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'[EE]: -15 volts from +9'
2002\03\09@075634 by michael brown

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I need an adjustable negative supply voltage in the -13 to -16 volt range
for the contrast control of an LCD module.  I would like to generate this
supply from a 9 volt battery that is also used for the positive supply of
the rest of the circuit.  I realize that I could just use a couple more 9
volt batteries and a pot to do it, but I would like it to be a little more
compact than that.  Can someone suggest some kind of simple charge pump that
will generate the voltage from the normal supply voltage, without using any
transformers?  I am quite weak when it comes to "funky" analog electronics.
;-)

Michael Brown
Instant Net Solutions
http://www.KillerPCs.net

"In the land of the blind, he who has one eye is king"

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2002\03\09@090801 by Olin Lathrop

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> I need an adjustable negative supply voltage in the -13 to -16 volt range
> for the contrast control of an LCD module.  I would like to generate this
> supply from a 9 volt battery that is also used for the positive supply of
> the rest of the circuit.  I realize that I could just use a couple more 9
> volt batteries and a pot to do it, but I would like it to be a little more
> compact than that.  Can someone suggest some kind of simple charge pump
that
> will generate the voltage from the normal supply voltage, without using
any
> transformers?  I am quite weak when it comes to "funky" analog
electronics.
> ;-)

Like you said, a charge pump, especially since your current requirements are
very low.  However, you will need two charge pumps to go from +9 to -16,
although that's still probably the best way to go.  Another alternative
would require a single inductor.


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2002\03\09@093446 by Roman Black

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Olin Lathrop wrote:
>
> > I need an adjustable negative supply voltage in the -13 to -16 volt range
> > for the contrast control of an LCD module.  I would like to generate this
> > supply from a 9 volt battery that is also used for the positive supply of
> > the rest of the circuit.  I realize that I could just use a couple more 9
> > volt batteries and a pot to do it, but I would like it to be a little more
> > compact than that.  Can someone suggest some kind of simple charge pump
> that
> > will generate the voltage from the normal supply voltage, without using
> any
> > transformers?  I am quite weak when it comes to "funky" analog
> electronics.
> > ;-)
>
> Like you said, a charge pump, especially since your current requirements are
> very low.  However, you will need two charge pumps to go from +9 to -16,
> although that's still probably the best way to go.  Another alternative
> would require a single inductor.


This was done in a rush, but I think something
like this would work?

Super cheap -15v smps psu.

  +9v------------------------*----*---------------------
  In                         |    |
                             R    |  BC337??
                             |    C  any 200ma NPN small
       L is a small 1500uH   *---B   transistor  (x2)
       (1.5mH) prewound RF   |    E
       inductor.             '----|------Rx------,
                                  |              |
                                  |              C
                                  *----L--*-*---B
                                  |       | |    E
                                  |       | |    |
                          1N4148  - D1    C R1   |
                       or 1N914   ^       | |    |
                                  |  0.1uF| |    |
                                  |       | |    |
  Gnd-----------------------------|-------*-*----*------
                                  |       |      |
          (Hopefully R1 sets the  |       |      | 15v 400mW?
          constant current, maybe |       C2     ~ Zener
          5 or 10mA ??)           |  10uF |      ^ diode
                                  |  25v  |      |
                                  |       |      |
                                  |       |      |
 -15v-----------------------------*-------*------*------
  Out

I think Rx may not be needed, depending how much
you need to keep the parts down. These inductors
are only slightly are only about 10mm x 5mm round
and not much bigger than a resistor. :o)
-Roman

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2002\03\09@102346 by Thomas C. Sefranek

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HA7660  Switched Capacitor Voltage inverter 8 Pin DIP



michael brown wrote:

{Quote hidden}

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2002\03\09@102553 by Dave Dilatush

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Roman Black wrote...

>This was done in a rush, but I think something
>like this would work?

