Date: Sun, 27 Oct 2002 14:07:06 -0800 (PST) From: Matthew Dillon <dillon@apollo.backplane.com> To: Terry Lambert <tlambert2@mindspring.com> Cc: David Nicholas Kayal <davek@saturn5.com>, nbari@unixmexico.com, freebsd-hackers@FreeBSD.ORG Subject: Re: i am looking for a 5 volt signal Message-ID: <200210272207.g9RM76sX091839@apollo.backplane.com> References: <Pine.BSF.4.44.0210270918380.364-100000@blackbox.yayproductions.com> <200210271919.g9RJJFEm091313@apollo.backplane.com> <3DBC42AF.C68F31B0@mindspring.com>
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:Matthew Dillon wrote:
:> Uh guys. Parallel port digital outputs do not generally have a whole
:> lot of drive. I really doubt a parallel port output could drive a
:> relay.
:
:Depends on the amperage the relay draws. 8-).
:
:I used to use the paralell port output to drive a pulse dialer and
:"off hook" for a modem that didn't have the capability itself (a
:300 baud accoustic couple modem). One of the reasons I laugh when
:I see the "war dialer" scene in War Games.
:
:That said, you damn well better not draw any real current from the
:parallel port, if you want to avoid cooking it. In other words, do
:*not* use it to power something external, and if you are using it
:for a 1/0 signal to something that wangts to switch a lot of volts
:and amps from a real power supply, consider using an SCR, instead.
:
:-- Terry
Huh. I would have expected you to use the current loop on the
phone line to power the dialer. There's a significant amount
of power available there, though you would have to isolate the
circuit since the common mode could be upwards of one or two
hundred volts.
In anycase, I can only concur on the danger of ganging the parallel
power output bits. A single chip is driving those outputs and is
almost certainly not rated to deal with a maximal current draw from
all of them at once for long periods of time. It could fry. Though
if it's a TTL output there isn't much current there anyway (TTL can
sink a lot of current but it can't source it, the equivalent
resistance to VCC is fairly high).
Serial port pins are a much better bet. Serial port outputs can handle
shorts far better then parallel port outputs and since the voltages
are higher you can draw a good chunk of current out of them. Use diodes
to rectify the serial port outputs (be sure to orient them properly,
BAR side is positive. Flip the orientation for the negative supply).
-o--------------o---- positive supply to circuit
| (bar) | (around +10V)
DIODE DIODE
| |
serial-pin serial-pin
| |
DIODE DIODE (OPTIONAL IF YOU NEED A NEGATIVE
| (bar) | SUPPLY)
-o--------------o---- negative supply to circuit
(around -10V)
Then:
positive supply to circuit -----[LINREG]---- +5V to circuit
Use a 100uF (or more), a 10uF, and 0.1uF capacitor on the input
side of the regulator and a 0.1uF capacitor on the output side of
the regulator. The 0.1's should be as close to the linear regulator
as possible. i.e.:
-------supply--------o-----o----o----[LINREG]------o---- +5V supply
| | | 5V |
CAP CAP CAP CAP
| | | |
gnd gnd gnd gnd
100 10 0.1 0.1
You can gang the serial outputs (by wire-dioding them together).
The driver chips are far better able to handle that sort of thing,
and since the output is +12V (well, usually +9 to +12) you can pull
more current before it drops down to < 5V, so you can power
5V circuitry from it (up to a point). I expect you can get at least
20mA @ 5V out of ganged serial port outputs, possibly more.
You could use an SCR but linear regulators are far, far better
(a little 3-pin TO92 package linear regulator). And linear regulators
have overcurrent protection as well (for themselves, not for whatever
is powering them). It's fairly difficult to blow one up.
-Matt
Matthew Dillon
<dillon@backplane.com>
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