Strange Volt regulator!?!

[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.

[lineup]

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

[richie00boy]

The funny voltage thing is a common problem with 78/79 series regulators. As you have found they need a minimum load.

[Wynand]

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.

[Wynand]

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.

[Wynand]

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.

[lineup]

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

[Wynand]

Thats what I learnt tonight..... aaggghhh, another day of getting cleverererer.

Enjoy your weekends everyone .

BYEBYE.
Wynand

[consort_ee_um]

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.

[cpemma]

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.