[GreenKeys] Loop supplies

[John Nagle]

> On Behalf Of W9DDD > Sent: Monday, October 29, 2018 1:59 PM
> Subject: [GreenKeys] Loop supplies or selector magnet drivers
> 
> Wonder what people are using these days loop supplies and keying the
> loop or selector magnet directly.  I was thinking of going old school
> with 120 VDC and a series 2,000 ohm 10 watt resistor. But, looking at
> the prices for 10 watt resistors, I'm re-thinking things.
> 
> I might get lucky with something already being in the LESU of the
> pending 28, but I'm not counting on it.  I'm trying to get setup so all
> I have to do is plug it in when the 28 arrives.

     I've been down this road several times, as many of you know.

     First I did a more or less traditional setup, with a 120VDC supply
and a 2K 10W resistor.

    http://www.aetherltd.com/connecting.html

These work fine. I've had two of them in use for almost a decade.
The 2K 10W resistor used is a strange thick-film device which can
be mounted on a PC board without scorching it, because it has so
much surface area.

     Then, as a technical tour de force, I built a 120V 60mA
driver for Model 14 and 15 Teletypes which is powered entirely
from a USB port.

    https://github.com/John-Nagle/ttyloopdriver

It's the size of a cigarette pack and generates little heat.
It uses a custom-designed special purpose switching power
supply. No energy wasted in a big resistor.

It's the ultimate solution to driving an old Teletype from
a computer. But it's hard for homebrewers to build.
It's all surface mount.  It has to be, or the switcher
wouldn't work. It's designed like a modern
production product, with protection circuitry and startup
power management.  This is how you'd do it if you were
having a thousand built in Shenzhen. But it's beyond most hams.
Steve Garrison has built some from this design.  I've built some.
But the market is too small for this to be a product.

			John Nagle