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

[Karl Ekdahl]

Thanks for your answers

I am using 1000uF caps before my regulators, and the 10uF / 220pF caps in the schematics are actually placed alongside each submodule within the circuitry.

JH also suggested a diode to ground just after the regulator for prohibiting reverse-current and this seems indeed to work and the circuit no more latches up. 

However i'm still not entirely sure i understand why this is happening in the first place, JH suggested he's seen it while doing circuits with unevens loads - but i've built literally 100s of circuits using this very same PSU scheme - that definitely have uneven loads - and i've never had this happen before.

Karl

--- Den ons 2010-04-28 skrev Laurie Biddulph <elby_designs at ozemail.com.au>:

> Från: Laurie Biddulph <elby_designs at ozemail.com.au>
> 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 22:30
> 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
> > 
> > 
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> > 
> 
>