ZERO to ~15v adjustable regulator part/schematic?

This is the place for any magazine-related discussions that don't fit in any of the column discussion boards below.
Post Reply
User avatar
VernGraner
Posts: 223
Joined: Sun Mar 02, 2003 1:01 am
Location: Austin Texas
Contact:

ZERO to ~15v adjustable regulator part/schematic?

Post by VernGraner » Tue Apr 08, 2008 2:11 pm

I'm looking for something similar to the LM-317 adjustable voltage regulator, but that will allow the voltage to be set to ZERO. All the examples I could find with the LM-317 show the regulator being adjustable from 1.2v to <x> voltage.

Does anyone know of a similar part (i.e. integrated, low external part count) that can be adjusted down to zero volts and that can handle about 1-1.5a of current?

I found the LM-338 but it also appears to only come down to 1.2v. :???:

Also, if anyone knows of a tricky way to wire up the LM-317/LM-338 so it can perform the task, I'd like to hear about it. Thanks!

Vern
--
Vern Graner

Robert Reed
Posts: 2276
Joined: Wed Nov 24, 2004 1:01 am
Location: ASHTABULA,OHIO
Contact:

Post by Robert Reed » Tue Apr 08, 2008 3:26 pm

All you need to do is take the set resistor to neg 1.2 VDC rather than ground. The neg 1.225 VDC references are commonly available and National semi for one makes them. Also they are dirt cheap and come in a TO-92 package.

User avatar
MrAl
Posts: 3862
Joined: Fri Jan 11, 2002 1:01 am
Location: NewJersey
Contact:

Post by MrAl » Tue Apr 08, 2008 3:39 pm

Hello,

I have to agree that connecting the lower set resistor to -1.25v instead
of ground allows the output to be adjusted to zero volts.

One way to do this is to simply use a separate supply for the minus
1.25v, or use a plus to minus voltage converter to get a minus
voltage, then regulate it to -1.25v.

Another way if the power of the design permits is to use a set of
series diodes. The diodes (maybe 3 in series) are connected in
series with the negative power supply lead and the output load.
The negative supply lead this way becomes about 2.1v more
negative than the output ground lead. You can then use a reference
diode with a current limiting resistor to develop a stable -1.25v
reference which can be connected to the bottom of the lower set
resistor (R2 in your example schematic). This method allows
a simple way to create a negative reference, but the drawback is
some power is wasted (about 2 or 3 watts) in the diodes, and the
total voltage available from the main supply is 2.1v or so less than it
was before. Another drawback is that the ground of the main supply
and the ground of the output are no longer common (they are connected
through three 3A rated diodes like 1N5401).
LEDs vs Bulbs, LEDs are winning.

Bigglez
Posts: 1282
Joined: Mon Oct 15, 2007 7:39 pm
Contact:

Re: ZERO to ~15v adjustable regulator part/schematic?

Post by Bigglez » Tue Apr 08, 2008 3:51 pm

Greetings Vern,
IlusionSYSOP wrote:I'm looking for something similar to the LM-317 adjustable voltage regulator, but that will allow the voltage to be set to ZERO.
As you know these regulators require a small ref voltage
(1.2V is typical) There are some buck converters that
can operate down to 600mV min.

To reach zero at rated load current would almost certainly
require a negative supply, if for no other reason to operate
the feedback loop.

There are a few tricks that can be done to offset the residual
output voltage. For example take a look at the NandV
issues for Mar and Dec (Pt.1) 2007, and Jan 08 (Pt 2).

I doubt there is a commercial "chip" that meets your requirement.

Comments Welcome!

User avatar
MrAl
Posts: 3862
Joined: Fri Jan 11, 2002 1:01 am
Location: NewJersey
Contact:

Post by MrAl » Tue Apr 08, 2008 4:09 pm

Hello again,


Here are the two basic types of circuits. One uses a separate
supply voltage source (low current) and the other uses the
main supply and some diodes.
Both of these circuits use the LM317 or similar IC chip.


Image




Image


In this last circuit if you cant get the reference diode to regulate
and stay within it's required current range you may have to increase
the number of diodes in series to four instead of three. This would
require an extra 2.8v instead of 2.1v so be aware of that.
LEDs vs Bulbs, LEDs are winning.

Yerry
Posts: 40
Joined: Wed Mar 02, 2005 1:01 am
Location: Kentwood, MI USA
Contact:

Post by Yerry » Tue Apr 08, 2008 4:48 pm

]But how will you know that it's working correctly; outputting zero volts instead of blowing itself up and outputting nothing?
:razz:

Bigglez
Posts: 1282
Joined: Mon Oct 15, 2007 7:39 pm
Contact:

Post by Bigglez » Tue Apr 08, 2008 6:14 pm

Greetings (No First Name Supplied),
Yerry wrote:]But how will you know that it's working correctly; outputting zero volts instead of blowing itself up and outputting nothing?
Even at "zero" output voltage the supply will deliver
current. The OP requested zero output volts and up to 1.5Amps.
I put "zero" in quote marks as there will be some residual
resistance. A lot depends upon where the output voltage is
measured. If a four-wire (AKA Kelvin) connection is made to
the load the voltage at the supply might be quite a bit higher
than at the load, as the supply drives current through the
interconnecting cables and connector(s).

Comments Welcome!

User avatar
MrAl
Posts: 3862
Joined: Fri Jan 11, 2002 1:01 am
Location: NewJersey
Contact:

Post by MrAl » Tue Apr 08, 2008 7:06 pm

Yerry wrote:]But how will you know that it's working correctly; outputting zero volts instead of blowing itself up and outputting nothing?
:razz:
Hi Yerry,

That's not as important as it might sound at first. For example,
turn the pot a tiny bit and you get 0.1 volts output, where with
a blown circuit you still get 0.0 volts.
Usually someone who wants a power supply that goes down to
0.0 volts really needs it to go down very low, like to 0.4 volts or
maybe even 0.2 volts, but rarely 0.0 volts. Without the mods
it only goes down to 1.2 volts, which isnt low enough for some
applications.

While we are on the subject, it's also interesting that when this
IC is used to supply a voltage that has to be current limited and
that current limiting is via the control signal at the bottom of R2,
sometimes the current stays high if the load still draws current
below 1.2 volts. This kind of situation also needs a negative
bias or the current might not actually get limited as intended.

As a side note, i build up a general purpose bench supply with the
LM317 a long time ago. I use it for many things including charging
Li-ion cells (or at least i did back then) so i built in a simple transistor
current feedback circuit (very common) that pulls the bottom of R2
low when the current gets above a preset level (base resistor sets
current limit). I didnt have a problem with the output only going down
to 1.2 though, as most of my loads only draw significant current
above that, like say 2.5 volts or so. This means i didnt need the
negative bias for this circuit to fit my purposes.
If an app does need the output to go to zero in order to properly
limit current however, the current feedback transistors emitter goes
to the -1.25v supply instead of ground...not too hard to do as long
as you have the -1.25v supply scheme worked out already. In this
setup however the -1.25v supply would have to handle the main
current flow. Better might be to use an op amp for sensing and
drive the bottom of R2 with the op amp output, using the negative
supply to power the negative power supply of the op amp, and use
an op amp whos output goes all the way down to the minus supply
rail. Another idea might be to bias the bottom of R2 in such a way
that the NPN transistor can still control it all the way down to -1.25v
even though its emitter is tied to ground. IF this works it would
require a few more resistors.
LEDs vs Bulbs, LEDs are winning.

Post Reply

Who is online

Users browsing this forum: No registered users and 51 guests