Roman, you're going to build this and test it and let us know what you
found.  Right?  :)

Dave

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2002\03\09@104058 by David Covick

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He did put a question mark after his sentence :)


----- Original Message -----
From: "Dave Dilatush" <dilatushspamKILLspamCOMCAST.NET>
To: <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
Sent: Saturday, March 09, 2002 7:23 AM
Subject: Re: [EE]: -15 volts from +9


{Quote hidden}

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2002\03\09@123015 by michael brown

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Subject: Re: [EE]: -15 volts from +9


> HA7660  Switched Capacitor Voltage inverter 8 Pin DIP

Thomas,

It looks like this device will not generate a high (low?) enough voltage for
me.  I see that you can cascade them, but I would need at least three in a
row, since each one generates -5 only(at least in the datasheet (for ME7660)
that I found, but someone else recommended this same chip, so I will look
for more datasheets).  Cascading might be a little too cumbersome or
expensive.  They look like they would be ideal for a text type LCD.  I have
a graphics LCD that needs a pretty high negative voltage (-15V).  It looks
like the MAX776 will do up to -15, I think this will be enough for me.

thanks for the help

michael

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2002\03\09@124059 by Dave Dilatush

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part 1 2051 bytes content-type:text/plain; charset=us-ascii (decoded 7bit)

Michael Brown wrote...

>I need an adjustable negative supply voltage in the -13 to -16 volt range
>for the contrast control of an LCD module.  I would like to generate this
>supply from a 9 volt battery that is also used for the positive supply of
>the rest of the circuit.  I realize that I could just use a couple more 9
>volt batteries and a pot to do it, but I would like it to be a little more
>compact than that.  Can someone suggest some kind of simple charge pump that
>will generate the voltage from the normal supply voltage, without using any
>transformers?  I am quite weak when it comes to "funky" analog electronics.

Maxim's LCD Bias Supply chip will do this job; it even has a built-in
6-bit DAC to allow the microcontroller to adjust the output voltage.  It
requires a 22 microhenry inductor, two small electrolytic caps, a pair
of Schottky diodes, and a few other R's and C's.

If you'd rather roll your own charge pumper using more readily available
parts, the attached diagram shows an approach using a MAX232A RS-232
interface chip that'll give you about -18V unloaded.  You can feed a
trimpot from that, to adjust the LCD contrast voltage.  If you use a
MAX232 instead of a MAX232A, make all the capacitors ten times bigger to
compensate for the slower switching frequency.  This design operates off
the +5V supply, which I assume you're regulating; and that eliminates
having to deal with a battery voltage that could be anywhere from 9V
(new) down to about 6V (discharged).

The MAX232A is a very useful chip.  The classical way of looking at it
is as advertised: it's an RS-232 interface chip with a built-in charge
pump for generating the interface voltages.  An alternate way of looking
at it is a general-purpose charge pump chip that can generate +/- 8-10V
from a +5V supply, and which also happens to have some built-in RS-232
I/O, just in case you need it.

They're real handy for generating +/- voltage for "funky" analog stuff.

Dave D.


part 2 3164 bytes content-type:image/gif; name=pumper.gif (decode)


part 3 144 bytes
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2002\03\09@133504 by michael brown

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Thanks Dave,

This looks like the best solution for me.  I happen to have a 232A laying
around, and I'm sure I can scrounge up the diodes and caps.  ;-)

Michael Brown
Instant Net Solutions
http://www.KillerPCs.net

"In the land of the blind, he who has one eye is king"

{Original Message removed}

2002\03\09@134125 by michael brown

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Thanks Dave, Olin, Roman and Thomas.  The 232A solution looks to be what I
will try first.  If it works well, then that will be that.  No coils and no
zeners.  Just easy to get stuff.  ;-D  I'm not too worried about any excess
current draw from the 232A, cuz I'll be running a high voltage inverter (for
cold cathode tube) from the same 9V battery (if the noise isn't too bad).
Total running time with display backlight on will be about 1 hour before a
9V battery is dead, but it's enough for now.  Thanks again.

