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'[EE]: Dropping 6V'
2003\03\12@195042 by Alex Kilpatrick

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I am planning to power a PIC project with two 3 V coin batteries.  I
don't want to add a voltage regulator to the project, so I was wondering
if I could just use a diode to drop .7 V, and give me something close to
5V.   My understanding of diodes is pretty limited, but I am assuming I
wouldn't lose much current this way.  Also, I assume I need a silicon
diode to get .7V, as most of the ones I have messed with lately only
drop .3V.
Any suggestions or alternatives would be most welcome.
Alex

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2003\03\12@201125 by SavanaPics

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How about a 5.1 volt zener.    that way you will acheive regulation as well  <A HREF="http://www.jameco.com">
http://www.jameco.com</A> has them for for 12 cents.  I know you mentioned not wanting a
regulator but the 78L05 low power version of the 5 volt is only 29 cents.  I
recommend the regulator or the zener, at least that way you will have some
regulation rather than  taking a chance on overheating the 5V ic's in your
project

Eddie, kc4awz

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2003\03\12@211551 by Jinx

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> My understanding of diodes is pretty limited, but I am
> assuming I wouldn't lose much current this way.

It's a similar question to the 7905 thread. Current going
through the diode is lost as heat

> Any suggestions or alternatives would be most welcome

Use an LF PIC. They'll work down to 2V, but check specs
to see if operating frequency 3V and under is high enough
for your project

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2003\03\12@212827 by Alex Kilpatrick

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>
> > Any suggestions or alternatives would be most welcome
>
> Use an LF PIC. They'll work down to 2V, but check specs
> to see if operating frequency 3V and under is high enough
> for your project
>
Unfortunately, that is not an option since I have an IR decoder that
requires 5V.  
Why don't they make 5V (or 2.5 V) batteries, with so many devices using
5V logic?

Alex

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2003\03\12@215655 by Herbert Graf

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> > > Any suggestions or alternatives would be most welcome
> >
> > Use an LF PIC. They'll work down to 2V, but check specs
> > to see if operating frequency 3V and under is high enough
> > for your project
> >
>
> Unfortunately, that is not an option since I have an IR decoder that
> requires 5V.
>
> Why don't they make 5V (or 2.5 V) batteries, with so many devices using
> 5V logic?

       Chemistry. A lithium cell outputs 3V, there is no way around it. Same as an
alkaline cell outputs 1.5V and a NiCD cell outputs 1.2V. There is no way
around it. TTYL

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2003\03\12@232005 by Sean H. Breheny

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Don't you think that more of the reason has to do with the fact that many
circuits require that the voltage not go much below 5V, so that you would
really need at least 6 V to start with to ensure that it stays above 5 for
the life of the battery?

You can approximate 5V fairly well with four NiCD cells (like what is used
for receiver packs in RC aircraft).

Also, IIRC, you cannot state that a particular chemistry gives exactly 3V,
for example, and you cannot do anything about it. I think it is a
logarithmic relationship with reagent concentration. In other words, you
COULD make a 5V lithium battery, but you would need insanely high solution
concentration, since the voltage goes as the log of the concentration. It's
sorta like saying that a silicon diode drop is 0.65 and you cannot do
anything about it. Actually you can, it depends on several things,
including dopant concentration, geometry of the diode, and the current
through it, but since they are all rather weak dependencies, it works out
that for practical values of these, you get a range of 0.5 to 0.8 volts.

Sean


At 09:54 PM 3/12/2003 -0500, you wrote:
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2003\03\12@233229 by Alex Kilpatrick

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>
> Don't you think that more of the reason has to do with the
> fact that many circuits require that the voltage not go much
> below 5V, so that you would really need at least 6 V to start
> with to ensure that it stays above 5 for the life of the battery?
>
OK, I will buy that.  But how are most consumer electronics designed?  I
was planning on making something pretty small, using coin batteries.
However, I just read that they are designed to run at tenths of
milliamps, with short peaks to tens of milliamps.  Not too good for a
PIC circuit.  So I could use AAA's, but I would need 4 to get 6V.  Or I
could use a 9V, but they are pretty big.

My PDA uses 2 AA's.  I don't think it is all 3V logic, so I am guessing
it uses some kind of step-up regulator?

Alex

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2003\03\12@233849 by Herbert Graf

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> OK, I will buy that.  But how are most consumer electronics designed?  I
> was planning on making something pretty small, using coin batteries.
> However, I just read that they are designed to run at tenths of
> milliamps, with short peaks to tens of milliamps.  Not too good for a
> PIC circuit.  So I could use AAA's, but I would need 4 to get 6V.  Or I
> could use a 9V, but they are pretty big.
>
> My PDA uses 2 AA's.  I don't think it is all 3V logic, so I am guessing
> it uses some kind of step-up regulator?

