[sdiy] 78xx regulator gives negative output half the time

[Laurie Biddulph]

I would have thought that the output capacitors here were excessive.
You need a large capacitor (>1000uF) along with a small one (~= 100nF) on 
the input and a smaller capacitor (~=10uF) and a small one (~=100nF) and 
these should all be mounted close to the regulator. Optionally you can put a 
diode from the output to GND but should always have the diode from output to 
input.

Each module should then, ideally, have a smallish capacitor (say around 
10uF) and either a single small one (100nF) at the power rail input or 
several spread around the board (these can be slightly smaller)

Regards
Laurie Biddulph

Mobile: 0400 257 645
Elby Designs
www.elby-designs.com
ABN: 70 022 727 605

--------------------------------------------------
From: "Karl Ekdahl" <elektrodwarf at yahoo.se>
Sent: Wednesday, April 28, 2010 10:44 PM
To: <synth-diy at dropmix.xs4all.nl>
Subject: [sdiy] 78xx regulator gives negative output half the time

> Hi list
>
> Soooo i stumbled upon something i've never seen before; a 7812 regulator 
> i'm using is *sometimes* latching up at ~-1.2V instead of its supposed 
> ~12V output.
>
> When i first discovered this, i could overcome it by unplugging the AC to 
> my rectifier and immediately plugging it back in - seemingly it'd un-latch 
> if there was some positive voltage stored in the rectifier caps.
>
> Another thing i noticed is that if i disconnected the over-voltage 
> protection diodes (going from the output to the input) the circuit would 
> not unlatch whatever i did.
>
> I've slowly desoldered piece by piece from my circuit board to try and 
> find the problem but to no avail. At the current state, without any ICs in 
> the circuit wse., the psu works fine 90% of the time and latches up the 
> restoring 10%.
>
> I'm using the most common PSU scheme ever that i've used many times before 
> (www.sdiy.org/knas/temp/psu.jpg) and i have no clue what's going on.
>
> Has anyone seen this before? Any common causes for this problem?
>
> Thanks
>
> Karl
>
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>