Michael Brown
Instant Net Solutions
http://www.KillerPCs.net

"In the land of the blind, he who has one eye is king"

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2002\03\09@134541 by Dave Dilatush

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Michael Brown wrote...

>Thanks Dave,
>
>This looks like the best solution for me.  I happen to have a 232A laying
>around, and I'm sure I can scrounge up the diodes and caps.  ;-)

You might want to consider using capacitors a bit larger than what I
showed.  The spec sheet for the 232A recommends 0.1 uF capacitors but I
suspect the output impedance of this circuit will be a little lower if
you used 0.47 uF or thereabouts; and that would help the output hold up
under load a little better.

Dave

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2002\03\09@184401 by Olin Lathrop

face picon face
{Quote hidden}

Umm, err, you didn't actually try this, did you?


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
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2002\03\09@191133 by Rick C.

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I tried it....

After the initial explosion all I could find of one of the transistors
were three leads sticking out of the breadboard!



:-0


Not really :-)

Something doesn't seem right though. Will have to look at it closer.
Rick

Olin Lathrop wrote:

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2002\03\10@072647 by Roman Black

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Olin Lathrop wrote:
{Quote hidden}

Ha ha! :o) Thanks guys, well I did say it was
done in a hurry, and no obviously not tested.
But i am curious as to what you see is the
problem with it? That it won't oscillate or that
Q1 (top) won't turn off? Or am I really thick
today and missing something? :o)
-Roman

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2002\03\10@090753 by Olin Lathrop

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{Quote hidden}

I was trying to encourage Roman to try it before anyone else spent time on
it.  I don't want to go into a detailed critique because last time we
discussed switching power supply circuits it degenerated into a fiasco.
People who couldn't even spell "NPN" crawled out of the woodwork and
proposed a whole parade of silly circuits that couldn't work and clearly had
never been tried.

All I'll say is consider the action of the top transistor if its emitter
were ever successfully pulled down to -15 by the inductor.


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2002\03\10@091347 by Dave Dilatush

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Roman Black wrote...

>Ha ha! :o) Thanks guys, well I did say it was
>done in a hurry, and no obviously not tested.
>But i am curious as to what you see is the
>problem with it? That it won't oscillate or that
>Q1 (top) won't turn off? Or am I really thick
>today and missing something? :o)

What's missing, Roman, is that you skipped a crucial step in designing
things like this: a walk-through of its operation, looking at the
voltages and currents that occur in each state to verify (1) that they
are what you need them to be and (2) that they are physically possible.

Had you done this, you would have discovered (referring to the redrawn
diagram below) that when Q2 turns on, the circuit node connecting the
emitter of Q1, the left end of L1, and the cathode of D1 cannot possibly
go more negative than [Vbe(Q1) - Vcesat(Q2)].  In order to function, and
deliver current to the -15V output, this node would have to go down to
-15.7 volts to forward-bias D1, and that it clearly cannot do because it
would require the collector of Q2 to go negative down to -15 volts which
would destroy Q2.

In other words, this circuit "fails by inspection" and a very brief
inspection, indeed, is all it should take to confirm that it doesn't do
the job.

Dave D.

 +9v------+----------------+
 In       |                |
          |                R2
      Q1  C                |
      NPN  B---------------+
          E                |
          |                |
          |                C  Q2
          +---L1--+---+---B   NPN
          |       |   |    E
          |       |   |    |
          - D1    C1  R1   |
          ^ 0.1uF |   |    |
          |       |   |    |
 Gnd------|-------+---+----+
          |       |        |
          |  10uF C2       ~ 15v
          |       |        ^ Zener
          |       |        |
-15v------+-------+--------+
 Out

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2002\03\10@095817 by Olin Lathrop

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> But i am curious as to what you see is the
> problem with it? That it won't oscillate or that
> Q1 (top) won't turn off?

