[GreenKeys] What is a polar relay for?

[WA5CAB at cs.com]

Joe,

An ordinary DC relay uses some mechanical means, usually a spring, to set the 
OFF state of armature in the relay.  Running the proper current through the 
coil changes the relay to the ON state.  Turn the current off and the relay 
returns to it's OFF or resting state.  A polar relay armature moves one way if 
the current is of one polarity and the other if the polarity is reversed.  A 
polar relay (or at least some of them) can also be keyed by a neutral circuit if 
it has a bias coil to set the OFF state of the neutral loop.  They were 
commonly found in Teletype sets into the 50's, and in things like regenerative relay 
units or repeaters.

It is incorrect or misleading to say that a polar relay reverses the polarity 
of a landline.  Reversing the polarity on the landline switches the polar 
relay.  Although you could certainly use a polar relay to do the line reversal, 
you can also use a normal DPDT relay to do it.  But the normal DPDT relay would 
not be usable at the other end of the line.  It would just stay ON.

$100 is excessive.  They generally go for I'll say $10-$25 in as-is where-is 
condition.  Freshly calibrated they would probably be more but few have the 
equipment to do that and some who do lack the time.

In a message dated 12/29/2008 11:19:56 PM Central Standard Time, 
amourdutigre at kc.rr.com writes: 
> I have a question that I really cannot find an answer for.
>   
>  In my readings about electromechanical TTY units, I keep seeing something 
> called a polar relay being referred to. I googled this, and found that it is a 
> device that can be used to revers the polarity on a landline telegraph 
> circuit (not the local loop). Also, most of what little I could find had to do 
> with hand (Morse) telegraphy, nothing with TTY.
>   
>  I have even seen polar relays referred to in amateur RTTY material from the 
> fifties and sixties.
>   
>  What were these used for? I found one (I guess, the seller called it a 
> "rolar relay, I suspect a mistype) on e-pay for well over a hundred bucks.
>   
>  Thank you in advance,
>  
> 

Robert Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mailman.qth.net/pipermail/greenkeys/attachments/20081230/d7b6623f/attachment.htm