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Coriolis
Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark
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Posted: Mon Aug 20, 2007 12:08 pm Post subject:
LM7912 acting funny Subject description: under what circumstances would it output more than 12V? |
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This is puzzling:
I put together a simple bipolar 12V psu, using LM7812 and 7912. Positive side (7812 - yes I did put the right one in) works just fine.
My CT transformer delivers 18Vac at the both inputs of the full-wave rectifier, and I'm getting 24Vdc at both outputs of it.
Pos reg outputs 12V - neg reg outputs 16V.
I've been over this thing (built on stripboard) 800 times now, cleaned the groove between traces, in case there were any shorts. Both sides measure exactly the same, but outputs differently. I even tried replacing the neg reg, but to no avail. It happily keeps on going - spitting out 16V.
Any thoughts?
C |
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Dan Lavin
Joined: Nov 09, 2006 Posts: 649 Location: Spring Lake, Mi, USA
Audio files: 21
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Posted: Mon Aug 20, 2007 12:20 pm Post subject:
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I'm assuming you've checked the 7912 pinout which is (gnd, in, out) not (in, gnd, out) like the 7812.
If that checks out, check your 7912's ground connection, it might be floating. Other than that, there's not a lot to go wrong with these. |
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Coriolis
Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark
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Posted: Mon Aug 20, 2007 12:34 pm Post subject:
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Yep, did check the pinout, along with the orientation of everything else.
It really should work, as far as I can see.
Could it possibly have any significance, that there is also a bipolar 15V psu connected to the same transformer? That one works fine though.
C |
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Dan Lavin
Joined: Nov 09, 2006 Posts: 649 Location: Spring Lake, Mi, USA
Audio files: 21
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Posted: Mon Aug 20, 2007 12:42 pm Post subject:
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No, I have a +/-5V, +/-12V and +/-15V 78xx/79xx-type power supplies all running in my rack. That shouldn't be a problem...they're essentually all running in parallel off of the main power supply caps on the back side of the diode bridge. That's not it.
I do remember once or twice when the voltage on a regulator 78xx/79xx running high when one of the pins was really weak from being bent back and forth a number of times....the connection I'm guessing wasn't really good. That's about all I can think of. |
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frijitz
Joined: May 04, 2007 Posts: 1734 Location: NM USA
Audio files: 54
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Posted: Mon Aug 20, 2007 12:51 pm Post subject:
Re: LM7912 acting funny Subject description: under what circumstances would it output more than 12V? |
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Coriolis wrote: | This is puzzling:
I put together a simple bipolar 12V psu, using LM7812 and 7912. Positive side (7812 - yes I did put the right one in) works just fine.
My CT transformer delivers 18Vac at the both inputs of the full-wave rectifier, and I'm getting 24Vdc at both outputs of it.
Pos reg outputs 12V - neg reg outputs 16V.
I've been over this thing (built on stripboard) 800 times now, cleaned the groove between traces, in case there were any shorts. Both sides measure exactly the same, but outputs differently. I even tried replacing the neg reg, but to no avail. It happily keeps on going - spitting out 16V.
Any thoughts?
C |
Some regulators require a load to operate properly. I never remember which ones. It should say in the data sheet, or just make sure you have a load of 5k or so.
Ian |
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Coriolis
Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark
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Posted: Mon Aug 20, 2007 1:06 pm Post subject:
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Thanks Ian, I might have to try that.
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Coriolis
Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark
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Posted: Mon Aug 20, 2007 1:13 pm Post subject:
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Hmm...looked in the datasheet, and it said that if using electros instead of tantalum for the stabilizing cap, it should be 10 times the value.
So 1uf tant = 10uf electro. Tried that - did nothing.
Still have to try that 5k load.
Thanks for the advice guys!
C |
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Randaleem
Joined: May 17, 2007 Posts: 456 Location: Northern CA, USA
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Posted: Mon Aug 20, 2007 1:15 pm Post subject:
Re: LM7912 acting funny Subject description: under what circumstances would it output more than 12V? |
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Coriolis,
The 7912 IS one of the ones which require a load (as Ian suggested).
But there may be another reason: I went through this recently with a gentleman in another thread here (MFOS power supply) and the only thing which worked was to change to a different mfr's 7912.
His bad ones were made by St Micro. They were putting out 21 volts. So if your's are ST also; you might want to change them for another brand.
Randal
Coriolis wrote: | Pos reg outputs 12V - neg reg outputs 16V.
I've been over this thing (built on stripboard) 800 times now, cleaned the groove between traces, in case there were any shorts. Both sides measure exactly the same, but outputs differently. I even tried replacing the neg reg, but to no avail. It happily keeps on going - spitting out 16V.
Any thoughts?
C |
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Coriolis
Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark
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Posted: Mon Aug 20, 2007 1:28 pm Post subject:
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Hi Randaleem!
Guess what: It IS from STMicro!
I was starting to feel REALLY bad about my troubleshooting skills, but I guess I have a couple of things to go by now.
Will try replacing with a different brand, but not tonight. Will post if that works. Other people might want to avoid these things, If the ones I got are bad (2 so far that output the wrong voltage).
Thanks for chiming in with that info!
C |
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Randaleem
Joined: May 17, 2007 Posts: 456 Location: Northern CA, USA
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Posted: Mon Aug 20, 2007 1:56 pm Post subject:
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Coriolis,
Glad to help. I was pulling my own hair out in that other thread in the MFOS section of EM. We tried everything!
But please DO try the dummy load Ian suggested.
Even though the previous problem was solved by using a different mfr's chip; I'd sure like to have the added data point of you trying it with the dummy load of a resistor ?
Randal |
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Coriolis
Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark
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Posted: Tue Aug 21, 2007 12:20 am Post subject:
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Yep, I will try that 5k load - I have to anyway, since the only other 7912's I have are from STMicro.
I hope to get back on this later today.
C |
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