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'[EE]: microcontroller based variable voltage regul'
2004\06\25@013949 by John Waters

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Hi All,

I want to use my car battery to provide a range of variable d.c. from 0-12V,
but a standard regulator circuit doesn't fulfill the requirement, as I want
the output voltage to be adjustable by software through a microcontroller,
could someone suggest how to do that?

Thanks in advance!

John

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2004\06\25@021631 by Ishaan Dalal

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John Waters wrote:

> I want to use my car battery to provide a range of variable d.c. from
> 0-12V,
> but a standard regulator circuit doesn't fulfill the requirement, as I want
> the output voltage to be adjustable by software through a microcontroller,
> could someone suggest how to do that?

The key is: digital POT. Question is whether you can afford a wasteful
linear regulator, or you need a switcher. The LM317 I believe is the
standard for this sort of thing. Of course, you might need an LDO (low
dropout) regulator...see TI, OnSemi, etc. for an adjustable LDO linear
regulator. All of these use some sort of resistive divider to fix
voltage, and with a digital POT in there (Microchip makes some nice
ones, the MCP4xxxx series), you're all set.

If you are looking for switchers, National makes a number of adjustable
ones. I've used the LM2576-ADJ, but that's a high current (3A) part.
They make low-current ones too. Check them out. "dropout", i.e. min
input-output differential is about 3V.

Cheers,
-Ishaan

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2004\06\25@092309 by Mike Hord

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Before you go willy-nilly sticking a digital pot in, though, you need
to be certain that the voltages on the pot are not past it's specs.

For example, with Microchip's digital pots (IIRC), you had BETTER
not put more than the supply voltage on any of the pins (at least,
according to the datasheet).

I don't know exactly what circuit you'll use, but just keep that in
mind.

Mike H.

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2004\06\25@095842 by Byron A Jeff

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On Fri, Jun 25, 2004 at 02:20:04AM -0400, Ishaan Dalal wrote:
> John Waters wrote:
>
> >I want to use my car battery to provide a range of variable d.c. from
> >0-12V,
> >but a standard regulator circuit doesn't fulfill the requirement, as I want
> >the output voltage to be adjustable by software through a microcontroller,
> >could someone suggest how to do that?
>
> The key is: digital POT.

Actually it isn't. The key is a controllable variable voltage. A digipot is
one mechanism for accomplishing that. However it does have its issues:

1) Digipots have variable impeadance.
2) Digipots may or may not have the proper resistive range for the application
3) Digipots may or may not have the same grainularity as other methods. 8
  bits only gets you 256 voltage steps.
4) Digipots may not be precisely linear over the its specified range.
5) And most importantly digipots may not be either easy or cheap to get.

So back to the main point, you need a controllable voltage. Fortunately the
PIC is quite helpful here, because many of them have high resolution PWM.
A simple one pole low pass filter consisting of a resistor and cap connected
to the PWM output can provide a stable reference voltage for the regulator.

Figure 4 of Microchip's appnote AN538 gives a perfect example of the process.
It also is followed bu an opamp voltage follower which solves the impeadance
problem because the opamp will provide whatever current is required to maintain
the given input voltage.

http://ww1.microchip.com/downloads/en/AppNotes/00538c.pdf

The opamp also provides another opportunity, which is amplifying the input
voltage. Since on a LM317 the output follows the ADJ voltage + 1.25V the
ADJ voltage needs to be able to get up to nearly 11V. The opamp with the
addition of a couple of resistors can easily double or triple the output
voltage. The maximize the precision of the 10 bit PWM you really want to
match the amplification for that 5V maps as close to 10.75V as you can get.
But even a straight double will get you a range of 1.25V up to 11.25V

> Question is whether you can afford a wasteful
> linear regulator, or you need a switcher. The LM317 I believe is the
> standard for this sort of thing.

Correct.

> Of course, you might need an LDO (low
> dropout) regulator...see TI, OnSemi, etc. for an adjustable LDO linear
> regulator.

