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

[JH.]

I just can say I ran into the same thing myself.
Never had problems with LM317/LM337 supplies, but for once had to switch to 78xx/79xx, and got them latched occasionally. Diode 
cured it.
I think the problem occurs if you have assymetric loads on + and - supply, and if you once run into a condition with reverse 
polarity during power-up (because of charges left in some caps from previous powering), it latches. The diodes will bring you over 
that short moment without latching. Or some such.

JH.


----- Original Message ----- 
From: "Karl Ekdahl" <elektrodwarf at yahoo.se>
To: "JH." <jhaible at debitel.net>
Sent: Wednesday, April 28, 2010 3:14 PM
Subject: Re: [sdiy] 78xx regulator gives negative output half the time


Hi Juergen

While i see how this would protect the output (and so far seems to work - thanks!) i don't see how this issue would come up in the 
first place? I'm not having an excessive current-draw or anything...

Karl

--- Den ons 2010-04-28 skrev JH. <jhaible at debitel.net>:

> Från: JH. <jhaible at debitel.net>
> Ämne: Re: [sdiy] 78xx regulator gives negative output half the time
> Till: "Karl Ekdahl" <elektrodwarf at yahoo.se>, synth-diy at dropmix.xs4all.nl
> Datum: onsdag 28 april 2010 14:54
> You need diodes that protect the
> output from reverse polarity.
> (Kathode at 7812 output, Anode at GND) and (Anode at 7912
> output, Kathode at GND).
>
> JH.
>
>
> ----- Original Message ----- 
> From: "Karl Ekdahl" <elektrodwarf at yahoo.se>
> To: <synth-diy at dropmix.xs4all.nl>
> Sent: Wednesday, April 28, 2010 2:44 PM
> 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
>
>
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