Use of MAX232 as power supply for operational amplifier in 5V powered system?

[Wojciech Zabolotny]

Hi,

I need to add simple amplifier to my digital system which is powered
from a single 5V battery.
To achieve proper operation of the operational amplifier I need at least
+8V and -8V voltages.
I can build a simple voltage multiplier and inverter like shown here:
http://upload.wikimedia.org/wikipedia/en/thumb/b/ba/Stacked_Villard_cascade.svg/400px-Stacked_Villard_cascade.svg.png
using the 6-CMOS inverter chip as an oscillator and a source of alternating
voltage.

However antoher idea is to use something cheap and standard like MAX232.
Has anybody tried to use this chip just as a voltage converter?
Unfortunately I can't find the output characteristics for V+ and V- pins.
Maybe I'll need also additional voltage regulator to improve the quality of
this power supply voltage (or maybe simple RC filter will be sufficient...).

Has anybody tried to do it?
--
TIA & Regards,
WZab

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[Steve Smith-2]

I have used this as a gate drive supply for IGBT's the +/- are pretty much
symmetrical and have their own bypass caps as part of the max. I uses 10uf
tants in addition to the 100n specified just hold a bit of energy.

Steve

-----Original Message-----
From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf Of
wzab
Sent: 11 October 2009 14:29
To: piclist@...
Subject: [EE] Use of MAX232 as power supply for operational amplifier in
5Vpowered system?

Hi,

I need to add simple amplifier to my digital system which is powered
from a single 5V battery.
To achieve proper operation of the operational amplifier I need at least
+8V and -8V voltages.
I can build a simple voltage multiplier and inverter like shown here:
http://upload.wikimedia.org/wikipedia/en/thumb/b/ba/Stacked_Villard_cascade.
svg/400px-Stacked_Villard_cascade.svg.png
using the 6-CMOS inverter chip as an oscillator and a source of alternating
voltage.

However antoher idea is to use something cheap and standard like MAX232.
Has anybody tried to use this chip just as a voltage converter?
Unfortunately I can't find the output characteristics for V+ and V- pins.
Maybe I'll need also additional voltage regulator to improve the quality of
this power supply voltage (or maybe simple RC filter will be sufficient...).

Has anybody tried to do it?
--
TIA & Regards,
WZab

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[Olin Lathrop]

wzab wrote:
> I need to add simple amplifier to my digital system which is powered
> from a single 5V battery.
> To achieve proper operation of the operational amplifier I need at
> least +8V and -8V voltages.

Why not use a amplifier that works on 0-5V?


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[Dario Greggio (in giro)]

wzab ha scritto:
> Hi,
>
> I need to add simple amplifier to my digital system which is powered
> from a single 5V battery.
>[...]
> However antoher idea is to use something cheap and standard like MAX232.
> Has anybody tried to use this chip just as a voltage converter?

I used it recently to power a LCD graphic module which needed some
-5..10V to operate.

I expect the current to be in the range of the 5..10mA - indeed one can
try it easily.
I also expect it to be a bit noisy for an OpAmp - but no worse than a
DC-DC like the one you suggest.

Dario
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[Zona-3]

HI:

as you say, you need a SIMPLE amplifier, so why you choice an amplifier which need +-8V power supply?
because of your 5V battery, if you use an +-8V power multiplier(no matter by the one reffered by wiki or with 232chips), the battery life need to  be caculated.
And last, if the amplifier is unable to change, you can study the 232 chips to make sure how many 232 client the chip can drive(it is different between MAX232 or LTC232 or DC232 bra,bra...), by this way you can caculate the max current ability of +-12V power. About the RC filter, em... maybe a big cap is better.
Only some idea, and you need to do experiment to verify.

