On Mon, 13 Aug 2001, Robert E. Griffith wrote:

> I thought I would need an AC (plus and minus) signal to drive a doubler
> bridge, but I guess what you are saying is that a periodic DC signal is just
> as good.  I am using both the PWM capable outputs for other things, but
> maybe I can juggle.  Is there another output on a PIC16F877 that can do a
> straight oscillator output (no PWM)?
>

Any pic pin can be used as oscillator output. The interrupt would not
affect too much the voltage if you choose capacitors value for minimum
output frequency. A buffer will allow you a good command ( resistor from
pic to npn base, resistor in colector, emiter to ground, tripler from
colector through a capacitor to ground)


> >> If you try to double the negative
> >> output of the max 232 you'll have
> >> some problems, first one is that voltage
> >> is already doubled and have a small
> >> output current capability.
>
> I need 2ma @ the doubled voltage, so that's 4ma at the input plus the
> inefficiency of the doubler.  I just checked the MAX232 data sheet to find
> that the typical output current is 22ma (that seems good) but the minimum is
> only 7ma (that's cutting it pretty close). I will give it a try and measure
> the input current (if it works at all).
>
> >> Try it with a simple doubler bridge
> >> ( but disconnect any capacitor from
> >> the max232 negative pin to ground before )
>
> So are you saying that if I remove that cap the MAX232 Tx output would
> oscilate making it suitable as input to a doubler bridge?

I'm not sure I've understood. The tx has nothing to do with this.
As I measure, you could take the oscillation direct from pins who have
doubler capacitor ( capacitor C1 between two max232 pins or capacitor C2
to ground for negative voltage) then you have a ripple voltage enough to
be tripled. But it's possible to not work.
I'll try to remove C1 and use that oscillation for a normal tripler. Also
you coud decrease the value of C1 and completely remove C2 and connect
from this output pin a voltage doubler.


> Thanks Dimitry and Vasile for your advise.
>
> --BobG
>
>
> -----Original Message-----
> From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On
> Behalf Of Vasile Surducan
> Sent: Monday, August 13, 2001 3:51 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE]: Negative 8-12 volts from -8 or +5V
>
> I really don't understood why you need an IC doubler when you can do it
> with or without using one pic pin. If you try to double the negative
> output of the max 232 you'll have some problems, first one is that voltage
> is already doubled and have a small output current capability.
> Try it with a simple doubler bridge ( but disconnect any capacitor from
> the max232 negative pin to ground before )
> Better is Dimitry's solution, or one pic pin as oscilator ( 1 to 10kHz is
> ok ) a npn buffer with a small resistor in colector ( 100...200 ohm) to
> +5V and a network tripler ( 3 diodes and 3 47...100 micro capacitors )
> connected for your desired polarity.
>
> Cheers, Vasile
>
> On Sun, 12 Aug 2001, Robert E. Griffith wrote:
>
> > Hello,
> >
> > Does anyone know of a simple, cheap DC-DC negative voltage doubler that
> can
> > double the -8.5 of a MAX232 to at least -12v at 2ma?
> >
> > Or, do you know of an inverting voltage tripler that would take a +5v
> > to -15v (less the V drop across the tripler)?
> >
> >
> >
> > Details...
> >
> > I got some help on this topic a couple of weeks ago that pointed me in the
> > right direction. After researching it on and off for these weeks I have
> > learned a lot and have several solutions, but I thought I would pursue
> this
> > one last way before settling on one.
> >
> > I am creating PIC base LCD circuit that has a +5v regulated supply coming
> > in.  The LCD needs a -8v to -12v bias at less than 2ma. The optimum value
> > varies from LCD to LCD so it needs to be set with a pot. Since the current
> > requirement is so low I can use a simple resistor and pot passive voltage
> > divider to adjust the voltage down, if I can just get a voltage equal or
> > less (more) than-12v.
> >
> > I can get -8.5v from the unused half of a MAX232 on the board.  Too bad
> the
> > MAX232 does not drive the output to RS232's full +-12v spec!
> >
> > So it seems that all I have to do it double that -8.5v from the MAX232
> > to -17v (less the V drop across the doubler) and then drop it down with a
> > passive voltage divider.
> >
> > What is a simple way to double a negative voltage?
> >
> > I am assuming that since I only have DC to start with that I cannot build
> a
> > simple discrete solution.
> >
> > The only negative voltage doubler I have found is the LT1045 and it is a
> > little more expensive than I would like (about twice the cost of other
> DC-DC
> > Ics I have looked at like the LTC1044) and it requires a lot of external
> > components when acting as a negative doubler.
> >
> > Does anyone know of a simple, inexpensive (<$2 in quantity 100) IC? I have
> > mainly looked into IC DC-DC inverters/doublers from Maxim, National, Toko
> > and TI.
> >
> > Thanks,
> >
> > --BobG
> >
> > --
> > http://www.piclist.com hint: The PICList is archived three different
> > ways.  See http://www.piclist.com/#archives for details.
> >
> >
>
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