µA7812 latch-up

[Steffen]

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

 

[gyraf]

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.

 

[Michael Krusch]

I had the same problem in my pultec. I changed the 7818 to another one and everything was fine.

 

[Steffen]

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

 

[Steffen]

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

steff

 

[gyraf]

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.

 

[mcs]

Would a schottky diode work instead of the germanium transistor?

Best regards,

Mikkel C. Simonsen

 

[sismofyt]

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.

 

[Michael Krusch]

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.

 

[matthias]

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

 

[gyraf]

Yes, I have had many a latch-up problem with the TS- regulators. The MC78XX are much more forgiving.

Jakob E.

 

[Steffen]

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

steff

 

[Steffen]

forgot the protection diodes (1N4004)

steff

 

[gyraf]

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.

 

[Michael Krusch]

Changed to ST brand from Segor and everything is fine now.

 

[gyraf]

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.

 

[matthias]

what about the 79xx ??? same problems ??

 

[gyraf]

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.

 

[saint gillis]

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

I had this problem with a 7815 on a Hartke amp, adapting an LM317 instead fixed the issue...

 

[JohnRoberts]

Dead thread walking...  quiet for 15 years.

JR

PS: Thanks for using the search function, latch up is an old problem.