Drawing Power from an RS232 Port

[S. Ethier]

I am interested in drawing power from an RS232 port so that it powers an
external circuit, only I am not too sure where to begin. The RS232 port
is not on the DTE side, but on the DTE side. I was able to measure
-7.59V DC and -25mA on the following PINs using a digital multimeter:
RI, DSR, DCD, CTS.

Is it possible to draw power from this DCE so that I have +3.3V DC at
approximately 140mA for my circuit? Let me know what you guys think.

Steph

 

[S. Ethier]

Sorry, I meant to say:
"The RS232 port is not on the DTE side, but on the DCE side."

Steph

S. Ethier wrote:

I am interested in drawing power from an RS232 port so that it powers an
external circuit, only I am not too sure where to begin. The RS232 port
is not on the DTE side, but on the DTE side. I was able to measure
-7.59V DC and -25mA on the following PINs using a digital multimeter:
RI, DSR, DCD, CTS.

Is it possible to draw power from this DCE so that I have +3.3V DC at
approximately 140mA for my circuit? Let me know what you guys think.

Steph

 

[Robert Scott]

On Mon, 11 Apr 2005 16:04:50 -0400, "S. Ethier"
<sethier@linuxmail.org> wrote:

I am interested in drawing power from an RS232 port so that it powers an
external circuit, only I am not too sure where to begin. The RS232 port
is not on the DTE side, but on the DTE side. I was able to measure
-7.59V DC and -25mA on the following PINs using a digital multimeter:
RI, DSR, DCD, CTS.

Is it possible to draw power from this DCE so that I have +3.3V DC at
approximately 140mA for my circuit? Let me know what you guys think.

No. RS-232 ports do not deliver that much current. If you got 25mA,
then you were lucky. I'll bet that the -7.59 v and -25mA did not
occur at the same time. You probably had to short the output to get
that much current.


-Robert Scott
Ypsilanti, Michigan

 

[budgie]

On Mon, 11 Apr 2005 16:04:50 -0400, "S. Ethier" <sethier@linuxmail.org> wrote:

I am interested in drawing power from an RS232 port so that it powers an
external circuit, only I am not too sure where to begin. The RS232 port
is not on the DTE side, but on the DTE side. I was able to measure
-7.59V DC and -25mA on the following PINs using a digital multimeter:
RI, DSR, DCD, CTS.

Is it possible to draw power from this DCE so that I have +3.3V DC at
approximately 140mA for my circuit? Let me know what you guys think.

Com ports vary enormously. I've had commercially designed interfaces looking
for 10mA at +5V from two handshake lines that would work on SOME pooters but not
others. Laptops are probably the worst in this regard, with rather wimpy
implementations.

But 140mA? No way. Read the EIA standard and see what a compliant DCE/DTE port
is expected to provide in the way of drive, and you'll understand why.

 

[Joerg]

Hello Steph,

Is it possible to draw power from this DCE so that I have +3.3V DC at
approximately 140mA for my circuit? Let me know what you guys think.

The only way I see to achieve 140mA for very short periods is if your
circuit could run on a low duty cycle and you provide a large cap that
is charged via a current regulator that limits things to a few mA.

Just curious: Why do you need so much? Can the circuitry be optimized
for lower power?

Regards, Joerg

http://www.analogconsultants.com