[sdiy] Update! Re: 78xx regulator gives negative output half the time

[Tim Ressel]

I had a problem with 7812 regulators not putting out full voltage. With no load 
they were fine but under load at startup they only put out 1-2V. It turns out my 
input voltage to the regulator was too high per the spec sheet. A lower voltage 
transformer fixed it.

I hope that helps.

--TimR




----- Original Message ----
From: Karl Ekdahl <elektrodwarf at yahoo.se>
To: synth-diy at dropmix.xs4all.nl; Ian Fritz <ijfritz at comcast.net>
Sent: Fri, October 29, 2010 6:39:03 AM
Subject: [sdiy] Update! Re: 78xx regulator gives negative output half the time

Hi List

I have once again gotten this weird problem; unit does not start once plugged in 
but if you immediately unplug it and plug it back in it starts up. The cuplrit 
is again a 7809 regulator.

Now this time the back-to-back diode network did *not* work to my big horror. I 
do however think i've found the problem; seems like the Fairchild LM7809ACT does 
not work, though the LM7809CT does! The only difference i can find inbetween the 
two is that the latter has a wider temperature range and slightly less awesome 
line/load regulation.

So before i order 50 of the LM7809CT as replacements i've got three questions:

1. Does this seem right? Can anyone refute my findings?

2. Do you think this is a manufacturing error on the part / a bad bath or just 
different behavior?

3. If this is not a bad batch of parts, how the hell can one make sure that you 
don't get the sensitive / weird parts if (like now) the part that first worked 
isn't in stock? There are literally 100's of 78xx/79xx regulators and i guess 
some work and some don't. The first time i had this problem (Re: this e-mail) i 
was working with some 7812 regulators - perhaps there's a 7812 regulator that 
will work?

Oh btw. i'll get back to y'all on this once i know all the hard facts, maybe 
it's time to watch out for any ACT regulators...

Thanks!

Karl

--- Den tors 2010-04-29 skrev Ian Fritz <ijfritz at comcast.net>:

> Från: Ian Fritz <ijfritz at comcast.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: torsdag 29 april 2010 02:13
> At 05:42 PM 4/28/2010, Karl Ekdahl
> wrote:
> > 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.
> 
> Different manufacturers?  There's been trouble with
> this reported in the past.
> 
> Ian 
> 


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