Power Supply for a LM317

G

goodfella

Guest
I have a 16 Volt, 4 amp wall I wart to use with a LM317 variable power
supply. The LM317 can only handle 1.5 amps, but the circuits that I
will be testing are rated at less than .5 amps. Will the 4 amp
overdrive the LM317 even though I will be using less than an amp?
Thanks
 
goodfella wrote:
I have a 16 Volt, 4 amp wall I wart to use with a LM317 variable power
supply. The LM317 can only handle 1.5 amps, but the circuits that I
will be testing are rated at less than .5 amps. Will the 4 amp
overdrive the LM317 even though I will be using less than an amp?
Thanks
YUP !!
Click to expand...

Rheilly P

(Dont forget to read books)
 
On Sep 18, 6:48 am, Rheilly Phoull <rhei...@bigslong.com> wrote:
overdrive the LM317 even though I will be using less than an amp?

YUP !!
Click to expand...
Er, I don't know much about electronics, but isn't current the actual
current being drawn and not necessarily the current that is
available ?

RP would be correct only if the device being powered suddenly drew
more than 1.5A.

Since you are only drawing 0.5A I can't see any problem, however
depending on what you are testing there might be an inrush that could
exceed 1.5A and then the 4A would come into play. A DC motor comes to
mind where at start up current for a fraction of a second can be many
times the run current.

I would think you could use a current limiting resistor in the 4A line
and keep your hands warm this Winter. <g>

I am NOT an expert on this so I suggest you wait for more definitive
answers.

Dave
 
On Thu, 18 Sep 2008 06:24:08 -0700 (PDT), goodfella
<jocar56@hotmail.com> wrote:

I have a 16 Volt, 4 amp wall I wart to use with a LM317 variable power
supply. The LM317 can only handle 1.5 amps, but the circuits that I
will be testing are rated at less than .5 amps. Will the 4 amp
overdrive the LM317 even though I will be using less than an amp?
Thanks
Click to expand...
No, it isn't a problem. The LM317 will limit the current.

Regards,
Bob Monsen
 
On Sep 18, 9:24 am, goodfella <joca...@hotmail.com> wrote:
I have a 16 Volt, 4 amp wall I wart to use with a LM317 variable power
supply.  The LM317 can only handle 1.5 amps, but the circuits that I
will be testing are rated at less than .5  amps.  Will the 4 amp
overdrive the LM317 even though I will be using less than an amp?
Thanks
Click to expand...
You will have no problem using the 16V wall wart with the LM317. The
current rating on the wall wart tells you want it is CAPABLE of. In
other words, it tells you the maximum power output of the device. In
your case, you'll never need that much. Of course, the LM317 will draw
a SMALL amount of extra power, and so you want a power supply that can
provide GREATER than 0.5A. So you're in good shape with your supply
and your IC.

*) On a related note, it is easy to increase the current carrying
ability of the LM317. If you look at the application examples at the
end of the datasheet, you'll see how you can add external pass
transistors around the LM317. As the LM317 draws more current, it will
signal to the external power transistors that they should provide that
current. So the LM317 becomes a control circuit driving pass
transistors that actually carry most of the current.

*) On another related note, if you look at the LM317 schematic, you'll
find that internally it uses a Darlington pair as a set of pass
transistors that do exactly the same as what I just mentioned. It has
a bandgap reference to hold the 1.25V, and that reference drives a
buffer which drives the base of the internal pass transistor which
delivers all of the power to the output.

--Ted
 
On Thu, 18 Sep 2008 06:24:08 -0700, goodfella wrote:

I have a 16 Volt, 4 amp wall I wart to use with a LM317 variable power
supply. The LM317 can only handle 1.5 amps, but the circuits that I will
be testing are rated at less than .5 amps. Will the 4 amp overdrive the
LM317 even though I will be using less than an amp? Thanks
Click to expand...
Not a problem. The load only takes what it needs - in this case, the
"extra" 3.5 amps are sort of "in reserve".

http://en.wikipedia.org/wiki/Ohm's_law

Do be careful that at low currents, the wall wart's output might want to
increase beyond the allowed input V of the regulator; some wall warts
have poor regulation.

Have Fun!
Rich
 
A great big THANK YOU to all of you who answered. I have always had a
passion for electronics, but not the education. Most of the things
that I have learned have been from generous people such as yourselves
on the net and from looking at schematics. Thanks again.
On Sep 18, 11:14 am, Robert Monsen <rcmon...@gmail.com> wrote:
On Thu, 18 Sep 2008 06:24:08 -0700 (PDT), goodfella

joca...@hotmail.com> wrote:
I have a 16 Volt, 4 amp wall I wart to use with a LM317 variable power
supply.  The LM317 can only handle 1.5 amps, but the circuits that I
will be testing are rated at less than .5  amps.  Will the 4 amp
overdrive the LM317 even though I will be using less than an amp?
Thanks

No, it isn't a problem. The LM317 will limit the current.