It all depends of if the battery is in a running car or not. Alternators
routinely output nearly 14V which is enough headroom for even the LM317.

But out of the box, and under any sort of load, it's closer to 12V.

> All of these use some sort of resistive divider to fix
> voltage, and with a digital POT in there (Microchip makes some nice
> ones, the MCP4xxxx series), you're all set.

You're already going to have a PIC. An opamp, a cap, and a handful of resistors
can give you the same effect.

>
> If you are looking for switchers, National makes a number of adjustable
> ones. I've used the LM2576-ADJ, but that's a high current (3A) part.
> They make low-current ones too. Check them out. "dropout", i.e. min
> input-output differential is about 3V.

Absolutely. You may need to check out SEPIC configurations which will buck
or boost providing the correct output voltage whether or not the input
voltage is above or below the output voltage.

BAJ

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2004\06\25@152629 by llile

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Well a straightforward way to do this is a PIC based power supply.  I
built some with the following rough outline:

Slew controls (two buttons, one up, one down)
Power up at 1.2 volts (about as low as it will go anyway
Range 1.2 to 24 volts at 1 amp
User interface consists of a digital voltmeter module
PIC checks the output voltage via a resistor divider and an RC filter
through analog input
PIC drives a series pass transistor at regular intervals  - on a timer
at about 20KHz the PIC decides if the output is too high or too low and
turns the pass transistor on or off.

Current regulation is crude and simple.  The PIC is powered from the same
supply as the power supply.  If you short it out, you short out the PIC
and it reboots and turns off the current.  Crude, Rude, and short proof,
so far meets my needs.  I was going to build in a current foldback feature
when I accidentally discovered that this works just fine.  If I was going
to sell this I would of course include current foldback, but this was a
quick one-off bench supply to replace the other one that fried.

PIC is on a simple 7805 regulator, the rest of the supply is on a 24V DC
power supply.  The whole thing is in a plastic box from radio shack and
took an afternoon to put together.  I added a dedicated 5V supply from a
simple 7805 just for kicks.

I've always built my bench power supply, partly because it galls me to pay
$200 or $300 for one that doesn't regulate any better than one I can build
out of spare parts.


-- Lawrence Lile





John Waters <spam_OUTjohn_fm_watersTakeThisOuTspamHOTMAIL.COM>
Sent by: pic microcontroller discussion list <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
06/25/2004 12:38 AM
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       Subject:        [EE]: microcontroller based variable voltage regulation, how?


Hi All,

I want to use my car battery to provide a range of variable d.c. from
0-12V,
but a standard regulator circuit doesn't fulfill the requirement, as I
want
the output voltage to be adjustable by software through a microcontroller,
could someone suggest how to do that?

Thanks in advance!

John

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2004\06\25@154120 by Paul James E.

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Lawrence,

I agree 100% from a regulation standpoint.  But from a noise perspective,
you'd be hard put to gain the same performance from such inexpensive
hardware.   We have spent many manhours designing a power supply for a
system we designed and build.   The noise performance for this system
is around the -140+ (or should that be minus for lower noise)db range.
This is not easily achieved by weither a linear type regulator or a
switching type regulator.  We have achieved a small switching supply that
has an input range of about 3 VDC to about 18 VDC with an output of
plus and minus 5 AVDC @ ~200ma (each leg).  And this with a noise figure
of better than -140db.   Not easily achieved, and not inexpensive, but
doable.

But as a practical matter, your design is sufficient for most lab uses
you will probably run into in general work.   But if you have to have
a very quiet supply, figure on spending a few more bucks than this.

                                              Regards,

                                                Jim



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2004\06\25@155440 by Ed Browne

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Jim,
Is this supply commercial and available for sale?  I have a project that
could use  quiet supplies.  Did you use a specific topology?  resonant mode,
for example?

