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'[EE]: Negative 8-12 volts from -8 or +5V'
2001\08\12@212956 by

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

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Hi Robert.

Use two diodes, one ceramic cap and one tantalum cap.
Drive them with some PWM freq. and measure somehow
output voltage. Changing PWM duty cycle will change
output negative voltage as well. PIC PWM module
and some small piece of software will do that trick
for you.

WBR Dmitry.

"Robert E. Griffith" wrote:
{Quote hidden}

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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:

{Quote hidden}

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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)?

>> 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?

Thanks Dimitry and Vasile for your advise.

--BobG

{Original Message removed}
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)

{Quote hidden}

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 removed}
> 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.

What you are calling a "periodic DC" signal is an AC and DC signal added
together.  For the purposes of a capacitive voltage doubler, the DC
component is irrelevant except that it effects the required voltage rating
of the capacitor.

********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinembedinc.com, http://www.embedinc.com

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