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

[Simon Brouwer]

Yes I have seen this before, and just as I had thought, your circuit uses
half wave rectification.

The problem is that when you switch it on while the input voltage is in
the negative half of the cycle, the 79xx is powered first, and through the
load may pull the positive rail to a negative voltage (the -1.2V you see),
from which the 78xx is not able to recover when it is powered on a moment
later.
As already proposed the solution is to simply mount a reversed diode at
the output of the 78xx to prevent it being pulled too low. I would use a
schottky diode such as 1N5817.

Best regards
Simon


Karl Ekdahl schreef:
> 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
>
>


-- 
Vriendelijke groet,

Simon Brouwer