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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
LM7912 dying out in series in MFOS Wall Wart PSU
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PERFORMANCE



Joined: Jun 11, 2020
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Location: Germany

PostPosted: Sun Feb 13, 2022 7:33 pm    Post subject: LM7912 dying out in series in MFOS Wall Wart PSU Reply with quote  Mark this post and the followings unread

Hey!

A couple of months ago I built a MFOS Wallwart PSU from scratch with each a LM7812 and LM7912 for testing purposes for eurorack modules (and to finally take that step empowering me to build everything from scratch - even the nasty mains stuff).

Getting it working took a little time - turned out the LM7912 was faulty. After that it ran just fine for a couple of months, no ripple, precise -12V/+12V and nothing smokey or sketchy.

Of course I tested a couple of faulty modules and unfinished stuff with it but in general it was very forgiving and never blew or let anything blow.

Last week I took another turn in my ever-failing MS20 filter project (which finally didn't fail after some serious troubleshooting,yay). When I plugged the module into the MFOS bench-supply for testing after most of the troubleshooting was done the LEDs (in place of average diodes in the module) behaved oddly so I measured the voltages and suddenly read -15V instead of -12V as before on the negative rail. Because I didn't want to risk anything I put away the PSU for good to eventually check what's wrong with it later.

Finished the filter, installed it into my actual Eurorack where it worked fine and received the right voltages.

Now today I finally found time and patience to have a look at the PSU again and since I suspected the regulator to be faulty in the first place, after inspecting the board (which was soldered together awfully, cosmetically speaking) I replaced both of them and - not so much to my surprise - now the voltages where right again (although the -V was slightly more off then with the still working almost perfectly precise previous regulator).

Now I suspect either by design of the PSU or by whatever reasons LM7912s tend to fail.

Is there anything I can do about that? I don't want to change the LM7912 every three months and I've already read that might be an issue with a specific brand but maybe there's anything else I can do about it?

Thanks a lot in advance!
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AlanP



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PostPosted: Mon Feb 14, 2022 9:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

I've got a few wallwart style PSUs, and the only problems I've had is when I've forgotten to add heatsinks.

If the soldering is cosmetically a bit dodgy, that might be my first suspicion.
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PostPosted: Tue Feb 15, 2022 5:28 am    Post subject: Reply with quote  Mark this post and the followings unread

AlanP wrote:
I've got a few wallwart style PSUs, and the only problems I've had is when I've forgotten to add heatsinks.

If the soldering is cosmetically a bit dodgy, that might be my first suspicion.


Mmmh, heatsinks are attached and seem to work as well and I "reflowed" the bits looking way too dodgy but intended to do a proper new version as well.

Thanks!
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PHOBoS



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PostPosted: Tue Feb 15, 2022 8:27 am    Post subject: Reply with quote  Mark this post and the followings unread

did you add the reverse voltage protection diodes on the outputs ?

I have one of those wallwart supplies that I use most of the time for testing circuits and I've probably been using for 10 years or so.
It doesn't even have any heatsinks on it and has been shorted a couple of times but it's still working fine.

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PERFORMANCE



Joined: Jun 11, 2020
Posts: 36
Location: Germany

PostPosted: Tue Feb 15, 2022 7:48 pm    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
did you add the reverse voltage protection diodes on the outputs ?

I have one of those wallwart supplies that I use most of the time for testing circuits and I've probably been using for 10 years or so.
It doesn't even have any heatsinks on it and has been shorted a couple of times but it's still working fine.


Ah, I don't think so (unless they're in the original design), will check that out next time i'm in the studio - good idea, also for modules (didn't have any problems with shorts anyway).

Thanks!
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PHOBoS



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PostPosted: Wed Feb 16, 2022 8:41 am    Post subject: Reply with quote  Mark this post and the followings unread

Those diodes are present in the orginal (MFOS) circuit. I actually just had a look at my PSU and I don't even have them on there. Embarassed
I knew I had some extra diodes but those are the ones connected between the in/output of the regulators which also add some protection.

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Psynth



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PostPosted: Sun Feb 20, 2022 3:53 pm    Post subject: Reply with quote  Mark this post and the followings unread

I've used the MFOS power supply for several synth cabinets for years, never had a problem. Though I have used LM317/337 adjustable regulators rather than the fixed voltage ones.

When I have used the 7912, I find it needs a minimum output current to regulate properly - I always include an LED on that side, to draw about 10mA. Without that it does tend to output about -15V or so.

Not sure how useful this is to you.
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