Switch wetting current

G

Glenn Garrett

Guest
I am designing a controller that needs to read the status of up to 40
pressure switches that will be mounted on irrigation pipes on farms.
The switches are already specified - SquareD brand and have large 10A 250Vac
open contacts. They are not well sealed.
The controller will be solar powered and I want to keep the supply current
at a minimum.
The contacts may only close once every week or so during the dry season,
closed for the entire wet season.
Web sites I have looked at say that 2 - 5mA of wetting current is needed -
for 40 switches that would be an excessive continuous current.
Has anyone had experience with this sort of thing?


--
Regards

Glenn Garrett

Industrial Dynamics Ballarat Pty. Ltd.
 
Glenn Garrett wrote:
I am designing a controller that needs to read the status of up to 40
pressure switches that will be mounted on irrigation pipes on farms.
The switches are already specified - SquareD brand and have large 10A 250Vac
open contacts. They are not well sealed.
The controller will be solar powered and I want to keep the supply current
at a minimum.
The contacts may only close once every week or so during the dry season,
closed for the entire wet season.
Web sites I have looked at say that 2 - 5mA of wetting current is needed -
for 40 switches that would be an excessive continuous current.
Has anyone had experience with this sort of thing?
Click to expand...
if they spec 2mA, you ought to respect that.

However, it doesn't have to be 2mA all the time and
it doesn't have to be 2mA in all the switches at the
same time.
 
In sci.electronics.design, Roy McCammon <rmccammon@austin.rr.com>
wrote:

Glenn Garrett wrote:
I am designing a controller that needs to read the status of up to 40
pressure switches that will be mounted on irrigation pipes on farms.
The switches are already specified - SquareD brand and have large 10A 250Vac
open contacts. They are not well sealed.
The controller will be solar powered and I want to keep the supply current
at a minimum.
The contacts may only close once every week or so during the dry season,
closed for the entire wet season.
Web sites I have looked at say that 2 - 5mA of wetting current is needed -
for 40 switches that would be an excessive continuous current.
Has anyone had experience with this sort of thing?

if they spec 2mA, you ought to respect that.

However, it doesn't have to be 2mA all the time and
it doesn't have to be 2mA in all the switches at the
same time.
Click to expand...
How about something like this (I'm not quite used to this ASCII
schematic editor thing, but it's readable). With a five volt supply it
gives a short pulse of 1/2 Ampere. Would a current pulse this short be
good enough for 'wetting' the contacts? I'd call up the manufacturer
or a rep, explain the problem and ask if something like the circuit
below (or maybe with a larger-value, electrolytic cap, if leakage
isn't a problem) would work.

100k 10 ohm
___ ___
.---|___|-----------|___|------.
' | |
- | |
--- | |
. | |
| ' |
| --- '
' --- ,22u | o
| | |=|>
- . | o
--- | .
| | '
---------------'---------------|
'
|
===
GND
created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-chat.de
 
Glenn Garrett <inddyn-info@iinet.net.au> wrote:
I am designing a controller that needs to read the status of up to 40
pressure switches that will be mounted on irrigation pipes on farms.
The switches are already specified - SquareD brand and have large 10A 250Vac
open contacts. They are not well sealed.
The controller will be solar powered and I want to keep the supply current
at a minimum.
The contacts may only close once every week or so during the dry season,
closed for the entire wet season.
Web sites I have looked at say that 2 - 5mA of wetting current is needed -
for 40 switches that would be an excessive continuous current.
Has anyone had experience with this sort of thing?
Click to expand...
Worst case, if you'r feeling paranoid.
You could simply drop the voltage to around a volt or so, and live with
2-5mW/switch.
 
Subject: Switch wetting current
From: "Glenn Garrett" inddyn-info@iinet.net.au
Date: 4/25/2004 6:25 PM Central Standard Time
Message-id: <408c48fa$0$16579$5a62ac22@freenews.iinet.net.au

I am designing a controller that needs to read the status of up to 40
pressure switches that will be mounted on irrigation pipes on farms.
The switches are already specified - SquareD brand and have large 10A 250Vac
open contacts. They are not well sealed.
The controller will be solar powered and I want to keep the supply current
at a minimum.
The contacts may only close once every week or so during the dry season,
closed for the entire wet season.
Web sites I have looked at say that 2 - 5mA of wetting current is needed -
for 40 switches that would be an excessive continuous current.
Has anyone had experience with this sort of thing?


