18VDC (5A) Regulator Needed

[Mike]

My system has a 28VDC supply which I need to step down to 18VDC (~5A).
Does anyone know of a regulator that will do the job. I have no
preference on type, switching, linear, etc. Almost anything will do
the job. Thanks.

 

[DaveM]

"Mike" <mep0716@gmail.com> wrote in message
news:1152221838.668816.225120@s16g2000cws.googlegroups.com...

My system has a 28VDC supply which I need to step down to 18VDC (~5A).
Does anyone know of a regulator that will do the job. I have no
preference on type, switching, linear, etc. Almost anything will do
the job. Thanks.

A National LM138 linear regulator will do the job for you. It's currently
stocked by many of the parts houses. Get the datasheet from www.national.com.
Be sure to mount it on a good heatsink. From your numbers, it will be
dissipating about 50 watts at full load.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Some days you're the dog, some days the hydrant.

 

[PeterSmith1954@googlemail]

DaveM wrote:

"Mike" <mep0716@gmail.com> wrote in message
news:1152221838.668816.225120@s16g2000cws.googlegroups.com...
My system has a 28VDC supply which I need to step down to 18VDC (~5A).
Does anyone know of a regulator that will do the job. I have no
preference on type, switching, linear, etc. Almost anything will do
the job. Thanks.


A National LM138 linear regulator will do the job for you. It's currently
stocked by many of the parts houses. Get the datasheet from www.national.com.
Be sure to mount it on a good heatsink. From your numbers, it will be
dissipating about 50 watts at full load.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Some days you're the dog, some days the hydrant.

Using a linear regulator for this would hardly be the best choice. At
5A, it will have 50W dissipation (and I don't know of a LM138 rated at
that, even with a heat sink less than the size of a small box). Apart
from that, it's very wasteful.

There are some commercial POL converters on plugins (although 18V is
not exactly a standard size, but I am sure one might be adapted).

Failing that, Maxim, TI and Linear tech have a nice line of controllers
(TI has some really nice ones with the compensation internally that
works for most cases).

Do you need a plugin solution or are you willing to lay out a switcher
(which is not that simple a matter)?

Cheers

PeteS

 

DaveM wrote:

"Mike" <mep0716@gmail.com> wrote in message
news:1152221838.668816.225120@s16g2000cws.googlegroups.com...
My system has a 28VDC supply which I need to step down to 18VDC (~5A).
Does anyone know of a regulator that will do the job. I have no
preference on type, switching, linear, etc. Almost anything will do
the job. Thanks.


A National LM138 linear regulator will do the job for you. It's currently
stocked by many of the parts houses. Get the datasheet from www.national.com.
Be sure to mount it on a good heatsink. From your numbers, it will be
dissipating about 50 watts at full load.

http://cache.national.com/ds/LM/LM138.pdf

The T-package (TO-220) has a thermal resistance of 4C/W from junction
to case - which would be a 200C temperature rise at 50W, so you can't
use that.

The K-package (TO-3) is a lot better at 1C/W, but you've got to limit
the junction temperature to less than about 125C (otherwise the LM338
will do it for you) so your total thermal resistance from junction to
ambient has to be less than 2C/W, which isn't easy to achieve - I'd be
looking at a 0.5C/W heatsink, like Farnell's150-019, which costs about
$25, stand 2"" high , 5" wide and 6" long.

Paralleling a couple of linear regulators might save you money and
heat-sink volume, or you might put a beefy power transistor in series
with a low-drop-out linear regulator, mounting it on the same heat-sink
as the linear regulator to exploit the linear regulators thermally
driven current limiter.
--
Bill Sloman, Nijmegen

 

[Phil Allison]

"DaveM"

A National LM138 linear regulator will do the job for you. It's currently
stocked by many of the parts houses. Get the datasheet from
www.national.com. Be sure to mount it on a good heatsink. From your
numbers, it will be dissipating about 50 watts at full load.

** Easily fixed by adding a series resistor of about 1.5 ohms.

Won't change the total dissipation, but shifts most of it off the LM338 so
it runs cooler and avoid approaching thermal shutdown. Means you can use
the cheap TO220 version OK.



........ Phil

 

Phil Allison wrote:

"DaveM"


A National LM138 linear regulator will do the job for you. It's currently
stocked by many of the parts houses. Get the datasheet from
www.national.com. Be sure to mount it on a good heatsink. From your
numbers, it will be dissipating about 50 watts at full load.



** Easily fixed by adding a series resistor of about 1.5 ohms.

Won't change the total dissipation, but shifts most of it off the LM338 so
it runs cooler and avoid approaching thermal shutdown. Means you can use
the cheap TO220 version OK.

I've used that trick - though not for many years - and it works really
well. Make sure that you don't mouint the reistor on the same heatsink
as the regulator - resistors are typically designed for a peak junction
temperature of 250C, and the heat sink can (and should) be sized to get
pretty hot (since resistors are cheap and heat-sinks are relatively
expensive).

--
Bill Sloman, Nijmegen

 

[Ken Smith]

In article <1152221838.668816.225120@s16g2000cws.googlegroups.com>,
Mike <mep0716@gmail.com> wrote:

My system has a 28VDC

How high can the nominal 28V go? If this is for aircraft, you'd better
defend against at least 50V spikes.

How low can the 28V go and do you need to maintain regulation under that
condition?

supply which I need to step down to 18VDC (~5A).
Does anyone know of a regulator that will do the job. I have no
preference on type, switching, linear, etc. Almost anything will do
the job. Thanks.

--
--
kensmith@rahul.net forging knowledge

 

[ratman]

On 6 Jul 2006 14:37:18 -0700, "Mike" <mep0716@gmail.com> wrote:

My system has a 28VDC supply which I need to step down to 18VDC (~5A).
Does anyone know of a regulator that will do the job. I have no
preference on type, switching, linear, etc. Almost anything will do
the job. Thanks.

There might be a SimpleSwitcher (R) that would work (National)