[GreenKeys] Polar Relay

[W2HX]

Well, it would seem to me that the ability to do things in parallel (voltage) instead of series (current) does have, as you point out, many benefits. There was an episode where I was pulling out my hair trying to understand why my loop was showing 0 current and nothing was working. After an hour I tracked it down to a ring terminal that I made that was intermittent. In a voltage-based environment, everything would have worked EXCEPT the faulty unit, greatly accelerating fault location!

-----Original Message-----
From: greenkeys-bounces at mailman.qth.net <greenkeys-bounces at mailman.qth.net> On Behalf Of Jim Haynes
Sent: Monday, February 6, 2023 9:40 PM
To: Tom Hunter <tomhenryhunter at gmail.com>
Cc: GreenKeys at mailman.qth.net
Subject: Re: [GreenKeys] Polar Relay

On Mon, 6 Feb 2023, Tom Hunter wrote:

> Eugene,
> the polar relay. I also believe a properly adjusted polar relay is 
> bistable (which would not matter in this situation.)
> 
Yes.  One of my first projects at Teletype was to work on, or at least think about, an electronic replacement for the polar relay.  That led to the low-voltage selector magnets of Model 32/33/35, but those were the solution to the wrong problem.  A real polar relay is terribly difficult to model electronically, especially in those days before we had opto- isolators.

In a real polar relay the windings are electrically isolated from the contacts and from each other.

A real polar relay is "side stable" meaning with no current in the windings the contacts will remain in whichever position they were before current was removed.

I think there is or was a paragraph in MIL-STD-188 which stated these were requirements for a replacement for a polar relay.

So actually I worked on a different problem, and came up with what I considered an excellent concept but was unable to sell it in the company.  The concept was that every selector magnet should have a selector magnet driver with an input something like RS-232.  RS-232 had not been created yet, but what I meant was with voltages, not current loops, inside the set.  So every keyboard or tape transmitter should similarly have a circuit for generating polar voltage signals.  What was different from RS-232 was that any signal source should be able to drive multiple loads.  What's beautiful about this is that it gets rid of the current loops inside the set, replacing them with voltage sources and sinks.  If you want a reperforator to copy what you are receiving you don't have to break open the printer current loop and insert the reperf selector; you just connect the reperf selector input to the "hub" that is driving the printer.  And there are things you can do with diode gates to simplify other switching internal to the set.
And you are all set up to add transistorized accessories such as a regenerative repeater to a set without having to go through current loop to voltage conversion and then back to current loop.  Then if the input to the set is a neutral or polar current loop go ahead and use the polar relay; it won't wear out with low voltages and currents on the contacts.

We did have unattractive ways of isolating circuits in the days before optoisolators.  The AN/FGC-5 time division multiplex, using tubes, used a gas diode enclosed in a solenoid as a relay.  With no current in the solenoid the gas tube would fire and conduct at the voltages applied to it from the loop.  With current in the solenoid the gas tube was unable to fire and made an open circuit.
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