Re: Need help identifying N.O. or N.C.

[not_real@xxxxxxx (Beachcomber)]

On Mon, 26 Sep 2005 16:16:01 GMT, "JW" <jweb@xxxxxxxxxx> wrote:

>Neither...NO/NC contacts are just that. Open or closed circuits.  The
>circuit you described is low voltage, on or off.
>
>"John Brummitt" <jdb5@xxxxxxxxxxxx> wrote in message
>news:hpmdnWoi8MxKhKXeRVn-pA@xxxxxxxxxxxxxxx
>> I'm having trouble figuring out the type of sensors I have ( N.O. or
>> N.C. )
>> Each zone has a 15K resistor from the loop power to the zone.
>>
>> The wireless receiver has 2 wires running to it to supply 12 Volts and
>> each zone has
>> 1 wire running back to the zone screw where the 15K resistor is attached
>> so that the 15K is always
>> supplying a little power to the zone screw.
>> When the zone is tripped the wire from the wireless receiver becomes a
>> ground for that zone.
>> So the power flows from the loop power through the resistor, then through
>> the now grounded wire.
>>
>> Would this be a N.O. or N.C. circuit?
>>

Strictly speaking, a Normally Open Switch refers to the position of
the switch (open or closed) when it is in its "normal" or relaxed
state.  i.e.  the magnet is away, or if it is a momentary switch, the
contacts are "open" until the switch is depressed.  If it is a relay,
the NO contacts are "open" when the coil is not energized.

In order to eliminate confusion, most alarm circuits are refered to as
normally-open or normally-closed,  and sensors are said to be for
normally open or normally closed circuits.

Most alarm circuits are normally closed loops with the sensors in
series and a maintained current flow until one sensor opens and stops
the flow of current which sets off the alarm.   The eol (end-of-line)
resistors are supposed to be installed at the end of the line in
series with the last sensor in the loop to provide fault supervision.
That way, a short somewhere along the loop will set off a trouble
indicator.   Lazy or inexperienced installers will sometimes not do
this and place the resistors in the control box.

Beachcomber



Some alarm circuits are normally open and must be wired in parallel
with other sensors.   The closure of any one circuit completes the
circuit, allows current to flow and sets off the alarm.