electro-music.com   Dedicated to experimental electro-acoustic
and electronic music
 
    Front Page  |  Radio
 |  Media  |  Forum  |  Wiki  |  Links
Forum with support of Syndicator RSS
 FAQFAQ   CalendarCalendar   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   LinksLinks
 RegisterRegister   ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in  Chat RoomChat Room 
 Forum index » DIY Hardware and Software
LM7912 acting funny
Post new topic   Reply to topic Moderators: jksuperstar, Scott Stites, Uncle Krunkus
Page 1 of 1 [11 Posts]
View unread posts
View new posts in the last week
Mark the topic unread :: View previous topic :: View next topic
Author Message
Coriolis



Joined: Apr 11, 2005
Posts: 616
Location: Stilling, Denmark

PostPosted: Mon Aug 20, 2007 12:08 pm    Post subject: LM7912 acting funny
Subject description: under what circumstances would it output more than 12V?
Reply with quote  Mark this post and the followings unread

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
Back to top
View user's profile Send private message Visit poster's website
Dan Lavin



Joined: Nov 09, 2006
Posts: 649
Location: Spring Lake, Mi, USA
Audio files: 21

PostPosted: Mon Aug 20, 2007 12:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

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.
Back to top
View user's profile Send private message
Coriolis



Joined: Apr 11, 2005
Posts: 616
Location: Stilling, Denmark

PostPosted: Mon Aug 20, 2007 12:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

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
Back to top
View user's profile Send private message Visit poster's website
Dan Lavin



Joined: Nov 09, 2006
Posts: 649
Location: Spring Lake, Mi, USA
Audio files: 21

PostPosted: Mon Aug 20, 2007 12:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

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.
Back to top
View user's profile Send private message
frijitz



Joined: May 04, 2007
Posts: 1734
Location: NM USA
Audio files: 54

PostPosted: Mon Aug 20, 2007 12:51 pm    Post subject: Re: LM7912 acting funny
Subject description: under what circumstances would it output more than 12V?
Reply with quote  Mark this post and the followings unread

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
Back to top
View user's profile Send private message
Coriolis



Joined: Apr 11, 2005
Posts: 616
Location: Stilling, Denmark

PostPosted: Mon Aug 20, 2007 1:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Ian, I might have to try that.

C
Back to top
View user's profile Send private message Visit poster's website
Coriolis



Joined: Apr 11, 2005
Posts: 616
Location: Stilling, Denmark

PostPosted: Mon Aug 20, 2007 1:13 pm    Post subject: Reply with quote  Mark this post and the followings unread

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
Back to top
View user's profile Send private message Visit poster's website
Randaleem



Joined: May 17, 2007
Posts: 456
Location: Northern CA, USA

PostPosted: Mon Aug 20, 2007 1:15 pm    Post subject: Re: LM7912 acting funny
Subject description: under what circumstances would it output more than 12V?
Reply with quote  Mark this post and the followings unread

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
Back to top
View user's profile Send private message Send e-mail
Coriolis



Joined: Apr 11, 2005
Posts: 616
Location: Stilling, Denmark

PostPosted: Mon Aug 20, 2007 1:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

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
Back to top
View user's profile Send private message Visit poster's website
Randaleem



Joined: May 17, 2007
Posts: 456
Location: Northern CA, USA

PostPosted: Mon Aug 20, 2007 1:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

Coriolis,

Glad to help. I was pulling my own hair out in that other thread in the MFOS section of EM. We tried everything! Very Happy

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
Back to top
View user's profile Send private message Send e-mail
Coriolis



Joined: Apr 11, 2005
Posts: 616
Location: Stilling, Denmark

PostPosted: Tue Aug 21, 2007 12:20 am    Post subject: Reply with quote  Mark this post and the followings unread

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
Back to top
View user's profile Send private message Visit poster's website
Display posts from previous:   
Post new topic   Reply to topic Moderators: jksuperstar, Scott Stites, Uncle Krunkus
Page 1 of 1 [11 Posts]
View unread posts
View new posts in the last week
Mark the topic unread :: View previous topic :: View next topic
 Forum index » DIY Hardware and Software
Jump to:  

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum


Forum with support of Syndicator RSS
Powered by phpBB © 2001, 2005 phpBB Group
Copyright © 2003 through 2009 by electro-music.com - Conditions Of Use