Strange Volt regulator!?! - diyAudio
Go Back   Home > Forums > General Interest > Everything Else

Everything Else Anything related to audio / video / electronics etc) BUT remember- we have many new forums where your thread may now fit! .... Parts, Equipment & Tools, Construction Tips, Software Tools......

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 3rd November 2006, 04:42 PM   #1
Wynand is offline Wynand  South Africa
diyAudio Member
 
Wynand's Avatar
 
Join Date: Mar 2005
Location: Cape Town
Default Strange Volt regulator!?!

I've read this before, but only saw it now...

I'm building a PSU for my Preamp, dual 5v and 9v

Starting it up I noticed that the -5 regulator reads -9v.

I checked for shorts and something melting/burning, nothing.

Then I put a 10K load on it. ~-7V.
With a 1K load on it it gives -5.03V

All the other regs read perfect with no load.

This regulator is the only one from ST, the other is fairchild and ON semi.

Just thought I'd share.
  Reply With Quote
Old 3rd November 2006, 05:02 PM   #2
lineup is offline lineup  Sweden
diyAudio Member
 
lineup's Avatar
 
Join Date: Dec 2005
Location: the north
Default Re: Strange Volt regulator!?!

Quote:
Originally posted by Wynand
I've read this before, but only saw it now...

I'm building a PSU for my Preamp, dual 5v and 9v

Starting it up I noticed that the -5 regulator reads -9v.

I checked for shorts and something melting/burning, nothing.

Then I put a 10K load on it. ~-7V.
With a 1K load on it it gives -5.03V

All the other regs read perfect with no load.

This regulator is the only one from ST, the other is fairchild and ON semi.

Just thought I'd share.
Checkout the pins of the negative regulator.
If it is the 3 pin type of 7905 alike,
the negative regulator PIN configuration is often DIFFERENT from positive regulator IC.

At least this is the case of the adjustable LM317, positive and LM337, negative regulators.
I think it is same issue, regardless if it is TO92 or TO220 case.

Datasheets:
http://www.fairchildsemi.com/ds/LM/LM7905.pdf
http://www.fairchildsemi.com/ds/LM/LM7805.pdf

====================================



edit:
The correct solution is in post below.
These 3 pin regulators will need normally
a minimum load of like 5 mA.
If your negative part draws too little current
you need to put in a load resistor,
to reach a total load of those ~ 5 mA.

There are some IC regulators, for example bigger TO3,
that even recommends a minimum load of 10 mA.

lineup
__________________
lineup
  Reply With Quote
Old 3rd November 2006, 05:05 PM   #3
Did it Himself
diyAudio Member
 
richie00boy's Avatar
 
Join Date: Nov 2003
Location: Gloucestershire, England, UK
The funny voltage thing is a common problem with 78/79 series regulators. As you have found they need a minimum load.
__________________
www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more.
  Reply With Quote
Old 3rd November 2006, 05:09 PM   #4
Wynand is offline Wynand  South Africa
diyAudio Member
 
Wynand's Avatar
 
Join Date: Mar 2005
Location: Cape Town
The positive chips voltages are ok, the -9v regulator is fine, it's just the -5v regulator that read -9v.

The negative regulators share the input and gnd connections. I've checked for shorts and there's none. With the reisistor on From the output of the -5v reg to GND the reading is fine.

That means the regulator will conduc fine with a load of 5V/1000R=0.005A

This wont be an issue because I'll have a LCD, MCU and two LM1973 volume control Ic's running off the regulator, I'll even get another -5v regulator if it runs too hot. I'll probable change the regulator though for another brand if it bothers me further.
  Reply With Quote
Old 3rd November 2006, 05:10 PM   #5
Wynand is offline Wynand  South Africa
diyAudio Member
 
Wynand's Avatar
 
Join Date: Mar 2005
Location: Cape Town
That's whats strange richie00boy. The rest of the regs all work fine. Even the 317/3317 combo in my other PSU.

This is my first one that doesn't read right without a load applied.

But your right, I've read this problem before on diyaudio.
  Reply With Quote
Old 3rd November 2006, 05:12 PM   #6
Wynand is offline Wynand  South Africa
diyAudio Member
 
Wynand's Avatar
 
Join Date: Mar 2005
Location: Cape Town
P.S I did check the pinouts, because in my first PSU I assumed that the pins was the same, needless to say, I've learnt my lesson back then.

Lots of other lessons to be learned.
  Reply With Quote
Old 3rd November 2006, 05:19 PM   #7
lineup is offline lineup  Sweden
diyAudio Member
 
lineup's Avatar
 
Join Date: Dec 2005
Location: the north
Quote:
Originally posted by richie00boy
The funny voltage thing is a common problem with 78/79 series regulators. As you have found they need a minimum load.
richie00boy is right!

These 3 pin regulators will need normally
a minimum load of like 5 mA.

If your negative part draws too little current
you need to put in a load resistor,
to reach a total load of those ~ 5 mA.

There are some IC regulators, for example bigger TO3,
that even recommends a minimum load of 10 mA.
They need it to start working okay.


lineup
__________________
lineup
  Reply With Quote
Old 3rd November 2006, 05:20 PM   #8
Wynand is offline Wynand  South Africa
diyAudio Member
 
Wynand's Avatar
 
Join Date: Mar 2005
Location: Cape Town
Thats what I learnt tonight..... aaggghhh, another day of getting cleverererer.

Enjoy your weekends everyone .

BYEBYE.
Wynand
  Reply With Quote
Old 3rd November 2006, 07:03 PM   #9
diyAudio Member
 
Join Date: May 2004
Location: Hertfordshire
I also found the negative regulator also needs a 1uF minimum capacitor on its output otherwise it oscillates whereas you can get away with 0.1uf on the positive regulators.
  Reply With Quote
Old 4th November 2006, 09:04 PM   #10
cpemma is offline cpemma  United Kingdom
diyAudio Member
 
Join Date: Jul 2003
Location: S Yorkshire OK
Quote:
Originally posted by Wynand
The rest of the regs all work fine. Even the 317/3317 combo in my other PSU.
With 317 regs, the 240R resistor typically fitted between Out and Adjust pins as part of the voltage adjustment chain carries 1.25/240=5mA, usually enough for decent regulation with no other load. 10mA minimum just guarantees it will meet spec.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Lithos regulator - A new super regulator? peranders Parts 7 8th May 2008 03:01 PM
Switching 230 Volt Transformer to 120 Volt OMNIFEX Parts 10 6th April 2008 05:34 PM
12Mhz crystal oscillator - 3,3 volt regulator ErikdeBest Swap Meet 0 28th April 2006 12:02 AM
5 volt version of the Updated super regulator. audiodoc Parts 18 27th March 2006 05:32 PM
Brazil, and some strange laws, not too much strange! destroyer X Everything Else 0 11th September 2004 01:38 PM


New To Site? Need Help?

All times are GMT. The time now is 04:35 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2