/Zona
On Sun, 11 Oct 2009 15:29:09 +0200
wzab <wzab@...> wrote:

> Hi,
>
> I need to add simple amplifier to my digital system which is powered
> from a single 5V battery.
> To achieve proper operation of the operational amplifier I need at
> least +8V and -8V voltages.
> I can build a simple voltage multiplier and inverter like shown here:
> http://upload.wikimedia.org/wikipedia/en/thumb/b/ba/Stacked_Villard_cascade.svg/400px-Stacked_Villard_cascade.svg.png
> using the 6-CMOS inverter chip as an oscillator and a source of
> alternating voltage.
>
> However antoher idea is to use something cheap and standard like
> MAX232. Has anybody tried to use this chip just as a voltage
> converter? Unfortunately I can't find the output characteristics for V
> + and V- pins. Maybe I'll need also additional voltage regulator to
> improve the quality of this power supply voltage (or maybe simple RC
> filter will be sufficient...).
>
> Has anybody tried to do it?
> --
> TIA & Regards,
> WZab
>
> --
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> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>

--
BR.
----
Zona <zano@...>

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[William "Chops" Westfield]

On Oct 11, 2009, at 6:29 AM, wzab wrote:

> However antoher idea is to use something cheap and standard like  
> MAX232.
> Has anybody tried to use this chip just as a voltage converter?

I've seen it done, somewhere.   I don't remember the details.
Here's one:  http://www.robot-electronics.co.uk/htm/srf04tech.htm

BillW

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[Jinx-4]

> However antoher idea is to use something cheap and standard like
> MAX232. Has anybody tried to use this chip just as a voltage converter?

Hi WZab. Yes, I've done this and it worked fine (for me)

http://home.clear.net.nz/pages/joecolquitt/white_noise.html

The +/- 8V is taken from a MAX232 between the PIC (a random
number generator) and a PC

> Unfortunately I can't find the output characteristics for V+ and V- pins

As there as so many clones and variants it might be best to pick a chip
and run some tests. If the only significant load is an op-amp then RC
filters and/or reservoir caps will probably be sufficient. Otherwise there
are LDO regulators with low quiescent currents for providing stable rails

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[Jinx-4]

> To achieve proper operation of the operational amplifier I need at least
> +8V and -8V voltages.

WZab, is that really necessary ? I've several applications using +/- 5V rails

http://home.clear.net.nz/pages/joecolquitt/lo_volt_DC.gif

Vee is got from a simple diode-capacitor inverter (which gets very close
to - Vcc) and an oscillating PIC pin

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[Olin Lathrop]

Steve Smith wrote:
> I
> uses 10uf tants in addition to the 100n specified just hold a bit of
> energy.

That's outside the datasheet spec if I remember right.  They give both a
upper and lower limit on the caps.  Violating either is bad design.

10uF 10V tantalum caps are also relics of a bygone era.  Nowadays we use
ceramics, which are cheaper, more reliable, less sensitive to abuse, and
have better electrical characteristics.


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[Peter Restall]

On Sun, 11 Oct 2009 15:29:09 +0200, wzab wrote:

> However antoher idea is to use something cheap and standard like MAX232.
> Has anybody tried to use this chip just as a voltage converter?
> Unfortunately I can't find the output characteristics for V+ and V- pins.
> Maybe I'll need also additional voltage regulator to improve the quality of
> this power supply voltage (or maybe simple RC filter will be sufficient...).
>
> Has anybody tried to do it?

I remember briefly playing about with this idea, but as you've found out
the output capabilities aren't too well specified.  Since it's a charge pump
there's also a fair bit of high-frequency noise in the output (probably not
too good if you're trying to do a low-noise op-amp application) and the
voltage seemed to droop quite quickly once any appreciable current was drawn
(and it was always less than 2x the supply for no loading anyway, usually
7-8V for a 5V input).  You might get away with this approach for a one-off or
for high-impedance loads (MOSFET gate drivers ?) without driving any RS232
lines as well, but I dare say the difference between units is marked if
you're planning on multiple devices.

If you're willing to spend a few sheckles on a MAX232, why not just spend
that much on a dedicated IC with better (and more defined) output
characteristics ?  Or if you've got a spare pin from your PIC (presuming
you're using one) then how about spitting out some PWM and using a few
jellybeans to make a buck-boost - I've recently done just this to get a -5V
supply for my op-amps ?

Regards,

Pete Restall
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[M.L.-2]

On Mon, Oct 12, 2009 at 1:10 PM, Peter Restall <pete@...> wrote:
>
> If you're willing to spend a few sheckles on a MAX232, why not just spend
> that much on a dedicated IC with better (and more defined) output
> characteristics ?  Or if you've got a spare pin from your PIC (presuming
> you're using one) then how about spitting out some PWM and using a few
> jellybeans to make a buck-boost - I've recently done just this to get a -5V
> supply for my op-amps ?
>

He could use a TC7660:
<http://ww1.microchip.com/downloads/en/DeviceDoc/21467a.pdf>

See page 6 for a bipolar supply using one chip.