       Yup. Almost all consumer devices these days have step-up regulators as the
first stage. Maxim has a great variety of converters that are quite easy to
work with. http://www.maxim-ic.com. TTYL

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2003\03\13@005244 by Alex Kilpatrick

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>
>         Yup. Almost all consumer devices these days have
> step-up regulators as the first stage. Maxim has a great
> variety of converters that are quite easy to work with. http://www.maxim-ic.com. TTYL


Just to save anyone following this thread some work.  The only 3V to 5V
DC-DC converter that I could find that wasn't SMT as the MAX608, which
comes in a 8-pin DIP package.

Cheers,

Alex

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2003\03\13@013453 by Sean H. Breheny

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Hi Alex,

I'm not suggesting that stuff doesn't use switching regulators, I'm just
suggesting a possible reason why we don't typically see 5V batteries to
cater to digital circuits. The reason given was that cell chemistry made it
difficult. My alternative reason is that many circuits require such a
narrow range of voltage for proper operation anyway that you would need
some kind of regulator. I.e., it's not that you can't get a 5V battery,
it's that you can't get a battery that stays close enough to 5V (say
between 4.5 and 5.5) for its whole useful life.

Sean

At 10:30 PM 3/12/2003 -0600, you wrote:
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2003\03\13@031443 by William Chops Westfield

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Batteries were first.  One might ask why logic chips weren't developed to
operate on convenient multiples of "typical" battery voltages.  Of course,
early logic was so power hungry that batteries were pretty much out of the
question (and battery tech sucked back then, too.)  On the other hand, I
can't for the life of me figure out why modern logic families have such
"odd" voltages (3.3V instead of 3, 1.8V ?!)

BillW

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2003\03\13@033642 by jim barchuk

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Hi BillW!

On Thu, 13 Mar 2003, William Chops Westfield wrote:

> Batteries were first.  One might ask why logic chips weren't developed to
> operate on convenient multiples of "typical" battery voltages.

Similar to how chemistry was mentioned as related to batteries, the origin
was based on physics, (chemistry is a subset of physics.) They needed 'x'
amount of energy/force to push the electrons (or holes) around. Same
reason as, prior to semis, tubes were powered by orders of magniture
higher numbers.

> Of course, early logic was so power hungry that batteries were pretty
> much out of the question (and battery tech sucked back then, too.)

I'm sure that when the earliest semis were around the -idea- of
portable/battery was only vaguely dreamed about. Oh sure, you can fit an
'instrument' in a truck and tow along a generator. LOL!

> On the other hand, I can't for the life of me figure out why modern
> logic families have such "odd" voltages (3.3V instead of 3, 1.8V ?!)

Physics. Technological evolution. Technolution? [tm] :) Nope, I just
looked it up, I didn't invent that word, there's already a
technolution.com. In any case they're not that odd anyway, adjustable
regulators are easy, if convenience is what you mean.

They make motors on silicon die and I'm sure the internal combustion folks
are talking about that the same way we are here.

Have a :) day!

jb

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2003\03\13@034302 by Jinx

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> Also, IIRC, you cannot state that a particular chemistry gives
> exactly 3V, for example, and you cannot do anything about it

My opinion is that a battery based on the simple electro-
chemical series does have a fixed voltage. The potential
produced between two dissimilar metals/conductors, eg
lead and sulphuric acid, will always be the same. Changing
the concentrations will not alter the potential of the individual
electrons but it will alter their number (cf voltage vs current,
water through small hoses / big hoses)

However, there are now a lot more complex chemistries than
good old accumulators that may have secondary or even
tertiary reactions after the initial redox

Quite an interesting read

http://www.wppltd.demon.co.uk/WPP/Batteries/Chemistry/chemistry.html

Back to Alex - 6V isn't terribly terribly out of spec and it's below
max rating - give it a go

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2003\03\13@035726 by erholm (QAC)

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I checked a collection of CR2016's some time ago,
and they was all betwen 3.2 and 3.4 V. So you might
have, with new batteries, between 6.4 and 6.8 V.

Jan-Erik Soderholm

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2003\03\13@085441 by Sean H. Breheny

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Hi Jinx,

Ok, so you made me go an look it up ;-)

Have a look at:

http://www.shodor.org/unchem/advanced/redox/index.html

This is not a reference about batteries, but about electrochemical
reactions in general, so there could be something specific to batteries
that I don't know about, but I don't see how it could be all that much
different from other redox reactions.

If you look about halfway down the page you come to the Nernst equation,
which tells you how to compute the potential in an electrochemical reaction.