I think it will start up as you intended.  The cap on the base of Q2 will
charge up and Q2 will turn on, which will turn off Q1, which forces the
inductor to suck current from somewhere else.  So far so good.  Obviously
you meant the inductor current to then come from the diode and contribute a
little negative charge to the -15V supply.  However, when the inductor
voltage gets negative, so does the emitter of Q1.  Even with Q2 off, the
base of Q1 will be driven when it gets below the Q2 collector on voltage,
around 200mV.  Q1 will therefore act like a -500mV voltage source on
attempts to pull its emitter lower, which will prevent the -15V from ever
being generated.


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(978) 742-9014, RemoveMEolinTakeThisOuTspamembedinc.com, http://www.embedinc.com

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2002\03\10@144059 by Peter L. Peres

picon face
Dave D., have you, or anyone else, tried to operate one or both of the
MAX232x RS232 drivers from its own switching frequency followed by a
doubler or triper to get even higher (or lower) voltage ? I have a project
where I could use 70Vdc at very low power. I think that this will work.
What do you think ? What does the MAX232 spec say about overloading the
outputs ?  Afair RS232 allows dead shorts on the outputs indefinitely.
True for MAX232 ?

Peter

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2002\03\10@172058 by Dave Dilatush

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part 1 1106 bytes content-type:text/plain; charset=us-ascii (decoded 7bit)

Peter Peres wrote...

>Dave D., have you, or anyone else, tried to operate one or both of the
>MAX232x RS232 drivers from its own switching frequency followed by a
>doubler or triper to get even higher (or lower) voltage ? I have a project
>where I could use 70Vdc at very low power. I think that this will work.
>What do you think ? What does the MAX232 spec say about overloading the
>outputs ?  Afair RS232 allows dead shorts on the outputs indefinitely.
>True for MAX232 ?

I haven't tried that, Peter, but I don't see why it wouldn't work
provided the loading is very light.  The MAX232 datasheet says the
outputs can withstand continuous short-circuit loads, so that's not a
problem.

The only possible difficulty I can see is that the internal charge pump
switching frequency might be too high for the transmit driver slew rate:
on the MAX232A I measured 120 KHz switching frequency.

But here's a circuit to try; it might be interesting to see what it
does.  (You'll have to find diodes/capacitors with sufficient voltage
rating.)

Dave D.



part 2 4059 bytes content-type:image/gif; name=hvpumper.gif (decode)


part 3 131 bytes
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2002\03\11@021938 by Alan Shinn

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IR (I think) makes a photovoltaic bias supply, you may need several to
get to -15V.
Has anyone mentioned a simple voltage doubler?

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2002\03\11@084611 by Roman Black

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Dave Dilatush wrote:
{Quote hidden}

Thanks Dave! Very nice looking circuit there too. :o)
R1 adjust (even a trimpot) should still give a
decent constant current control and the thing should
oscillate well and be pretty efficient even with a
flat 9v battery. :o)
-Roman

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2002\03\11@123551 by michael brown

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It works! It works! ;-))))))  Way to go Dave, it puts out -17.3V into an
open load (-15.3 at 1.5-2.0ma load).  This generates the a great looking
contrast without even adjusting it.  I used a regular 232N and 1.0uF caps
and tiny little RadioShack 1N914/4148 glass diodes.

Thanks again, it works great.  ;-D  Plus, the added benefit of the 232
running off of a regulated supply keeps the contrast looking good until the
9V battery is just about dead.

michael brown

Dave D the genius wrote:

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2002\03\11@170800 by Dave Dilatush

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Roman Black wrote...

>Thanks Dave! Very nice looking circuit there too. :o)
>R1 adjust (even a trimpot) should still give a
>decent constant current control and the thing should
>oscillate well and be pretty efficient even with a
>flat 9v battery. :o)

OK, Roman, we've each had our say on how this circuit of yours will
behave, and our accounts are obviously at odds: you say it should be
"pretty efficient", and I say its efficiency will be exactly zero
because the circuit's just going to sit there doing nothing.  If it
oscillates at all, it will be at far too low an amplitude to be useful.

