[GreenKeys] Loop supply voltage

[John Nagle]

On 1/19/25 09:00, greenkeys-request at mailman.qth.net wrote:
> From: steve bennett<raleigh_ranger at yahoo.com> Subject: Re:
> [GreenKeys] Loop supply voltage 
> Message-ID:<680590144.1729907.1737259588685 at mail.yahoo.com> 
> Content-Type: text/plain; charset="utf-8"
> 
> Harold, I get it about the inductance.But once the coil reaches
> steady state you are just left with the resistance in a DC circuit.So
> if the goal is to have 60mA at steady state 100v is way too much.
> What am I missing?The loop supply is DC correct? Maybe that is where
> I'm confused. Is the loop AC? -Steve

The loop is DC. For model 14 and 15 machines, it should be 60mA,
constant current. The voltage varies. If you look at this on
a scope, on the transition to MARK, you see 120V across the
selector magnet. This drops off in a few milliseconds and settles to
12 to 15 volts.

The inductance of the selector magnet is about 5 henries.
(Usually you see inductances listed in millihenries. This is
big inductance.) It takes a lot of voltage to push 60ma
through that big magnet.

The traditional Teletype way to get a constant current 60mA is to put a
big 2K ballast resistor in series with a 120 VDC source. About 90% of
the energy goes into heating up the ballast resistor, which will
dissipate about 10 watts and get hot.

There are more efficient solutions, but they are more complex. See:

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

This can drive a Model 15 or 14 powered only from a USB port.

What I did on my driver board was to charge up a capacitor
to 120V, using a tiny switching power supply. On transition
to MARK, the capacitor dumps into the selector magnet.
This pulls in the selector magnet. Once the capacitor discharges,
a small 15V supply keeps the magnet pulled in.

I and two other people have built this, but it's kind of
overkill. I did it because I used to bring these machines
to steampunk conventions, For smaller conventions, I had
a cute portable rig with a Model 14 in an Anvil case,
a small laptop, and my tiny interface box.

				John Nagle