When you look at an electrochemical series (table of half reactions and
their potentials), it is important to notice that some assumptions are
made. The table is given under "standard conditions", which is a specific
temperature and concentration (1 molar). You must use the Nernst equation
to determine what the potential would be in your particular situation.
However, the dependence on concentration is logarithmic so for a wide range
of practical concentrations, you get roughly the same voltage. Note,
though, that the temperature dependence is linear, which is part of why (I
think) it affects batteries so much (although this is absolute temp not
Celsius). The Nernst equation also explains why batteries go down in
voltage as they discharge :-)

It's been a few years since I last took a chem class and there is part of
this that I don't remember, though, and that is what role "n" plays in this
equation. This web does not really explain what it is. In one spot they
call it number of moles of electrons transferred, and in another place they
call it number of moles _being_ transferred, almost as if it were a rate.

Sean

At 09:42 PM 3/13/2003 +1300, you wrote:
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2003\03\13@090217 by Micro Eng

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Nope...Linear Tech makes one LT1173 in a minidip

For what its worth....I REFUSE to design in any sort of Maxim part to a
design that is going to be produced in a qty of 10 or more.  Why? Because
they don't like to do business with anyone that doesn't order millions of
the same part year after year.

However, Linear Tech will and do....provides samples just as easy and will
support the small product runs.





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2003\03\13@104726 by Alex Kilpatrick

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>
>
> Nope...Linear Tech makes one LT1173 in a minidip
>
Thanks.  It is always good to have alternatives.  It seems like a more
versatile chip.

> For what its worth....I REFUSE to design in any sort of Maxim
> part to a design that is going to be produced in a qty of 10
> or more.  Why? Because they don't like to do business with
> anyone that doesn't order millions of the same part year after year.
>
But Digikey sells all the Maxim stuff, and they don't care about
quantity 1, as long as you spend $25.

Alex

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2003\03\13@151439 by Jinx

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> Ok, so you made me go an look it up ;-)
>
> Have a look at:
>
> http://www.shodor.org/unchem/advanced/redox/index.html
>
> This is not a reference about batteries, but about electrochemical
> reactions in general, so there could be something specific to
> batteries that I don't know about, but I don't see how it could be
> all that much different from other redox reactions

One basic difference is that a battery cell is a closed system and
redox by-products will interfere with current flow to the terminals.
eg sulphation of Pb plates

Can I suggest then, in the spirit of half-reactions, that we're
both half-right ?

If I had more time I'd look into actual cell reactions and practical
application. It probably is possible to make a 5V battery (or better,
5.5V) and it's undoubtedly something the battery makers think
about. It may simply be that the reactants are unacceptable for
any number of reasons. Perhaps the new generation of conductive
dopable polymers will see a range of batteries more suitable for
low-power devices. In Alex's case, the micro is quite happy at 3V
but other devices aren't. So the fault isn't entirely with the battery
is it ?

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2003\03\13@170814 by Matt Pobursky

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On Thu, 13 Mar 2003 09:45:18 -0600, Alex Kilpatrick wrote:
> > For what its worth....I REFUSE to design in any sort of Maxim
> > part to a design that is going to be produced in a qty of 10
> > or more.  Why? Because they don't like to do business with
> > anyone that doesn't order millions of the same part year after year.
> > 

> But Digikey sells all the Maxim stuff, and they don't care about
> quantity 1, as long as you spend $25.

Maxim also sells small quantities w/credit card on their web site too.

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2003\03\13@172653 by

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For *real* small quantities, Dallas/Maxim also has a
free "sample service". Used it late last year...

Jan-Erik Söderholm.



Matt Pobursky wrote:
>Maxim also sells small quantities w/credit card on their web site too.

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2003\03\13@173146 by Alex Kilpatrick

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>
>
> For *real* small quantities, Dallas/Maxim also has a
> free "sample service". Used it late last year...
>
> Jan-Erik Söderholm.
>
In my experience, they give you exactly two, and you don't get any choice in the matter.  You also have to tell what project you are using it for.  Of course, you could make that up.

I think it is a great service.  Half of the time I just want to try out a part to see if it fits my needs, and this way I don't have to order a bunch of stuff from Digikey (to get around the $25 thing), just to get the one part.

Alex

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2003\03\13@192907 by Ian McLean

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Thanks for the tip!  Just ordered samples of the DS1233 5V EconoReset chips
from Dallas this way.  Expecting 2 free samples in post sometime in near
future.  I agree this service is much better than having to order a minimum
quantity from somewhere like Digi-key just to try a part out.  A few times I
have done this and ended up with a part that turned out to not quite fit my
needs.

Rgs
Ian.

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