µA7812 latch-up

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Steffen

Well-known member
Joined
Jun 3, 2004
Messages
399
Location
Germany
some of us had problems with 7812 latchup in the ssl clone (7812 output hangs @ -0.6V). are LM317 regs known to be safe?

steff
 
Hi Steff,

It was me who had the problem. It is mainly when double-single-rectifying a single AC voltage to acheive symmetric supplies that the problem occurs. It happens when the negative half-wave is coming first, charging the negative side first, dragging down the positive side to less than -0.6V before this gets started.

http://wwwd.national.com/national/PowerMB.nsf/88bc465ea04c97a58525620b0040cc4a/FB19F9D1B2E1D9B888256DE2003DB534?OpenDocument

For some unknown reason, this only happens with certain brands of 78xx's - and not with 79xx's.

I don't know if the 317 acts the same..

Jakob E.
 
yepp....I made a new ssl pcb and used double-single-rectifying wich is definately the reason for this bahavior. the funny thing is this only seem to happen with 12V regs since I use this PSU scheme a lot and never had that kind of trouble.....

so I´ll check out if lm317 will work...otherwise I´ll do some sort of neg rail soft start circuit....

steff
 
have to check this out: I´ll let ya know how it turned out....

steff



78xx.gif
 
No, those protection diodes won't do the trick, as their forward voltage is high enough that latch-up still occurs. I've been there.

You would need germaniums for this - power types. National recommends a 2N3612, 10A germanium power transistor (!) in their application note:

http://wwwd.national.com/national/PowerMB.nsf/(approved-attachments)/E335D52E00D4244D88256DFD00627DE5/$File/AN-103.pdf

..page 10..

Jakob E.
 
Would a schottky diode work instead of the germanium transistor?

Best regards,

Mikkel C. Simonsen
 
A 78XX is not just a 78XX, there can be quite a variation between different manufactures and batches! :evil:

LM3X7 seem to be much more constant quality, so do the LTs.
 
Just working on my SSL and the same problem occured.
On my 7815 is written:
TS7815
S
3A1 CZ

Maybe we can collect some informations about affected brands.
Monday I will get a different brand and test again.
 
hi,

last week I built the power supply for my calcrec eqs

everything was fine but I had only 10,6 V at the uotput...

I changed the 7815 from another brand and everything was working fine...


the bad 7815 is labled:


TS7815
S
3A1 CZ


no wondering anymore.... seems that "reichelt" has a bad series of 78xx


what about the 79xx are they ok ??


matthias
 
I´m using this little softstart circuit to get rid of that latchup thing....no problems since then...

steff

latchupfix.gif
 
The simple - but not as power efficient - way would be to charge the first reservoir electrolytic slower - through a resistor - ensuring that V- won't be charged much faster than V+.

When we had this discussion on the old forum, someone pointed to a commercial circuit (I think it was a Rane) that did this.

Jakob E.
 
It seems like we could conclude that the TS78xx series of regulators are marginally designed. Yes, they work fine under conservative voltages/load, but they are very easy to latch up.

Stay away from those.

Jakob E.
 
No, never.

I haven't got a clue why.

Maybe it's because the 79xx types are not so well protected as the 78xx's. You often see burnt-out 79xx's, rarely 78xx's

Jakob E.
 
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