Current loops for switch sensing, what's a sensible current range to aim for? - Page 1
Forum section: electronics / projects

author: e100
author_rank: Frequent Contributor
timestamp: March 23, 2024, 03:58:31 pm
content: 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.

author: temperance
author_rank: Super Contributor
timestamp: March 23, 2024, 04:06:12 pm
content: 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

author: e100
author_rank: Frequent Contributor
timestamp: March 23, 2024, 04:52:54 pm
content: Quote from: temperance on March 23, 2024, 04:06:12 pmInteresting, 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.

author: temperance
author_rank: Super Contributor
timestamp: March 23, 2024, 05:27:18 pm
content: Indeed. Evey switch or relays with a descent datasheet will specify a minimum current/voltage.

author: MarkT
author_rank: Frequent Contributor
timestamp: March 24, 2024, 08:21:26 am
content: 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.

author: max_torque
author_rank: Super Contributor
timestamp: March 25, 2024, 01:22:42 pm
content: 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

author: inse
author_rank: Super Contributor
timestamp: March 25, 2024, 03:32:13 pm
content: Reed contacts or relays have hermetically sealed contacts and don’t need a wetting current.

author: temperance
author_rank: Super Contributor
timestamp: March 27, 2024, 12:28:47 am
content: QuoteDoing 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.