[GreenKeys] What is a polar relay for?

[Jeffrey D Angus]

amourdutigre wrote:
> I keep seeing something called a polar relay being referred to.

The magnets in your TTY machine, and most "normal" relays are operated
by on and off current. Or what is called "on off" keying. The selector
magnet
in a TTY machine is normally "unenergized" in the space position. And when
energized it is in the mark position. Typically with 60 ma of current to
cause
it to actuate.

In a polar circuit, it has three states. On one way, on the other way,
and off
(in the middle) which is sort of a disallowed condition.

For example, 20 mA positive current causes the relay to close in the mark
condition. Reversing the current in the opposite direction causes the relay
to swing over to the space condition.

The idea behind polar keying was to require current to be flowing for both
the mark and space signal, not JUST in the mark condition. And of course,
the lack of any current was detected and acted upon as a fault condition.

Jeff-1.0
wa6fwi