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

 

[aman]

First of all max current RS232 can supply is 50mA. That means if you
need more current you will require an amp with an external power
supply. Also RS232 voltage levels do not include 3.3V

So think about some voltage conversion also.

 

[Michael Black]

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

It doesn't matter which side it's on, since each pin on the interface
is either sending or receiving, and all you need is to look at the outputs.
That sounds like too much current you want, but simply look up the
line drivers inside the device, and see how much current they are talking
about supplying.

The trick when this was done is to put diodes on the output lines, so you
can combine the signals.

Michael

 

[S. Ethier]

First of all max current RS232 can supply is 50mA. That means if you
need more current you will require an amp with an external power
supply. Also RS232 voltage levels do not include 3.3V

So think about some voltage conversion also.

Dosent the current supply depend on the RS232 line driver? I also
realize that the RS232 level does not include 3.3V but couldn't the use
of a regulator be used to lower the voltage to 3.3V?

 

[aman]

Use voltage regulator for positive voltage. Be careful about RS232
negetive voltages. Some regulator dont like negetive voltages.So you
have to have some circuitry to take care of these. Also use amp for
current with external supply. This will solve both voltage and current
problems.

 

[Michael Black]

Use voltage regulator for positive voltage. Be careful about RS232
negetive voltages. Some regulator dont like negetive voltages.So you
have to have some circuitry to take care of these. Also use amp for
current with external supply. This will solve both voltage and current
problems.

At which point he doesn't need an amplifier, since the external supply
will furnish the power.

Michael

 

[mike]

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

There are a bunch of issues here.
RS-232 is a STANDARD, but hardly anybody follows it.
If you have a dedicatd device, you can just measure it.
If you want to work on ANY RS-232 port, you'll find that there
are a bunch of devices that use +/-5V. +5V and ground often works too.
When I try to do this, I limit the current to 10mA and use a 5V
regulator...and it doesn't work on all my laptops.

mike

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[GotCoffee]

Why not pull the power from the 5V lead from the power supply or the 5V
from a USB port, if you have a USB port?