Regards,
 Bob Monsen
Click to expand...
 
On Thu, 18 Sep 2008 06:24:08 -0700 (PDT), goodfella
<jocar56@hotmail.com> wrote:

I have a 16 Volt, 4 amp wall I wart to use with a LM317 variable power
supply. The LM317 can only handle 1.5 amps, but the circuits that I
will be testing are rated at less than .5 amps. Will the 4 amp
overdrive the LM317 even though I will be using less than an amp?
Click to expand...
---
The 4 amps won't overdrive the LM317, but the difference in voltage
between the input and output of the LM317 and the current through the
load might.

For instance, with 16V from your wall-wart into the LM317, 5V out of the
LM317, and 0.5A into the load, the LM317 will dissipate:

P = IE = I * (Ein - Eout) = 0.5A * (16V - 5V) = 5.5 watts

The LM317 in a TO-220 package has a maximum thermal resistance (from
junction to ambient, with no heat sink) of 50C per watt and a maximum
junction temperature of 150C:

http://cache.national.com/ds/LM/LM317HV.pdf?

That means that with an ambient temperature of 25C and a maximum
junction temperature of 150C , the maximum power which the LM317 can
dissipate before it goes into thermal shutdown will be:

50C * watts 50C * 5.5W
Pd = ------------- = ------------ = 2.2 watts
150C - 25C 125C

With 5.5 watts being dissipated by the LM317, that means you'll need to
use a heat sink to keep the junction temperature below 150C.

Which one?

From National's data sheet, the LM317A has a maximum thermal resistance,
(Rtjc) from junction to case, of 5C per watt.


Assuming the LM317 is mounted to the heat sink using grease and has a
thermal resistance through the interface (Rtcs) of 1C/W,

Then we can write:

Tj - Ta
Pd = --------------------
Rtjc + Rtcs + Rtsa

and solve for Rtsa:


Tj - Ta
Rtsa = --------- - (Rtjc + Rtcs)
Pd


150C - 25C
= ------------ - ((5C/W) + (1C/W)
5.5W



= 22.7C/W - 6C/W

= 16.72C/W

Which is the highest thermal resistance required.

http://www.aavidthermalloy.com/cgi-bin/stdisp.pl?Pnum=tv1500g

shows a likely candidate, and one which I've used in the past.

Nice heat sink.

JF
 
On Thu, 18 Sep 2008 16:49:13 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Thu, 18 Sep 2008 06:24:08 -0700 (PDT), goodfella
jocar56@hotmail.com> wrote:

I have a 16 Volt, 4 amp wall I wart to use with a LM317 variable power
supply. The LM317 can only handle 1.5 amps, but the circuits that I
will be testing are rated at less than .5 amps. Will the 4 amp
overdrive the LM317 even though I will be using less than an amp?

---
The 4 amps won't overdrive the LM317, but the difference in voltage
between the input and output of the LM317 and the current through the
load might.

For instance, with 16V from your wall-wart into the LM317, 5V out of the
LM317, and 0.5A into the load, the LM317 will dissipate:

P = IE = I * (Ein - Eout) = 0.5A * (16V - 5V) = 5.5 watts

The LM317 in a TO-220 package has a maximum thermal resistance (from
junction to ambient, with no heat sink) of 50C per watt and a maximum
junction temperature of 150C:

http://cache.national.com/ds/LM/LM317HV.pdf?

That means that with an ambient temperature of 25C and a maximum
junction temperature of 150C , the maximum power which the LM317 can
dissipate before it goes into thermal shutdown will be:

50C * watts 50C * 5.5W
Pd = ------------- = ------------ = 2.2 watts
150C - 25C 125C

With 5.5 watts being dissipated by the LM317, that means you'll need to
use a heat sink to keep the junction temperature below 150C.

Which one?

From National's data sheet, the LM317A has a maximum thermal resistance,
(Rtjc) from junction to case, of 5C per watt.


Assuming the LM317 is mounted to the heat sink using grease and has a
thermal resistance through the interface (Rtcs) of 1C/W,

Then we can write:

Tj - Ta
Pd = --------------------
Rtjc + Rtcs + Rtsa

and solve for Rtsa:


Tj - Ta
Rtsa = --------- - (Rtjc + Rtcs)
Pd


150C - 25C
= ------------ - ((5C/W) + (1C/W)
5.5W



= 22.7C/W - 6C/W

= 16.72C/W

Which is the highest thermal resistance required.
Click to expand...
---
That's confusing; sorry.

