Connect with us

Drawing Power from an RS232 Port

Discussion in 'Electronic Basics' started by S. Ethier, Apr 11, 2005.

Scroll to continue with content
  1. S. Ethier

    S. Ethier Guest

    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.

  2. S. Ethier

    S. Ethier Guest

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

  3. aman

    aman Guest

    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.
  4. 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.

  5. S. Ethier

    S. Ethier Guest

    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?
  6. aman

    aman Guest

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

  8. mike

    mike Guest

    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.


    Return address is VALID but some sites block emails
    with links. Delete this sig when replying.
    Wanted, PCMCIA SCSI Card for HP m820 CDRW.
    FS 500MHz Tek DSOscilloscope TDS540 Make Offer
    Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
    Wanted 12" LCD for Compaq Armada 7770MT.
    Bunch of stuff For Sale and Wanted at the link below.
  9. GotCoffee

    GotCoffee Guest

    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?
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day