[ARDF] Fuses in foxboxes?

[Kuon & Dale Hunt]

If you are going to use a fuse, the automotive "blade" type
is a good choice from a reliability point of view - no spring
contacts to bounce open.  Remember to leave several spares
inside the case for when you need them in the field.

My favorite is the PTC Thermistor.  Looks like a big disk
ceramic capacitor, protects against short circuits, and
automatically resets when the short is cleared.  For
example the BC1359 is listed in the Digi-Key catalog as
passing 1 Amp without triggering but shuts down at 1.5 amps.
Resistance is 1.3 ohms until it kicks in, when it passes
a couple mils if I remember correctly.  This particular one
is still comparatively expensive (over $5) but I've started
seeing them in the surplus catalogs such as BG Micro at
several for a dollar.  (I got a handful of the 2 amp ones
from some catalog at a very reasonable price a few years
ago.)

Another option is a small circuit breaker:  some include
a switch function built in, which could also be useful.
I found some in the "CardGuard" series designed for
protecting each card in a system that were around 1.5 amps
from a surplus catalog.  This doesn't reset itself, but at
least you don't have to worry about being out of spares if
(when) it trips in the field.

I use the Montreal controllers, and the board provides for
two power sources with diode switching.  A 9V battery makes
a simple backup in case something happens to the main
power source.  (Even with a resetable breaker, a temporary
short will drop the main voltage down below usable limits.)
The Ron Graham 2m receivers have problems when the battery
voltage drops to 7.5V or so, but these batteries will still
run a controller for a long time at a 3ma load.




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