Quick question - voltage regulators in parallel?

Hi, just a quick question, I'm working on a power supply for a project
and I need a +5v @ 2amp supply. I've got some LM7805's handy, but
they're rated at 1amp each. This question probably sounds stupid, but I
don't know the inner workings of these regulators so I'm not sure if
it's safe: can I hook them up to the same supply and connect the output
pins together to get my +5V @ 2A output?
Thanks!

-Eric A.

 

Nope. I would definitely not do that. They never track each other exactly.

 

Alright, thanks Joerg. Do you know of any 2A-rated regulators like the
trusty 7805 series?

 

No. They'll probably end up having an argument over who's boss.

Graham

 

There is a version up to 3A but I don't know the part number.

- Henry

 

There used to be 5A versions but they might have been obsoleted. There
is no big market for 5V anymore and today everyone uses switchers. When
I designed big 5V supplies in the old days I did it in discretes since
the higher power regulators were hard to get and quite expensive.

 

They might argue, but try putting a small R, say 0R2 in series with
the o/p of each regulator, it might even the balance, maybe, possibly,
if they are from the same batch.

Of course this will affect the Vout, it depends on your requirements

But check that they have the same o/p voltage at the same load first


martin

 

The 78H05 was the 5A version but AFAIK it's long since obsolete. For a
one-off hobby project it may be possible to find one somewhere.

 

There is even an option to use a 7805 with a PNP transistor and the 7805
does regulation where the PNP powers.
If the OP goes behind the 78xx part number there is surely enough equivalent
parts around with higher current spec.

And there is a 7805-like switcher chip made by TI.


- Henry

 

Unfortunately if this works I'll be making a lot of them, so the
one-off solution won't work.. however after digging around the
datasheet for this 7805 I see it says it's fine to go over 1A so long
as you provide adequate heatsinking... so I'll just slap a nice
heatsink on it & some thermal paste and hope for the best...

- Eric A.

 

Most 'standard' 7805s are 1.5A rated these days but you may of course have old
ones.

Graham

 

Well, not a great idea.

What's going to happen is one of them will have a setpoint a few
millivolts higher than the other one. The one with the higher setpoint
will try to pull the voltage up to its desired level. Since it's
unlikely to put out 2 amps, it will go into current limiting and
effectively become just a say 1.25 amp current source.

Then the one with the lower setpoint will become the voltage regulator,
putting out 0.75 amps at it's setpoint voltage.

This might be a stable situation, for small values of the word
"stable".

All of course in theory. In reality they might oscillate, or the
higher one might go into a full thermal shutdown. Some of them have a
designed in thermal hysteresis-- so it might go into wild on-off cycles.

 

There is an adjustable one, I think it's the LM338T
http://www.national.com/pf/LM/LM338.html

You will need an extra two resistors, and I suggest metal film, 1% or
better.

Chris

 

Why not look into the PTH series from TI? Switching regulator modules
in a variety of packages and input voltages. Plus they don't heat up
that much.

 

Awesome, looks like that would be my best option, as I will also be
producing a few other voltages.. now for another quick question, I'll
have 12volts coming in to the circuit, but it's going to run from a car
so I'm expecting a LOT of noise on the line. Would, say, five 6800uF
Electrolytic capacitors in parallel with the incoming 12V (about 0.034
Farad) work well to filter it? I've been playing with the idea of using
"super capacitors", but they're pretty pricey and because most of them
are rated around 2.5volts that I can find, I'd have to use them in
series and though the overall capacitance would be higher by a decimal
place or so, I understand putting caps in series should be avoided..
Any thoughts from you experts out there? Thank again!!

-Eric A.

 

I remember the PNP trick, but couldnt remember which data sheet it was
is, prolly Natsemi, a suitable place to retire old 2N3055s' maybe


martin

 

Ohhh neat, never seen these before. But! - I'm on an extremely tight
budget and these linear regulators are only about $1 each, whereas the
PTH-series seems to run about $11 to $30 each (mouser.com being my
source), but I've got 'em bookmarked for future projects now, thanks!

-Eric A.

 

About as well as trying to stop an elephant with ummmm.... anything not very
big.

Have you heard of these things called LC filters ?


Graham

 

You can use the extra series resistors for a quick test / prototype,
but for longer term you might as well get some 2A 5V regulators:

http://rocky.digikey.com/scripts/ProductInfo.dll?Site=US&V=497&M=L78S05CV

Also possible: use a high-current bypass transistor as shown in Figure
14 here:
http://www.fairchildsemi.com/ds/LM/LM7805.pdf
You will need a pnp transistor that can handle the extra 1 Amp.

Mark

 

Why not calculate the heatsinking required ?

What's the input voltage ?

Graham