Need help with circuit :

C

coulyo

Guest
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.
 
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


"coulyo" <nomail@btinternet.com> wrote in message
news:bkd4na$e79$1@hercules.btinternet.com...
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.
Click to expand...
 
On Thu, 18 Sep 2003 20:31:38 +0000 (UTC), "coulyo"
<nomail@btinternet.com> wrote:

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.
Click to expand...
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.


--
Peter Bennett VE7CEI
GPS and NMEA info and programs: http://vancouver-webpages.com/peter/index.html
Newsgroup new user info: http://vancouver-webpages.com/nnq
 
On Thu, 18 Sep 2003 20:31:38 +0000 (UTC), "coulyo"
<nomail@btinternet.com> wrote:

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 ?
Click to expand...
---
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 100ľF 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.

--
John Fields
 
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).
Click to expand...
1. Where are you finding this information for RS-232, is it online somewhere ?

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 100ľF 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.
Click to expand...
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?
 
Personally, I'd probably add a series resistor and another 100 uF
capacitor to ground, between the diode and regulator.
Click to expand...
Is the series resistor to protect the serial port ?
 
On Fri, 19 Sep 2003 16:10:04 +0000 (UTC), "coulyo"
<nomail@btinternet.com> wrote:

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).

1. Where are you finding this information for RS-232, is it online somewhere ?
Click to expand...
---
http://www2.rad.com/networks/1995/rs232/pins.htm#f2
---


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 100ľF 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.

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?
Click to expand...
---
The negative part of the signal is referred to as the marking condition
and is called "mark" or "1", not "space".

No.

--
John Fields
 
On Fri, 19 Sep 2003 16:12:24 +0000 (UTC), "coulyo"
<nomail@btinternet.com> wrote:

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

Is the series resistor to protect the serial port ?
Click to expand...
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.



--
Peter Bennett VE7CEI
GPS and NMEA info and programs: http://vancouver-webpages.com/peter/index.html
Newsgroup new user info: http://vancouver-webpages.com/nnq
 
On Fri, 19 Sep 2003 16:10:04 +0000 (UTC), "coulyo"
<nomail@btinternet.com> wrote:

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).

1. Where are you finding this information for RS-232, is it online somewhere ?
Click to expand...
Google is your friend - go to http://www.google.com, and search for
RS-232 (or anything else you may be curious about)

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?
Click to expand...
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)




--
Peter Bennett VE7CEI
GPS and NMEA info and programs: http://vancouver-webpages.com/peter/index.html
Newsgroup new user info: http://vancouver-webpages.com/nnq
 
Peter Bennett wrote:

The RS-232 data pins typically swing between +12V
(for a logic 0) and -12V (for a logic 1).
Click to expand...
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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