🚀 go-pugleaf

In article <5417@cbnews.ATT.COM>, knt@cbnews.ATT.COM (kirk.n.trost) writes:
> Does anyone have a simple circuit for pulse dialing a telephone
> without using a relay (i.e. non-mechanical)?
> I want to be able to control the pulsing via a TTL signal from
> a microprocessor.

	For all intents and purposes, a pulse-dialing circuit is ALSO the
circuit which determines whether the associated telephone, modem or whatever
is on-hook or off-hook.

	Your request is interesting because, offhand, I cannot recall ever
seeing a solid-state dialing circuit except in a self-contained telephone.
Every piece of externally-powered equipment that I can recall used a relay
for this purpose, with the relay type being either reed, mercury-wetted
reed or conventional "low-mass" contacts.

	Mind you, there is no reason why a solid-state circuit cannot be
used; it's just that several design criteria need to be met in order for
the circuit to be reliable.  While these design criteria are non trivial,
they are not overly difficult, either.  Most designers seem to say the
hell with it, and use a relay.  I must admit that I, too, fall in this
category and use a relay - even though I explicity understand the required
design criteria and am capable of designing such a circuit with minimum
effort.

	I am assuming that you wish to have a dial pulsing circuit that
is COMBINED with the on-hook/off-hook control function.  The design criteria
for a solid-state circuit is the same for both functions, and it would seem
contrary to your requirements to say, use a relay for the on-hook/off-hook
control.

	So, to answer your question, the following design criteria need to
be met for a solid-state dialing circuit (not necessarily in order of
importance):

1.	The portion of the circuit which actually connects to the telephone
	line must be optically isolated from the TTL control line and its
	power supply.  In my opinion, there is just no way around this
	requirement that will protect BOTH the telephone line AND the
	dialing apparatus from electrical fault.

2.	The switching device is a suitably-selected transistor.  I would
	consider an NPN silicon transistor with an hfe of at least 100 at
	150 mA; you want this transistor to saturate quickly and conduct
	with a minimum of base current.   The Vceo MUST be at least 200
	volts.  The collector current should be rated at least 250 mA to
	eliminate having to use a heat sink. 

	You are going to switch a current that varies between 25 mA on a
	very long CO loop to 125 mA if you are sitting right next to a
	PABX.  While a typical current may be more of a mean between
	these two values, you need to consider the whole range for a
	reliable, conservative circuit design.

	The Vceo rating is particularly important.  While the DC on-hook
	voltage of a telephone circuit is typically between 48 and 52
	volts, there are four circumstances which can result in the
	switching of MUCH higher voltages.  First, an AC voltage as
	high as 120 volts @ 20 to 30 Hz _superimposed_ upon 48 to 52
	volts DC can be present during the ringing cycle.  If the
	designed circuit is idle this transistor must withstand these
	voltages without getting upset and conducting through breakdown.
	Second, during dialing there may be inductive transients on the
	line as a result of the inductive reactance present in bridged
	mechanical ringers; these inductive transients can be as high
	as 120 volts.  Third, inductively and capacitatively coupled
	transients may exist on the telephone line as a result of
	lightning; these transients can exceed 200 volts DC.  Fourth,
	long central office loops may have loop extenders which can
	result in on-hook voltages as high as 100 volts DC instead 
	the usual 48 to 52 volts DC.

	You can and perhaps should protect the transistor with a metal
	oxide varistor (MOV) or a zener diode having a voltage rating of
	say, 180 volts DC.  The use of a MOV or zener is NOT a substitute
	for a Vceo of less than 200 volts.
	
	Under no circumstances do you want any false conduction of the
	transistor or its protective MOV or zener at a voltage less than
	150 volts since this may cause false ring tripping.
	
	While I have suggested a silicon NPN power transistor, you could
	consider a darlington device, a power MOSFET or a VMOS power device.

3.	The polarity of the telephone line voltage presented to the
	transistor above needs to be assured through the use of a full
	wave bridge rectifier circuit (i.e., telephone line in series
	with AC leads of bridge).

4.	The effective voltage drop across the dialing circuit should be
	less than 5 volts.  You are going to lose 2 volts across the
	bridge, so the transistor cannot have a collector-emitter drop
	of more than 3 volts with applied base current in the on state.

5.	The collector-emitter of an optoisolator will switch the base
	current to turn on the switching transistor.  BEWARE that in
	many circuit designs the optoisolator Vceo rating must be as
	high as the switching transistor itself.

	This optoisolator will be driven by the TTL signal through a
	suitable driver capable of providing 20 mA or so through the
	LED.  This optoisolator provides the requisite electrical
	isolation between the TTL circuit and the telephone line.

6.	Assuming an effective 60% break +/- 2% for the dial pulsing
	function at a dialing rate of 10 pps, the effective switching
	ON and OFF times of the dialing circuit should be well under
	2 milliseconds.  Your microprocessor or whatever will be
	required therefore to open the circuit for 60 ms followed
	by a 40 ms closure for one dial pulse; this gives a 60% break.

7.	The dialing circuit must also pass the voice frequency signal
	on the telephone line when it is in an off-hook state.  For a
	decent circuit design, the AC insertion loss between 300 and
	3,000 Hz should not exceed 0.2 dB.  The traditional way to
	assure this is to bridge a capacitor across the dialing circuit.
	HOWEVER, this requires some careful circuit design since you
	do not want to create an excessive transistor collector current
	by discharging a capacitor each time you go off-hook or dial
	pulse; a series resistor may be in order.  Also, the presence of
	too much capacitance will result in effective dial pulse distortion.

	This area can be the tricky part...

	Following the above design criteria will result in a good, reliable
and conservative circuit design.  As I said in the beginning of this
article, the design effort is non-trivial, but not that difficult, either.

	Most of these problems can be readily solved in one fell swoop using
a relay, which is why most people say screw it and do it that way. :-)

<>  Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
<>  UUCP:  {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry
<>  VOICE: 716/688-1231, 716/773-1700        {att|hplabs|utzoo}!/
<>  FAX:   716/741-9635, 716/773-2488      "Have you hugged your cat today?"