On May 27, 10:56*am, bud-- wrote:
On 5/26/2011 10:24 AM, Smitty Two wrote:
In ,
* *wrote:
On 5/25/2011 12:45 PM, wrote:
Another thread discussing whether a switch in something was normally
open or normally closed reminded me of a similar question I have.
If I want to buy a reed switch/magnet pair to use as a door or window
contact in an alarm system and the system requires that the contact be
closed when the window or door is closed, do I buy a N.O. switch or
N.C. switch? *I assumed N.C. until I started to over-think it. *A
normally open switch is one that is open until it gets activated. Reed
switches are activated by the magnet getting close. *So, a normally
open switch would be closed when the magnet is near and that happens
when the window is closed - which is normal - so normally, a normally
open switch is closed - aaaaaargh! *I need a replacement switch a few
months ago and ended up paying extra for a double throw switch. *It
wasn't labeled so I still don't know if I wanted N.O. or N.C. *I just
used the contact that worked.
So, what is "normal" for a reed switch? *When the magnet is near or
not?
Pat
There was a thread on this about a year ago.
RBM's reply was:
As an electrician, I've been through this numerous times over the years.
It clearly depend on who you are talking to, and what trade they're in..
IMO, the "normal" position is the state the switch is in when nothing
affects it. Alarm people have the opposite take on it.
My reply was:
I agree.
In all the following the contacts are closed with the magnet next to the
reed switch.
* *From an alarm perspective *NC* ("normal" is when the door is closed):
http://en.wikipedia.org/wiki/Burglar..._control_panel
"Most switching devices are N.C. (normally closed) circuits, so when the
device is not in an alarm condition, the circuit is closed."
http://www.alibaba.com/product-gs/21...or_Alarm_Switc....
"Output signal: normally closed (switch contact is closed when the door
closed)"
* *From a component manufacturer *NO*
C&K: "ELECTRICAL CIRCUIT: SPST NO (Contact Form A). Reed switch
opens when magnet is removed from proximity. Contacts are
held closed when magnet is within actuation range."
To know what the switch does you need to have a description like all of
the above.
end reply.
Interesting. My take on that is twofold:
One, alarm people don't know what the hell they're talking about. Words
become meaningless when one group of people decide to throw out the real
definition and use their own.
IMHO alarm terms make sense from within the industry. For a series loop
circuit you need devices that are NC while the alarm is armed. That is a
NO reed switch with the magnet adjacent. A NO reed switch in its
"normal" condition (while armed) is NC, and is called NC. If you buy
from sources in the alarm industry you probably won't have a problem.
That was the point I made a while back, ie if you buy a window/door
switch sold for alarm use, it's very hard to go wrong.
There is a problem using devices from outside the alarm industry.
Which is why I don't understand the need to buy a reed switch
from any other source if you're intending to use it as a door/window
switch in an alarm system They come made specifically for that
purpose, with self-adhesive and screw mounting capability, cost
$3 and are widely available, including Ebay.
It is not the only instance where terminology changes. Electronics
circuits have (had?) black as "ground", green was vacuum tube grid. A
neon transformer that is "overloaded" has too long a length of neon
tubes connected (current is too low). What is "high voltage" is
different for a low voltage tech, an electrician, a TV (CRT) repairman,
and a power utility engineer.
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