[ARDF] Fuses in foxboxes?

[Charlesscharlau at aol.com]

In a message dated 9/30/2003 5:28:09 PM Eastern Standard Time, 
kharker at cs.utexas.edu writes:

>  I'm getting closer to getting my first hidden transmitter built.
> 
>   I plan to use a 7Ah 12 VDC gel cell in each box.  Each gel cell will
> be connected to an HT, an 80M transmitter, and a PicCon.  I think I would
> like to uses fuses in the + lines, just in case something goes wrong.
> Should I (a) use a single fuse near the battery and before I split off 
> to the three devices, (b) use three separate fuses one near each of the 
> three devices in the box, or (c) do something else? 
> 
> -- 
> ----------------------------------------------------------------------------
> Kenneth E. Harker    "Vox Clamantis in Deserto"    kharker at cs.utexas.edu
> University of Texas at Austin           Amateur Radio Callsign: WM5R
> Department of the Computer Sciences      Central Texas DX &Contest Club
> Taylor Hall TAY 2.124              Maintainer of Linux on Laptops
> Austin, TX 78712-1188 USA       http://www.cs.utexas.edu/users/kharker/
> ----------------------------------------------------------------------------
> _______________________________________________

I like using a thermal fuse, or a similar self-resetting current-limiting 
device. It has a negligible voltage drop, protects your equipment, and keeps 
working even after "blowing." As with other fuses, place it in series with the + 
battery line as close to the battery terminal as possible.

Other suggestions:

1. Use reliable keyed connectors between your battery and transmitter. This 
also makes it easy to swap out batteries when necessary. The fuse goes between 
the battery and the connector. I like the red-and-black powerpole style 
connectors.

2. Keep the ammo can (or conductive box) at a DC floating potential. This 
might not be practical, depending on your transmitter and antenna connectors. But 
if you can accomplish it, you won't have a short circuit when that +12V wire 
gets clamped between the lid and the box.

3. Mount an on/off switch in series with the battery's negative terminal, as 
close to the battery terminal as possible. I like to use a key-operated SPST 
switch, like you sometimes find on computers.  This acts as a master power 
switch. You can mount it through the wall of the ammo box so that it is accessible 
from outside the box, yet because it is key operated, it is unlikely to be 
turned off unintentionally, or mischievously! Because it disconnects the 
negative battery terminal, you can be sure that any shorts between ground and +12V 
won't result in battery drain, fire, or blown fuses.

4. Provide a way to turn on/off the controller separately from the 
transmitter. This is especially handy if you are using Montreal-style controllers, or 
similar synchronized devices. It is helpful because it allows you to keep the 
controllers running (synchronized) yet have the transmitters powered down while 
you are transporting them. This might be of no use for the PicCons.