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'[EE]: Suggestion for voltage regulator.'
2002\06\04@114508 by A.J. Tufgar

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Hello All,
         I have a project that I'm working on that runs of two D cells
in series.  I'm using a 16C711 that works from 2.5-5.5V.  On the ADC
port I have a 3VDC pressure transducer with a output of 0-60mV.  When a
certian pressure is reached a solenoid valve opens.

Here's my problem.  I want this to be effecient on battery power so I'd
like to run both sensor and pic at 2.5V trouble is even LDO's will
dropout at about 2.7V.  The solenoid works to about 2.4V.

What I was thinking of doing is not using a regulator so the voltage
would vary over battery life about 1.6-1.25.  Vref (I have it set at
about 70mV) should drop with respect to battery and so should the output
of the sensor (it's a wheatstone bridge configuration).  I also have the
pic calibrate on boot.  So these three attributes combined I would
believe would be enough to not have to use a regulator.

Any thoughts?  Any ideas on a reg that would output 2.5V until the
voltage dropped below 2.5V?

Aaron

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2002\06\04@124100 by Thomas C. Sefranek

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On 4 Jun 2002 at 11:41, A.J. Tufgar wrote:

> Hello All,
>           I have a project that I'm working on that runs of two D cells
> in series.  I'm using a 16C711 that works from 2.5-5.5V.  On the ADC
> port I have a 3VDC pressure transducer with a output of 0-60mV.  When a
> certian pressure is reached a solenoid valve opens.
>
> Here's my problem.  I want this to be effecient on battery power so I'd
> like to run both sensor and pic at 2.5V trouble is even LDO's will
> dropout at about 2.7V.  The solenoid works to about 2.4V.
>
> What I was thinking of doing is not using a regulator so the voltage
> would vary over battery life about 1.6-1.25.  Vref (I have it set at
> about 70mV) should drop with respect to battery and so should the output
> of the sensor (it's a wheatstone bridge configuration).  I also have the
> pic calibrate on boot.  So these three attributes combined I would
> believe would be enough to not have to use a regulator.
>
> Any thoughts?  Any ideas on a reg that would output 2.5V until the
> voltage dropped below 2.5V?

Shunt Regulator. (Zener).
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2002\06\04@204121 by Rick C.

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A fresh set of alkalines will give you a pretty constant voltage that you
should be able to calibrate to upon power up and provide useful accuracy.
Once the batteries reach the end of their useful life, the voltage will take
a steep plunge and there is no real advantage of tracking or calibrating
your pic reference to it. Two aging cells in series providing 2.5 volts
means it is already on a rapid avalanche to zero. A fresh set of cells is
the only reliable fix.

Rick

"A.J. Tufgar" wrote:

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2002\06\05@024827 by Peter L. Peres

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A Whetstone bridge type pressure transducer is ratiometric. You don't
really need a regulator for that. You do need to keep track of the voltage
and apply a multiplier to fix the decreased readout for lower voltage.

I do not know of a regulator that goes that low but they should exist.

Peter

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2002\06\05@094236 by A.J. Tufgar

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I was looking at the duracell site and I'm thinking of using two 3V
batteries to power my application apparently the 1.5V will drop almost
immediately.

Using a Linear Technology regulator LT1754-3.3 will give me 3.3V for
alot longer but this is specfic to my app I guess.

Anyweays if anyone is interested:
http://www.duracell.com/oem/Pdf/others/ATB-full.pdf

Aaron

{Original Message removed}

2002\06\05@120652 by Roman Black

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> On 4 Jun 2002 at 11:41, A.J. Tufgar wrote:
>
> > Hello All,
> >           I have a project that I'm working on that runs of two D cells
> > in series.  I'm using a 16C711 that works from 2.5-5.5V.  On the ADC
> > port I have a 3VDC pressure transducer with a output of 0-60mV.  When a
> > certian pressure is reached a solenoid valve opens.
> >
> > Here's my problem.  I want this to be effecient on battery power so I'd
> > like to run both sensor and pic at 2.5V trouble is even LDO's will
> > dropout at about 2.7V.  The solenoid works to about 2.4V.

> > Any thoughts?  Any ideas on a reg that would output 2.5V until the
> > voltage dropped below 2.5V?


It shouldn't be that hard to make a low-dropout
regulator with three transistors. What sort of
current does this circuit require? If it's only
<100mA etc than this should do the trick:

In 3.4v to 2.52v
   ------*---*-----------,
         |   |           |
         R   |  PNP      R
         |   E  lo-sat   |
         *--B            |
         |   C-----------|------*------ 2.5v out
         |               |      |
         |               |      |
         C               |      R
          B--------------*-,    |
         E                 |    |
   any   |                 C    |
   NPN   |           any    B---*
         |           NPN   E    |
         |                 |    R
         |                 |    |
gnd -----*-----------------*----*------ gnd


I've tested lo-sat transistors here that will do
better than 20mV C-E at 50mA, the bad news is that
they are TO-220 pack power trannys. I'm sure you
can find a decent lo-sat tranny in a small pack
like standard TO-92 etc. A fet will do the low
drop but will be almost impossible to drive with
only 2.5v as your input voltage. :o)
-Roman

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2002\06\06@032121 by Vasile Surducan
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Roman, I saw the feedback but I don't see anywhere the reference, maybe
you should explain a little. My impresion is that stabiliser is strongly
dependent with the load parameters. Could you point also what transistors
did you found with 20mV Vce at Ic = 50mA ?

thanks,
Vasile
http://www.geocities.com/vsurducan


On Thu, 6 Jun 2002, Roman Black wrote:

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2002\06\06@092447 by Roman Black

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Vasile Surducan wrote:
>
> Roman, I saw the feedback but I don't see anywhere the reference, maybe
> you should explain a little. My impresion is that stabiliser is strongly
> dependent with the load parameters. Could you point also what transistors
> did you found with 20mV Vce at Ic = 50mA ?


Hi Vasile! :o) The transistors I have the most of
are BD203 (NPN) and BD204 (PNP). I often use them
as low-sat drivers, at 50mA Ic you get about 20mV
C-E on the NPN but the PNP is a fraction worse.
The BD203 is TO-220, 60v 8A 60w. Used in the TV
trade for vertical power amp in ancient TVs. About
$0.20 US. Absolutely brilliant for the $$, almost
indestructable and about as good as trannys got
before the fet fad.

A similar "modern" transistor is the TIP41C, ok for
low-sat use at 170mV C-E at 1A, 260mV C-E at 2A.
Not quite as good as BD203 for very low currents
though. Both these transistors will drive 1A loads
and stay cool with no heatsink, and are more robust,
easier to drive and cheaper than similar fets (but
that's a whole new argument!)

The circuit I proposed is a simple low-dropout
regulator to give regulated 2.5vdc from 2x 1.5v cells.
The regulation should be quite independent of load
parameters, it's determined by the B-E junction of
the last transistor and since that transistor passes
very little current and won't get hot it should
regulate quite well. There is plenty of gain and
voltage headroom to drive the main tranny well into
saturation even with 2.52v input and 2.50v output.
:o)
-Roman

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