Since neither of our opinions on what this circuit "should" do counts
anywhere near as much as what it actually does, and since you were the
one to propose it to the PICLIST, how about you do us the courtesy of
building your circuit, testing it and reporting back to us with the
results.

Dave D.

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2002\03\11@170954 by Dave Dilatush

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Michael Brown wrote...

>It works! It works! ;-))))))

OK, glad I could be of some help.

In the event your LCD module needs a bit more voltage at high/low
temperatures, let me know and we'll add another "compressor stage" onto
the charge pump to get you another 4 or 5 volts.

Dave D.

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2002\03\14@115245 by ards, Justin P

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Dave,

I found it difficult to judge your tone when suggesting Roman build and test
the cct, so just in case I have to add my $0.02.

Many members of the list appear to me brilliant, experienced, do the stuff
professionally and busy.

I am a hobbyist, (pipe) dreams of professional involvement (any jobs out
there, I'll sweep the floors just to be near some of this stuff)not so busy,
so if people are willing to read my requests for help and throw me a bone I
am willing to run with it even if it is untasted.

I have learnt alot this way (actually I have learnt how little I know), and
keen to race home and experiment with this cct.  In the past tid-bits have
seen me thru experimenting and solving my amateurish problems.  Even if they
don't work I have still learnt something.

Didn't Edison respond something like "Ah yes, but now I know a thousand
different ways it wont work so there are only so many possibilities
remaining one of which must work" and also along the lines of "90%
perspiration..." or was it 99%.

Just saying I am happy to do the perspiration and welcome those tid bits no
matter how inaccurate from any one and everyone willing to share their time.

Once again I may have misinterpreted your post, so hard to accurately convey
and interpret those subtle nuances  when trying to minimise the word count.

Justin


<snip>

Since neither of our opinions on what this circuit "should" do counts
anywhere near as much as what it actually does, and since you were the
one to propose it to the PICLIST, how about you do us the courtesy of
building your circuit, testing it and reporting back to us with the
results.

Dave D.

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2002\03\14@182811 by Dave Dilatush

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Hi Justin,

I understand your concerns; perhaps this will address them.

The issue here is one of context.

If the original poster had written the PICLIST expressing an interest in
learning about switching regulators and doing some experimenting with
them, and if he had asked for some suggestions on circuits he could play
around with for starters, this discussion would have gone differently.

But he didn't ask for that: he asked for help-- help in solving a
problem that he said was outside his particular area of expertise.
Although he didn't say so in so many words, I inferred from what he
wrote that he wasn't much interested in experimenting-- he wanted
something that he could rely on to get the job done.

And it's because of this context that Roman's offering got the rather
pointed comments it did from several of us who recognised the design as
fatally flawed.  Giving a person who has asked for help a circuit design
that absolutely cannot work, is a disservice; and this could have been
avoided had he tried out his circuit before posting it.

I don't think making distinctions between "hobbyists" and
"professionals" is all that useful: in the end, every one of us is a
newbie at something, and the trick is recognising what you're good at
and distinguishing it from what you're not good at (yet).

In my case, that "something" is PIC programming.  That's why I don't
give out very much PIC programming advice here: because I'm still as
dumb as a bag o' hammers when it comes to software, compared to the
experts on the PICLIST-- even those experts who consider themselves to
be "hobbyists".

On many hardware questions, particularly those that involve analog
circuit design, I can often dash off a quick circuit diagram and post it
to the PICLIST with near-total certainty that it will work exactly as
advertised.  But I won't post even the tiniest snippet of PIC code,
without having thoroughly tested it first.

Know what you know; and, more importantly, know what you don't know.

Dave D.


Justin wrote...

