[GreenKeys] Fusing requirements for loop

[John Nagle]

On 9/20/19 12:59 PM, greenkeys-request at mailman.qth.net wrote:
> How forgiving are selector magnets? Has anyone stress tested them?
> Maybe even Teletype themselves?
> 
> I had an unfused loop burn out an optoisolator in a prototype board
> and get up to something like 150 - 200mA. My 15's magnets started
> smoking. But since it all happened so quickly and I shut off power
> when I realized what was happening, I may not have measured the
> current properly, but I thought I glimpsed something in that range.
> Also, the melted component in question was rated for 150mA. The other
> component on the board that did NOT burn out was rated for 200mA, I
> believe. So the current had to be somewhere in that range, and I know
> the magnets were getting dangerously hot.

If you melted a 150mA component in seconds, you were way over 150mA.
Probably several times that.

My early interface board used a largish open-frame 120VDC supply
I bought in a surplus store. I used a 125mA fast-blow fuse on that.
Never blew one.

My USB-port powered interface board has no fuses, but some protection
circuitry.

https://raw.githubusercontent.com/John-Nagle/ttyloopdriver/master/board/images/schematic.png

That unit has a custom switcher which charges up 2uF to 120V to provide
the initial current needed to push though the 5 Henries or so of
inductance in the selector magnet. That supply takes a whole SPACE
time to charge up, and can't deliver enough current continuously to
cause overheating.

The sustain supply, which keeps current flowing after the pull-in
pulse is from U8, a 5VDC to 15VDC DC-DC converter.  That's only rated
for 66mA at 15 volts, so it can't produce enough output to cause
overheating, either.

At the output end, Q2 and R5 form a 60mA current limiter.
That can survive a short circuit indefinitely; Q2 will heat up
but it stays well within its thermal limits. I checked with an
IR thermometer.  That's the main short circuit protection,
more to protect the electronics rather than the Teletype.

At the input end, there's a AP2553W6 USB power interface part.
That limits current drawn from the USB side to 400mA, and
that's mostly to prevent the protection circuitry in laptops
from shutting down the power.  (If you draw too much power
from a USB port, even for a millisecond, it turns off
power to prevent crashing the laptop. Which is why you
need a power interface part with current limiting on
anything that has a filter cap which needs to charge up
at power up.)

				John Nagle