Current loops for switch sensing, what's a sensible current range to aim for?

[e100]

I plan to use a current loop for sensing a remote switch because it allows for the detection of short and open circuit fault conditions.

I'm guessing that industrial systems use 4-20mA, however house alarm sensor circuits seem to operate OK at much lower currents (presumably to prolong battery life during a power outage). End of line resistors in 12v alarm circuits are usually in the range of 1-6.8k which I'm guessing results in loop currents well under 6mA.

Is 6mA a sensible number to aim for?
In my specific application the distance from the switch to the micro will be about 10m over unshielded CAT5 and will be running at 3.3v.

[temperance]

All mechanical switches require a minimum current and voltage to operate reliable which is called wetting current.

http://www.aeroelectric.com/Reference_Docs/Microswitch/TB13_Low_Energy_Switching.pdf

[e100]

All mechanical switches require a minimum current and voltage to operate reliable which is called wetting current.

http://www.aeroelectric.com/Reference_Docs/Microswitch/TB13_Low_Energy_Switching.pdf

Interesting, so house alarm sensor circuits work reliably at low currents because they use sealed reed switches, whereas regular unsealed push button switches need more current.

[temperance]

Indeed. Evey switch or relays with a descent datasheet will specify a minimum current/voltage.

[MarkT]

I think its mainly about oxide film build up on the contacts - which is why gold plated or gold-alloy contacts are used for small-signal relays, reed-switches and the like.

And the frequency of operation matters too - a switch sitting on the shelf for 20 years will build up a film on the contacts that might make it unreliable when first used.  The wetting current is enough to disrupt the oxide coating formation in regular use.

[max_torque]

you can use a lower continuous current and use a small capacitor to provide a short higher current pulse for contact cleaning, ie when the contacts are open, the CCD tries to push it's constant current through the system, which it can't do, so it charges the "output" cap, and when the switch is closed, that cap drives a higher short pulse of current through the contacts at the moment of closing.

If you put a resistor across the switch to act as a diagnostic path, then you can still use this arrangement

[inse]

Reed contacts or relays have hermetically sealed contacts and don’t need a wetting current.

[temperance]

you can use a lower continuous current and use a small capacitor to provide a short higher current pulse for contact cleaning, ie when the contacts are open, the CCD tries to push it's constant current through the system, which it can't do, so it charges the "output" cap, and when the switch is closed, that cap drives a higher short pulse of current through the contacts at the moment of closing.

Doing so requires a proper current limiting resistor to limit the circuit current to something below the switch capability otherwise you'll damage the contact surface.