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

• Current loops for switch sensing, what's a sensible current range to aim for?
  Posted by e100 on 23 Mar, 2024 15:58
• 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.
• #1 Reply
  Posted by temperance on 23 Mar, 2024 16:06
• 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
  
• #2 Reply
  Posted by e100 on 23 Mar, 2024 16:52
• Quote from: temperance on 23 Mar, 2024 16:06

  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.
• #3 Reply
  Posted by temperance on 23 Mar, 2024 17:27
• Indeed. Evey switch or relays with a descent datasheet will specify a minimum current/voltage.
• #4 Reply
  Posted by MarkT on 24 Mar, 2024 08:21
• 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.
• #5 Reply
  Posted by max_torque on 25 Mar, 2024 13:22
• 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
• #6 Reply
  Posted by inse on 25 Mar, 2024 15:32
• Reed contacts or relays have hermetically sealed contacts and don’t need a wetting current.
• #7 Reply
  Posted by temperance on 27 Mar, 2024 00:28
• Quote

  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.