Maker Pro
Maker Pro

dual opamp & negative voltage

Hi All
I am getting -5V off a max232, however i need more current. Now i read
that putting a dual opamp buffer can help me here but when i tried
using lm328, with both omps acting as inverters, all i got was a
positive supply. Am i missing something here or using the chip
incorrectly somehow?
TIA
 
E

Eeyore

Jan 1, 1970
0
Hi All
I am getting -5V off a max232, however i need more current. Now i read
that putting a dual opamp buffer can help me here

Putting it where ? Why would that help ? Where did you read this ?

Graham
 
T

Tam/WB2TT

Jan 1, 1970
0
Hi All
I am getting -5V off a max232, however i need more current. Now i read
that putting a dual opamp buffer can help me here but when i tried
using lm328, with both omps acting as inverters, all i got was a
positive supply. Am i missing something here or using the chip
incorrectly somehow?
TIA
Are you sure you mean an LM328. That contains a positive step down voltage
regulator. Not sure what a buffer is going to do for you; you need to power
it from somewhere.

Tam
 
M

mpm

Jan 1, 1970
0
Hi All
I am getting -5V off a max232, however i need more current. Now i read
that putting a dual opamp buffer can help me here but when i tried
using lm328, with both omps acting as inverters, all i got was a
positive supply. Am i missing something here or using the chip
incorrectly somehow?
TIA

Do you mean to say you need more current on the RS232 Interface, or
are you trying to use the RS232 signal and/or handshaking lines to
power something else?? Please clarify.

Also, I believe a "truly compliant" RS232 device would have to output
-7 volts to -15 volts, and +7 to +15 volts.
Everyting in-between is "no-man's land".
I'm pretty sure I remember reading that in the RS232 book, but I also
know most receivers these days will work to half that number and
below.

Why do you need more current?
 
J

Jamie

Jan 1, 1970
0
Hi All
I am getting -5V off a max232, however i need more current. Now i read
that putting a dual opamp buffer can help me here but when i tried
using lm328, with both omps acting as inverters, all i got was a
positive supply. Am i missing something here or using the chip
incorrectly somehow?
TIA
With out knowing your connections. I'll guess.
You have connected the op-amps to the 5 volts + that
supplies the MAX chip ?
if so, that is all you'll get is 0..5 volts.
in order to get a -5 volts out, you need the op-amp
to operate from a +/- dual rail with respect to common.

if you look at the MAX chip, you'll notice it has an
inverter on it to give you the +/- rails for the chip
it self internally.
This current is just about enough for the standard specs.
but all is not lots how ever! :)
if you're operating from a single supply which I think you
are ? You can put one of your own little inverters on board.
either construct an osc via a higher wattage simple op-amp
that can drive a cap and be rectified for a - Volts or, get one
of those ready made DC-DC inverters capable of the current you need.
 
J

Jamie

Jan 1, 1970
0
mpm said:
Do you mean to say you need more current on the RS232 Interface, or
are you trying to use the RS232 signal and/or handshaking lines to
power something else?? Please clarify.

Also, I believe a "truly compliant" RS232 device would have to output
-7 volts to -15 volts, and +7 to +15 volts.
Everyting in-between is "no-man's land".
I'm pretty sure I remember reading that in the RS232 book, but I also
know most receivers these days will work to half that number and
below.

Why do you need more current?
Oh don't go there unless you want the young engineers to attempt
chopping off your weenie when you state the proper output voltages
for 232 as they should be from years ago.
Of course, many are use to the marketing schemes used these days to
cut corners and end up getting serial ports on devices that barely
output 5 volts..That saves on parts for devices that don't have the
supplies internally for the proper voltages like laptops etc..

so there you go!

Glad to see that some one knows the difference.
also, the other problem is current. it use to be that 20 ma's was the
max output before fold back.. now, it seems that 20 ma's is what you
get at short circuit in many implements.

oh well.. It's all falling apart in a hand basket!
 
Top