[GreenKeys] Polar Relay

[Harold Hallikainen]

On Tue, February 7, 2023 9:30 am, W2HX wrote:
> 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!
>

Just like series string Christmas lights! Each has its advantages. In a
series fed current loop, an open kills the loop. In a shunt fed voltage
system, a short kills the system.

I have heard that Teletype liked mechanical stuff and wanted to minimize
electrical/electronic circuitry. They used a mechanical UART (though
electronic ones did not exist at the time) and directly drove the selector
magnet instead of including an electronic driver. They could have made it
a voltage driven system by putting a resistor in series with the selector
magnet such that a keyed voltage bus would result in the correct current
through the selector magnet. This would require additional current from
the keyed voltage bus as additional devices were added.

I think the current loop works out well, especially with shorting jacks. I
also think holding magnets are VERY CLEVER. Since not as much current is
required, the coils are in parallel instead of series substantially
reducing the inductance (to about 1/4 that of the series magnets).

Harold
https://w6iwi.org