I'd suggest having a look at the MAX232, GD75232 and other RS232 output buffer/driver
chips.
The datasheets mostly contain an voltage/load characteristic that should give you and
idea
of how much current you can consume from one or more of the output buffers of an RS232
output.
One thing that might help is a set of circuits for interfacing *your* circuit to
RS232. Generally
speaking, a normal RS232 buffer/interface chip is not such a good idea :-) Using one
RS232 interface
chip to povide the power for another one tends to result in requesting too much power
from the
chip that provides the power...
I have had good results by generating RS232 signals from your device to a serial port
using a 74HC14
(nicely low-power) using some circuit like this:
1/6
74HC14 100ohm
signal >>----|>o------|||||--->> RS232
NB: a 5.1 Volts zener at the RS232 side is a good idea if you have the board space to
spare.... It
absorbs EMI and transients that the 74HC14 might not like..
An 74HC14 can also be used as RS232 input buffer (use diode/zener diode clamping and
some resistors...)
which gives you a quite low-power RS232 interface reciever circuit:
1/6
74HC14
+----|>o------>> signal
|
diode 10K | 100K
RS232 >>----|>|---|||||----+-----|||||-----| gnd
|
+------|<|------| gnd
4.7 Volts
zener
The schmitt-trigger inputs of the 74HC14 are used as anti-noise-suppression inputs
here..
I have used a circuit with a few (shottky) diodes from the RS232 port -> a capacitor +
one LM317
(the 100mA version) to get 5 Volts. A word of caution: some RS232 output circuits
(laptops mostly)
are limited to either 0 and 5 Volts , or -5V and +5V as 'RS232' outputs. That makes
them quite
troublesome when you want to use them as supply for a small circuit with 5 Volts.
MAX232 chips are (used to be??) quite liable to burn their voltage mutiplier when
their RS232 output
buffers are overloaded.
> -----Original Message-----
> From: Peter Vogel [mailto:[EMAIL PROTECTED]
> Sent: vrijdag 28 november 2003 7:29
> To: Multiple recipients of list CHIPDIR-L
> Subject: Stealing power from RS232 port
>
>
> Does anyone have any experience in using an RS232 port to power a
> periphereal device? The device includes a small PIC micro that will
> need to send data back to the computer it's plugged into, so I need to
> power the RS232 driver as well.
>
> Any practical advice such as how much current and voltage I
> can rely on
> will be appreciated.
>
> Peter Vogel
> ZapTV Pty Ltd
> 30 Adeline St, Faulconbridge 2776
> Australia
> Tel: 02 4751 8735
> Fax: 02 4751 2601
> email: [EMAIL PROTECTED]
>
> --
> Author: Peter Vogel
> INET: [EMAIL PROTECTED]
>
> Fat City Network Services -- 858-538-5051 http://www.fatcity.com
> San Diego, California -- Mailing list and web hosting services
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--
Author: Faasse, P.R.
INET: [EMAIL PROTECTED]
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