Quick question - voltage regulators in parallel?

L

lain

Jan 1, 1970
0
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.

J

Joerg

Jan 1, 1970
0
lain said:
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!

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

L

lain

Jan 1, 1970
0
Alright, thanks Joerg. Do you know of any 2A-rated regulators like the
trusty 7805 series?

E

Eeyore

Jan 1, 1970
0
lain said:
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?

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

Graham

H

Henry Kiefer

Jan 1, 1970
0
There is a version up to 3A but I don't know the part number.

- Henry

J

Joerg

Jan 1, 1970
0
lain said:
Alright, thanks Joerg. Do you know of any 2A-rated regulators like the
trusty 7805 series?

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.

M

martin griffith

Jan 1, 1970
0
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.
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

J

Joerg

Jan 1, 1970
0
Henry said:
There is a version up to 3A but I don't know the part number.

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.

H

Henry Kiefer

Jan 1, 1970
0
martin griffith said:
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

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

L

lain

Jan 1, 1970
0
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.

E

Eeyore

Jan 1, 1970
0
lain said:
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...

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

Graham

A

Ancient_Hacker

Jan 1, 1970
0
lain said:
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.

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.

C

Chris Jones

Jan 1, 1970
0
lain said:
Alright, thanks Joerg. Do you know of any 2A-rated regulators like the
trusty 7805 series?

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

A

[email protected]

Jan 1, 1970
0
lain said:
Alright, thanks Joerg. Do you know of any 2A-rated regulators like the
trusty 7805 series?

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.

L

lain

Jan 1, 1970
0
You will need an extra two resistors, and I suggest metal film, 1% or
better.

Chris

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.

M

martin griffith

Jan 1, 1970
0
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
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

L

lain

Jan 1, 1970
0
in a variety of packages and input voltages. Plus they don't heat up
that much.

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.

E

Eeyore

Jan 1, 1970
0
lain said:
Would, say, five 6800uF
Electrolytic capacitors in parallel with the incoming 12V (about 0.034
Farad) work well to filter it?

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

R

redbelly

Jan 1, 1970
0
lain said:
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.

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

E

Eeyore

Jan 1, 1970
0
lain said:
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...

Why not calculate the heatsinking required ?

What's the input voltage ?

Graham

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