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Need help with circuit :

Discussion in 'Electronic Basics' started by coulyo, Sep 18, 2003.

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  1. coulyo

    coulyo Guest

    I was wondering if someone is a little familiar with the circuit
    which can be found at the following link

    If you scroll down about 40 percent there is a circuit on a yellow-ish
    background with squares, which is called COM84.

    If someone can find this circuit, my question is regarding the "power stealing"
    from a pc's serial port technique. Pin 3 (tx) of the serial port can be connected
    to a 78L05 componenet to generate a regulated voltage. How does this work ?
    I mean, any transmission from the tx pin will be a series of 0's and 1's . Is it
    like a switching power supply or what, I don't quite find the explanation ?
  2. Dana Raymond

    Dana Raymond Guest

    When the tx pin is +V the regulator is powered. When it is -V the diode
    protects the regulator, and it is unpowered.
    BTW I'm not sure about the absence of a capacitor on the Vin of the
    regulator. The bulk capacitance (100uF here) should be on the input and a
    smaller cap on the output, depending on the regulator.

    Dana Frank Raymond
  3. The RS-232 data pins typically swing between +12V (for a logic 0) and
    -12V (for a logic 1).

    While the TX line is at +12, the diode and 78L05 voltage regulator
    conduct, and charge the 100 uF capacitor. The capacitor will supply
    power to the rest of the circuit when the TX line is at -12.

    Personally, I'd probably add a series resistor and another 100 uF
    capacitor to ground, between the diode and regulator.
  4. John Fields

    John Fields Guest

    According to RS-232, the voltage on the TX pin can vary from 5 to 15
    volts for a "space" (0) and from -5 to -15 volts for a "mark" (1).

    What this circuit does is allow the positive voltage generated when the
    signal is a space to go into the regulator and appear at the output of
    the regulator as 5VDC. The 100µF capacitor on the output of the
    regulator is used as a reservoir and allows the circuit to which it is
    connected to keep operating (for a while) when the data goes to mark
    condition. The problem with the circuit is that there's no guarantee
    that it'll always work because there's no guarantee that the data signal
    during space will be greater than the dropout voltage of the regulator
    plus the forward voltage of the diode. The diode is in there to keep
    the input of the regulator from going more negative than the output,
    which would be a bad thing.
  5. coulyo

    coulyo Guest

    According to RS-232, the voltage on the TX pin can vary from 5 to 15
    1. Where are you finding this information for RS-232, is it online somewhere ?
    2. Would it be usefull to utilise the negative cycle ("space") as well
    (i.e. use another regulator for the negative) to have more current?
  6. coulyo

    coulyo Guest

    Personally, I'd probably add a series resistor and another 100 uF
    Is the series resistor to protect the serial port ?
  7. John Fields

    John Fields Guest

  8. No - an RS-232 driver should be able to drive into a short circuit
    without damage. However, since the TX data line is carrying data, I
    don't want to put an excessive load on it - the receiver should still
    see a high, even if the input capacitor is discharged.
  9. Google is your friend - go to, and search for
    RS-232 (or anything else you may be curious about)
    Yes - you could easily obtain -5 volts this way, using a 79L05
    regulator. Getting more current at +5 would be more difficult (but
    probably not impossible)
  10. Jim Large

    Jim Large Guest

    Ancient history! The current standard requires that
    receivers work with input voltages as as small as
    +/-3V. Commonly used EIA-232 interface chips
    generate +/- 5V.

    Twelve volt signalling is still acceptable (in fact,
    anything up to 25V is acceptable), but I don't think
    twelve has been NOMINAL since the early 1980s.

    -- Jim L.
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