[GreenKeys] Loop electronics - a request for assistance

[Ralph Mowery]

----- Original Message ----- 
From: "gil smith" <gil at baudot.net>
To: "Don Robert House" <drhouse at nadcomm.com>
Cc: <greenkeys at mailman.qth.net>
Sent: Monday, January 09, 2006 12:20 AM
Subject: Re: [GreenKeys] Loop electronics - a request for assistance


> Hi Don:
>
>  From this description, I would presume that the loop keyer is an NPN
> transistor to ground.  They say it is open-collector, so it will be
> grounding the loop to complete the circuit.  Since there is no mention of
> opto-isolation, there is no floating return output.  Be sure to provide
> current-limiting resistance or you will blow the transistor.
>
> I would connect the negative side of the loop supply to their circuit
> ground (pins 10/11/12/13/14), connect the positive side of the loop supply
> to one side of the tty, connect the other side of the tty to one side of
> the loop resistor, and connect the other side of the loop resistor to the
> open-collector output (pin 15).
>
> Try it with a low-volt supply first -- no tty, just the supply and the
> resistor.  Maybe a 12VDC adapter and a 10K resistor (1/8 watt is plenty),
> for a measly 1.2 ma that won't hurt anything.  Connect a scope ground to
> their circuit ground, and connect the scope to the transistor output -- 
you
> should see a nice clean keying waveform from 0V (mark) to 12V (space).  If
> you don't have a scope handy, but you can set the unit's output to a
static
> space or mark, put a meter on it -- you should see 12V (no current) for
> space, and 0V (current flowing) for mark.  Remove the meter or scope
before
> connecting a high-voltage supply.
>
> You can now try the full HV tty if you'd like, or you can test a bit more
> incrementally -- eg: drop the resistor to 200 ohms (at 1W) to see if it is
> sinking 60-ma properly (provided your 12VDC source can provide it).  If
all
> is well you can put a full loop supply on it, with the appropriate
resistor
> (eg: 120V with  2000-ohm/10W, 60V with 1000-ohm/5W...)
>
> Be careful with those nasty loop voltages!  I'd opt for 60 to 80V to keep
> it a bit safer, and keep the heat down, and I'd bet that even 30V would be
> just fine for most machines.  I have not seen a tty yet that needed 120V.
>
> gil
>

Gil, I have to agree with Robert.  Maybe for a local loop you can run low
voltages, but for the older 15 gear there is several resons to run 100 to
150 volts on the loop especially if comming off the air via radio.  The main
one as Robert mentioned is the range.  It has to do with the dv/dt or how
fast the coil current can rise to pull in the magnets.  Lower voltages take
too long to get enough current in the loop .   Also you need a high enough
voltage to keep the dirt and oil burnt off the contacts of the keys.

de KU4PT