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2002\03\14@212036 by 859-1?Q?Alexandre_Guimar=E3es?=

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face
Hi, Dave

> On many hardware questions, particularly those that involve analog
> circuit design, I can often dash off a quick circuit diagram and post it
> to the PICLIST with near-total certainty that it will work exactly as
> advertised.  But I won't post even the tiniest snippet of PIC code,
> without having thoroughly tested it first.
>
> Know what you know; and, more importantly, know what you don't know.

   Sorry to disagree.... Some of us on the list do not have all the time we
would like to have to answer the questions and test the suggestions. It is a
common think for people that work on our area to have long 20 hours work
days and sometimes the piclist is a pleasure that is hard to keep. I have
used some of the "crazy" ideas that appear on the list on many ocasions,
both professionaly and in my hobby ( that happens to deal with the same
things I do for food :-) ). I do not have the time to really check all the
few answers I am able to give to the list but I think some of them have
helped people on the list.

   The kind of harsh words like you used in your message are, in my opnion,
one of the reasons why we have lost some very nice people that contributed a
lot to the list in the past. I am not saying in any way that you are wrong
technically but you could have expressed that in a nicer way. We are all
here to have fun with electronics and microprocessor not to figth with each
other for so little.

   Please do not take this as personal, it is not. You have been always a
very nice people ( to most of us ) but your message really sounded much
harsher than they needed to be.

   I hope I have not missed the point because of my poor english and wish
that all list members take the time to think how much we can hurt each other
fellings and "egos" when we write our messages. This is a community and in
my opnion a very nice one and we should be more "fraternal" with each other.

best regards,
Alexandre Guimaraes

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2002\03\14@222406 by ards, Justin P

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Dave,

you have made some good points.

I am keen not to restrict the free flow of information so perhaps we could
agree that it maybe ok to respond with a fanciful fatally flawed solutions
or for purely academic reasons if it was clearly documented as such.

This may often be of more service to others like myself than the person who
originally posted the request.

Regards Justin

{Original Message removed}

2002\03\14@225118 by Dave Dilatush

picon face
Alexandre,

I get the message, and I'll keep it in mind in the future.

Thanks,

DD

Alexandre wrote...

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2002\03\14@225532 by Dave Dilatush

picon face
Justin wrote...

>I am keen not to restrict the free flow of information so perhaps we could
>agree that it maybe ok to respond with a fanciful fatally flawed solutions
>or for purely academic reasons if it was clearly documented as such.

I agree completely; being very clear about when you're very sure
something will work (and can therefore help someone who has asked for
assistance) and when you're not sure something will work, is always a
good thing.

DD

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2002\03\15@025518 by Vasile Surducan

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On Fri, 15 Mar 2002, Dave Dilatush wrote:

>
> I agree completely; being very clear about when you're very sure
> something will work (and can therefore help someone who has asked for
> assistance) and when you're not sure something will work, is always a
> good thing.
>
 Unfortunately this "sure is working" and "no definitely it doesn't work"
is extremely relative. On this list are various people with large
aknowledgements variation ( from "how to connect an ammeter, in series or
in parallel..." to "how to improve a FFT performances into a DSP" )
so, you must admit, even you draw a schematic and explain carefully to
someone who never heard before about operational amplifiers, that power
supply rejection ratio and common mode rejection ratio are very important
things, he will never understood even is clear like daylight...
 Sometime this is piss you (and me ) off, you ( and me )are bored and the
best thing is to shut off the list in that moment and not argue with me ( or
you...)
 Some people, like Roman, have a gift: to explain with details something
I will never have patience to do. Other peoples ( like Dave or Olin ) are
very good professional, but haven't patience to explain details.
I understood them very well.
I hope you'll not be angry. The best thing for all is not to argue, but to
understood at the every end of the wire there is a living human being with
feelings, with various problems. Which can make mistakes or not.
But if you shut him, you don't do nothing good. ( this is valable for me
too...)
best regards to all,
Vasile

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