Thanks,
Ed

On 6/25/04 you wrote:
We have achieved a small switching supply that has an input range of about 3
VDC to about 18 VDC with an output of plus and minus 5 AVDC @ ~200ma (each
leg).  And this with a noise figure of better than -140db.   Not easily
achieved, and not inexpensive, but doable.

But as a practical matter, your design is sufficient for most lab uses you
will probably run into in general work.   But if you have to have a very
quiet supply, figure on spending a few more bucks than this.

                                              Regards,

                                                Jim

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2004\06\25@160100 by llile

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Yep, if you want better performance you will have to pay for it.  I've
been thinking about building another in-house bench supply with a National
simple switcher just to get the experience using them.   Of course, this
is always on the back burner unless the old power supply is fried.


-- Lawrence Lile





"Paul James E." <@spam@jamespKILLspamspamintertex.net>
06/25/2004 04:41 AM


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       cc:     RemoveMEllileTakeThisOuTspamSALTONUSA.COM
       Subject:        Re: [EE]: microcontroller based variable voltage regulation, how?



Lawrence,

I agree 100% from a regulation standpoint.  But from a noise perspective,
you'd be hard put to gain the same performance from such inexpensive
hardware.   We have spent many manhours designing a power supply for a
system we designed and build.   The noise performance for this system
is around the -140+ (or should that be minus for lower noise)db range.
This is not easily achieved by weither a linear type regulator or a
switching type regulator.  We have achieved a small switching supply that

has an input range of about 3 VDC to about 18 VDC with an output of
plus and minus 5 AVDC @ ~200ma (each leg).  And this with a noise figure
of better than -140db.   Not easily achieved, and not inexpensive, but
doable.

But as a practical matter, your design is sufficient for most lab uses
you will probably run into in general work.   But if you have to have
a very quiet supply, figure on spending a few more bucks than this.

                                              Regards,

                                                Jim



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2004\06\25@161009 by Tom

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Jim,

If you don't mind giving away some secrets, what was the general method you
used to achieve these numbers?  Not looking for schematics or anything,
just curious how you could get a switcher so quiet.

And also, how did you verify the -140db?  It seems to me not a trivial
achievment.

Tom

At 02:41 PM 6/25/04 +0500, you wrote:
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2004\06\25@163812 by Harold Hallikainen

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Not a DC supply, but I did a PIC based regulated AC supply to run a
halogen lamp in a dental curing lamp. I use a triac and phase control to
vary the RMS voltage to the lamp. The 250W 24V lamp runs on an
autotransformer that also has an isolated secondary to power the PIC. An
opto isolator carries the gate signal from the PIC back to the line
connected triac. The tap on the autotransformer for the lamp is about 30V.
We then phase control that down to 24V. The tricky part was figuring out
how to measure the RMS voltage the lamp was getting to adjust the firing
time on the triac. I finally ended up saying the transformer secondary
voltage is proportional to the primary and lamp voltages. So, I use the
PIC A/D to measure the instantaneous voltage on the transformer secondary
(isolated from line). Each A/D sample is squared and added to a sum. 256
samples are taken, then the average computed by throwing away the bottom 8
bits. The square root of this average is taken to give the RMS voltage. A
compare register in the PIC is incremented or decremented to move the
triac trigger time as required. Since the PIC knows when the triac is on
(it's telling the triac when to run), the RMS computing routine adds in a
zero when the triac is not on, and the appropriate squared instantaneous
secondary voltage when it's on.

Works pretty well and has been shipping for several years.

Harold

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2004\06\25@170336 by Paul James E.

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Tom,

It was done with a combination of Switching and linear regulation.
Also, the switching frequency of the switching regulator is >1Mhz.
This allows for small components and easier filtering.  There is a
common mode choke used.  Plus the transformers are part of the filter
circuit.   Beyond that, I can't say much more.

As far as measuring the noise floor, we used a SRS Spectrum analyzer.



                                          Regards,

                                            Jim




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