of the case.

To clarify your first suggestion about putting 'two diodes reverse biased across the output' - are those the diodes marked D3 and D4 in the schematic I posted above?

I'll also try removing one of the 4,700uF capacitors on the -ve side and replacing the L7812 in case that is faulty.

Thanks again for your help - much appreciated.

Re: Possible issue with L7812 voltage regulator

PostPosted: Tue Oct 09, 2018 4:32 pm
by ef37a
" Cased, switched, fused" and EARTHED I trust?

No, the diodes for start up protection need to go across C9,C10 reverse biased of course.

These four diodes are often omitted on self contained equipment because the capacitances are known and won't change (except some folks HAVE to "improve" things!"If 2,200 mfd is good, 10,000 MUST be better!" "Oh! Why does that primary fuse keep blowing?").

Dave.

Re: Possible issue with L7812 voltage regulator

PostPosted: Tue Oct 09, 2018 5:34 pm
by dwebb
Understood and thanks again.

PS Yes, earthed.

Re: Possible issue with L7812 voltage regulator

PostPosted: Wed Nov 07, 2018 11:59 am
by dwebb
... a quick addendum should it be of use to anyone. I replaced the L7812 and all has been working well for a couple of weeks. Thanks again to all who offered assistance.