--
Regards

Glenn Garrett

Industrial Dynamics Ballarat Pty. Ltd.
Click to expand...
1) Multiplex the switches.
2) Use small caps to achieve wetting current at turn-on.
3) Only activate the multiplexer when you're reading the switches -- you
probably don't have to read them all the time.

Good luck
Chris
 
Glenn Garrett wrote:
I am designing a controller that needs to read the status of up to 40
pressure switches that will be mounted on irrigation pipes on farms.
The switches are already specified - SquareD brand and have large 10A 250Vac
open contacts. They are not well sealed.
The controller will be solar powered and I want to keep the supply current
at a minimum.
The contacts may only close once every week or so during the dry season,
closed for the entire wet season.
Web sites I have looked at say that 2 - 5mA of wetting current is needed -
for 40 switches that would be an excessive continuous current.
Has anyone had experience with this sort of thing?
Click to expand...
If those switches are not well-sealed, have 10A rating, and operate in
an outdoor -wet- environment, then over time you may be looking at 1/2A
or more for reliable readings. As Chris Foley suggested, the best option
is to read the status with a current pulse- something like 250ms
duration- and be sure to account for the line run resistance- round trip
distance. The 2-5mA figure is ridiculously inadequate- you could have
higher leakage than this over time. It sounds like you're using the
wrong pressure switches anyway- those SqD things are probably designed
to throw line voltage onto a 1.5HP pump- and not for sensing- so looks
like you paid extra for the wrong part.
 
Ben Bradley wrote:
In sci.electronics.design, Roy McCammon <rmccammon@austin.rr.com
wrote:



Glenn Garrett wrote:

I am designing a controller that needs to read the status of up to 40
pressure switches that will be mounted on irrigation pipes on farms.
The switches are already specified - SquareD brand and have large 10A 250Vac
open contacts. They are not well sealed.
The controller will be solar powered and I want to keep the supply current
at a minimum.
The contacts may only close once every week or so during the dry season,
closed for the entire wet season.
Web sites I have looked at say that 2 - 5mA of wetting current is needed -
for 40 switches that would be an excessive continuous current.
Has anyone had experience with this sort of thing?

if they spec 2mA, you ought to respect that.

However, it doesn't have to be 2mA all the time and
it doesn't have to be 2mA in all the switches at the
same time.


How about something like this (I'm not quite used to this ASCII
schematic editor thing, but it's readable). With a five volt supply it
gives a short pulse of 1/2 Ampere. Would a current pulse this short be
good enough for 'wetting' the contacts? I'd call up the manufacturer
or a rep, explain the problem and ask if something like the circuit
below (or maybe with a larger-value, electrolytic cap, if leakage
isn't a problem) would work.
Click to expand...
I don't know, but I'd guess that 1 second would be enough.


created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-chat.de
Click to expand...
 
Thanks for everyones' replies - I used this info to convince the other
engineer that we needed a new pressure switch. Gems make an ideal sealed
switch with gold contacts for much the same price as the farm one.

Glenn

"Fred Bloggs" <nospam@nospam.com> wrote in message
news:408D0AD8.4000103@nospam.com...
Glenn Garrett wrote:
I am designing a controller that needs to read the status of up to 40
pressure switches that will be mounted on irrigation pipes on farms.
The switches are already specified - SquareD brand and have large 10A
250Vac
open contacts. They are not well sealed.
The controller will be solar powered and I want to keep the supply
current
at a minimum.
The contacts may only close once every week or so during the dry season,
closed for the entire wet season.
Web sites I have looked at say that 2 - 5mA of wetting current is
needed -
for 40 switches that would be an excessive continuous current.
Has anyone had experience with this sort of thing?



If those switches are not well-sealed, have 10A rating, and operate in
an outdoor -wet- environment, then over time you may be looking at 1/2A
or more for reliable readings. As Chris Foley suggested, the best option
is to read the status with a current pulse- something like 250ms
duration- and be sure to account for the line run resistance- round trip
distance. The 2-5mA figure is ridiculously inadequate- you could have
higher leakage than this over time. It sounds like you're using the
wrong pressure switches anyway- those SqD things are probably designed
to throw line voltage onto a 1.5HP pump- and not for sensing- so looks
like you paid extra for the wrong part.
Click to expand...
 
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