It's not going to boost the voltage with a single chip, but at least
it's designed to be a power supply.

In my experience, using a charge pump converter to power an analog
circuit is just not a great idea.
If it is possible to make your circuit use a single-supply analog
circuit, it would be a much nicer solution.

--
Martin K.

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[Dwayne Reid]

At 07:35 AM 10/12/2009, Olin Lathrop wrote:

>10uF 10V tantalum caps are also relics of a bygone era.  Nowadays we use
>ceramics, which are cheaper, more reliable, less sensitive to abuse, and
>have better electrical characteristics.

That said: there are still many applications where tantalum
capacitors are a better choice than ceramic - mostly audio-type
stuff.  One of the major problems associated with ceramic capacitors
in audio applications is microphonics.

But ceramic capacitors are my preferred choice for most of my SMPS stuff.

dwayne

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[phone] voice          [phone] fax
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Custom Electronics Design and Manufacturing

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[Forrest W Christian-2]

As others have said, why not just use a 0 to 5V opamp.   I use a MCP6024
in a lot of my circuits (2.5V to 5.5V power supply, rail to rail inputs
and outputs).

And, in case you are worried about measuring stuff outside of the 0 to
5V range on the input, you can usually level shift with resistors on the
front end of the OPAMP.

Lots cheaper than a +- power supply.

-forrest

wzab wrote:

> Hi,
>
> I need to add simple amplifier to my digital system which is powered
> from a single 5V battery.
> To achieve proper operation of the operational amplifier I need at least
> +8V and -8V voltages.
> I can build a simple voltage multiplier and inverter like shown here:
> http://upload.wikimedia.org/wikipedia/en/thumb/b/ba/Stacked_Villard_cascade.svg/400px-Stacked_Villard_cascade.svg.png
> using the 6-CMOS inverter chip as an oscillator and a source of alternating
> voltage.
>
> However antoher idea is to use something cheap and standard like MAX232.
> Has anybody tried to use this chip just as a voltage converter?
> Unfortunately I can't find the output characteristics for V+ and V- pins.
> Maybe I'll need also additional voltage regulator to improve the quality of
> this power supply voltage (or maybe simple RC filter will be sufficient...).
>
> Has anybody tried to do it?
>  

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[Richard Pytelewski-2]

Didn't see the original thread but,

If you have + voltage source the good old 555 can give you a negative
voltage at more ma than the 232.  

I Googled this one:    http://www.csgnetwork.com/ne555c1.html       it'll
give you a start. And 555's are cheap.  I have made negative power supplies
and been able to get clean (-) voltage out of it with filtering.

Rich

-----Original Message-----
From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf Of
Peter Restall
Sent: Monday, October 12, 2009 1:10 PM
To: piclist@...
Subject: Re: [EE] Use of MAX232 as power supply for operational amplifier in
5V powered system?


On Sun, 11 Oct 2009 15:29:09 +0200, wzab wrote:

> However antoher idea is to use something cheap and standard like MAX232.
> Has anybody tried to use this chip just as a voltage converter?
> Unfortunately I can't find the output characteristics for V+ and V- pins.
> Maybe I'll need also additional voltage regulator to improve the quality
of
> this power supply voltage (or maybe simple RC filter will be
sufficient...).
>
> Has anybody tried to do it?

I remember briefly playing about with this idea, but as you've found out
the output capabilities aren't too well specified.  Since it's a charge pump
there's also a fair bit of high-frequency noise in the output (probably not
too good if you're trying to do a low-noise op-amp application) and the
voltage seemed to droop quite quickly once any appreciable current was drawn
(and it was always less than 2x the supply for no loading anyway, usually
7-8V for a 5V input).  You might get away with this approach for a one-off
or
for high-impedance loads (MOSFET gate drivers ?) without driving any RS232
lines as well, but I dare say the difference between units is marked if
you're planning on multiple devices.

If you're willing to spend a few sheckles on a MAX232, why not just spend
that much on a dedicated IC with better (and more defined) output
characteristics ?  Or if you've got a spare pin from your PIC (presuming
you're using one) then how about spitting out some PWM and using a few
jellybeans to make a buck-boost - I've recently done just this to get a -5V
supply for my op-amps ?

Regards,

Pete Restall
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