Contact wetting current
David Gibson
I am looking for some good information on wetting current in
switches and relays, and cannot find any serious references in the
search engines.
Specifically, I'm interested in the effectiveness of using an R-C
circuit to provide an initial "jolt" to the contacts, where it's
impractical to provide a high steady state current.
In the last month we have "caught out" two of our customers using
15A micro-switches as inputs to logic circuits. One circuit (not
designed by us!!) provides just 800uA wetting current, and the
switch maker, Omron, quotes a minimum contact current of 100mA.
In both cases we can insist on the customer using a more appropriate
switch or relay, i.e. gold plated, bifurcated contacts. However, I'd
like to learn more about the subject.
If anyone knows of a good on-line discussion of the topic, I'd
appreciate a pointer.
TIA
David Gibson, http://microconsultants.com
Makers of SPLat easy to use programmable controller
(Simply Programmed Logic automation tool)
and GaugeLink digital gauge to computer interface.
Free software/tutorials for machine control and SPC.
*** Remove anti spam block from return address ****
David Lesher
dgi...@microconsultants.com.nojunkmail (David Gibson) writes:
>I am looking for some good information on wetting current in
>switches and relays, and cannot find any serious references in the
>search engines.
You might look in the telco art over "sealing currents" as I supect
this is a parallel. Sealing Current is the small static current run
on local loops to keep splices from "getting noisy"....
--
A host is a host from coast to coast.................wb8foz@nrk.com
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433
David Lesher
Alan Fowler
dgi...@microconsultants.com.nojunkmail (David Gibson) wrote:
>I am looking for some good information on wetting current in
>switches and relays, and cannot find any serious references in the
>search engines.
"Wetting Current" is used to break down any insulating film that
has built up on the contact surfaces while the contacts are
open. You need a high enough voltage to break down the film,
and sufficient current to clear the remains of the film away
from the actual contact area.
Contacts do not make contact over a large area because they are
not perfectly smooth. The surface roughness produces aspherites
(I think that's how it is spelt) and the tips of these on one
contact meet with similar raised bits on the other contact.
There has to be sufficient voltage between the two contacts
after they meet (the softening voltage) for sufficient current
to flow to soften the high spots enough to produce a large
enough contact area to carry the current.
>Specifically, I'm interested in the effectiveness of using an R-C
>circuit to provide an initial "jolt" to the contacts, where it's
>impractical to provide a high steady state current.
I would be more inclined to design the input circuit to detect
an open circuit condition. i.e. the normal condition is a
closed contact feeding a small current into a resistor to ground
to hold the input gate high.
I've seen contacts permanently welded together from the
discharge of an inappropriate R-C combination.
>In the last month we have "caught out" two of our customers using
>15A micro-switches as inputs to logic circuits. One circuit (not
>designed by us!!) provides just 800uA wetting current, and the
>switch maker, Omron, quotes a minimum contact current of 100mA.
>In both cases we can insist on the customer using a more appropriate
>switch or relay, i.e. gold plated, bifurcated contacts. However, I'd
>like to learn more about the subject.
>If anyone knows of a good on-line discussion of the topic, I'd
>appreciate a pointer.
>TIA
> David Gibson, http://microconsultants.com
>Makers of SPLat easy to use programmable controller
> (Simply Programmed Logic automation tool)
> and GaugeLink digital gauge to computer interface.
>Free software/tutorials for machine control and SPC.
>*** Remove anti spam block from return address ****
regards, Alan.
.
My correct mail address is amfo...@melbpc.org.au
Please delete anything between the "@" and "melbpc" when replying.
,-._|\ Alan Fowler. (Alan M. Fowler FIEAust CPEng)
/ Oz \ Mail Address: PO Box 1008G, North Balwyn 3104 Vic, AUSTRALIA.
\_,--.x/ Phone: +613-9857-7128 Member, Melbourne PC User Group.
v +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Paul E. Bennett
In article <34b2b054....@news.ocean.com.au>
dgi...@microconsultants.com.nojunkmail "David Gibson" writes:
>
> I am looking for some good information on wetting current in
> switches and relays, and cannot find any serious references in the
> search engines.
[%X]
> If anyone knows of a good on-line discussion of the topic, I'd
> appreciate a pointer.
Hi David,
I haven't come across much discussion on net yet. However, one of my
previous railway contracts required comms links down the length of
a train with an auto-coupler mid-train that was already old enough to
be in a technology museum (about 18 years). The specification called
for a minimum of 10mA wetting current for all circuits going through
the autocoupler and all switch and relay contacts. This included the
data comms circuits.
With modern materials, in a cleanish environment and reasonable
protection of open contacts I would expect that a wetting current of
1mA would be reasonable. The dirtier and less protected the contacts
are the higher the wetting current might need to be. I would not
expect any circuit to have to provide greater than 50mA for the
purpose of wetting contacts (some very big contactors perhaps).
Of course wetting currents are the minimum current flow that will
provide a reliable indication of a circuit closure.
--
Paul E. Bennett ................... <p...@transcontech.co.uk>
Transport Control Technology Ltd. <http://www.tcontec.demon.co.uk/>
+44 (0)117-9499861 <enq...@transcontech.co.uk>
Going Forth Safely
Alan Sanderson
10 to 20 ma is a typical current in low voltage circuits that is
sufficient to burn through the oxidation layer that can form on the
contact materials while the contacts are open. If sufficient energy
is not available to burn through the oxidation, the contact closure
will not pass any current. This will vary with contact design and
material.