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

Dave Kendall davekendall at ntlworld.com
Fri Oct 29 15:58:10 CEST 2010

This rings a bell.

There was a bad batch of LM7812s a while back - I forget where this was 
discussed - either here, or on electromusic.com, or possibly on the 
9090 forum. I seem to remember they were outputting +21V.... not 
exactly what you want.....I can't remember whether national semi or 
fairchild parts were the culprits.


On Oct 29, 2010, at 14:39, Karl Ekdahl wrote:

> 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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