# Need help with circuit :

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

1. ### coulyoGuest

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

http://www.rentron.com/Myke4.htm

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 ?
Tia.

2. ### Dana RaymondGuest

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. ### Peter BennettGuest

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 FieldsGuest

---
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. ### coulyoGuest

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. ### coulyoGuest

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

8. ### Peter BennettGuest

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. ### Peter BennettGuest

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 LargeGuest

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.