What I meant to say was that that's the highest thermal resistance which
can be used. Lower is better and will keep the part cooler.

JF
 
goodfella wrote:

I have a 16 Volt, 4 amp wall I wart to use with a LM317 variable power
supply. The LM317 can only handle 1.5 amps, but the circuits that I
will be testing are rated at less than .5 amps. Will the 4 amp
overdrive the LM317 even though I will be using less than an amp?
Thanks
No.
Click to expand...
The amp rating is what you are able to connect to the supply before
it damages the supply.

Voltage is the factor when being concerned about the connected device
how ever, for safety matters and learning etc..., I have found it
cheaper when working with youngsters getting into the field to use a
series incandescent lamp rated for the supply or connected device in the
event of a direct short. The lamp will illuminate at full brightness to
indicate this condition and create a high resistance in the circuit
there by protecting the components.. This also impresses the little kiddies

Of course, one could always use a selectable current regulated supply :)


http://webpages.charter.net/jamie_5"
 
On Thu, 18 Sep 2008 19:33:50 -0400, Jamie
<jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

goodfella wrote:

I have a 16 Volt, 4 amp wall I wart to use with a LM317 variable power
supply. The LM317 can only handle 1.5 amps, but the circuits that I
will be testing are rated at less than .5 amps. Will the 4 amp
overdrive the LM317 even though I will be using less than an amp?
Thanks
No.
The amp rating is what you are able to connect to the supply before
it damages the supply.
Click to expand...
---
The current rating of the load, don't you mean?
---

Voltage is the factor when being concerned about the connected device
how ever, for safety matters and learning etc..., I have found it
cheaper when working with youngsters getting into the field to use a
series incandescent lamp rated for the supply or connected device in the
event of a direct short. The lamp will illuminate at full brightness to
indicate this condition and create a high resistance in the circuit
there by protecting the components..
Click to expand...
---
And when the circuit is operating perfectly, the series connected lamp
will be dim, no matter what the load?
---

This also impresses the little kiddies.
Click to expand...
---
That may be true, but I'm sure that doesn't impress the rest of us
grown-ups who aren't interested in impressing little kiddies.

Me, at least, since I don't represent the rest of "us".
---

Of course, one could always use a selectable current regulated supply :)
Click to expand...
---
And what would you know about that?

JF
 
John Fields wrote:

On Thu, 18 Sep 2008 19:33:50 -0400, Jamie
jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:


goodfella wrote:


I have a 16 Volt, 4 amp wall I wart to use with a LM317 variable power
supply. The LM317 can only handle 1.5 amps, but the circuits that I
will be testing are rated at less than .5 amps. Will the 4 amp
overdrive the LM317 even though I will be using less than an amp?
Thanks

No.
The amp rating is what you are able to connect to the supply before
it damages the supply.


---
The current rating of the load, don't you mean?
---


Voltage is the factor when being concerned about the connected device
how ever, for safety matters and learning etc..., I have found it
cheaper when working with youngsters getting into the field to use a
series incandescent lamp rated for the supply or connected device in the
event of a direct short. The lamp will illuminate at full brightness to
indicate this condition and create a high resistance in the circuit
there by protecting the components..


---
And when the circuit is operating perfectly, the series connected lamp
will be dim, no matter what the load?
---


This also impresses the little kiddies.


---
That may be true, but I'm sure that doesn't impress the rest of us
grown-ups who aren't interested in impressing little kiddies.

Me, at least, since I don't represent the rest of "us".
---


Of course, one could always use a selectable current regulated supply :)


---
And what would you know about that?

JF
ha, A lot more than you obviously know about me..
Click to expand...

I'm sorry you have the wrong impression of me John. I always placed you
above most others here when it came to judgment calls how ever, in this
case, maybe I am the one that made an error in judgment?

If i'm correct in this assumption, please place me in your ignore list
because I don't need to see any disgruntle users here on account of me.

Have a nice day if you can stand your self.

http://webpages.charter.net/jamie_5"
 
On Thu, 18 Sep 2008 06:24:08 -0700 (PDT), goodfella <jocar56@hotmail.com> wrote:

:I have a 16 Volt, 4 amp wall I wart to use with a LM317 variable power
:supply. The LM317 can only handle 1.5 amps, but the circuits that I
:will be testing are rated at less than .5 amps. Will the 4 amp
:eek:verdrive the LM317 even though I will be using less than an amp?
:Thanks


NO!
The current required by the load device is all that will be drawn from any of
the preceding power supply units even if those units have very high current
output capability.

As an example, a telephone exchange battery has current capability in the
thousands of amps range yet the small electronic devices being driven by that
supply don't get fried.
 
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