Francois Robbertze wrote:

>I want to test a 2km (1.6mm dia) steel wire that runs around a residential home for continuity.
>
>There are several wires parralel to each other, but aren't connected together. All the wires are tightened to wooden poles - no isolators.
>
>I was thinking of connecting a 12V power supply on the one end. The other end I want to connect to one of the PIC's pins. (through voltage devider or what ever...)
>
>It must use relatively little current (powered by a 12V battery) and must be robust agains lightning.
>
>Can someone help me with connections.
>
>Must I use optocouplers, MOV's or Transorps
>
>Regards
>
>Francois
>  

Francois,

I would use constant current (say 1 mA) through the (steel wire) loop.

This puts you in the best position to keep lightning away (from your electronics). Because :
Capacitors give you bypass or shorting (short-circuiting) capability ...
Inductors give you blocking capability ......
   ...... both without basically affecting your constant-current system.

Then there are MOV's (Metal-Oxide-Varistors). I see the telephone-company using them. Between wires (not sure) and from each wire to ground, in the terminalbox at every house. There is a wire to a groundrod. Our tel.system is mostly overhead wires.
We have serious lightning here in the tropics. I have regular twinlead wire here to show it. Insulation melted away by lightning in many spots ... while in use for telephone service.

With only 12Volt (max) you can stop lightning getting in (capacitors, inductors) and whatever gets past the capacitor/inductor protection you can clamp with 12V MOVs.

For 1 mA and 200 Ohm loop resistance (resistance assumed), you would need less than a Volt to maintain the 1 mA current (plus voltage-drops over poor connections in the loop).
The voltage-drops serve to find the poor connections with a voltmeter.

If you generate the constant-current from the 12V powersupply you mentioned, the voltage will shoot up to 12V when the loop is broken.
You can make a 1 mA constant current generator with just 2 parts .....
 an LM317 voltage-regulator and a 1200 Ohm (1 to 2 kOhm) resistor.

Herman in PHL.

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist