# Beginner question on resistance

Discussion in 'Electronic Basics' started by John Popelish, Mar 23, 2007.

1. ### John PopelishGuest

The limit depends on how much current the alarm passes
through the contacts (and the resistance of their wiring)
and at what total voltage drop across that circuit, it
decides that some switch is open. For instance, if the
switch loop is fed with 10 mA and decides that an open
condition is indicated at a voltage drop of 2 volts, then
the total resistance of the loop and all the switch contacts
must drop less than 2 volts while 10 mA passes through them.
The limiting resistance would be 2/.01= 200 ohms. For any
reasonable gauge wire, that is a lot of length, but this
example is completely hypothetical.

I would want to find out the limiting resistance by test,
using a variable resistor in place of the loop, and
measuring the resistance that is interpreted as an open, by
experiment. Then, I would try not to make any loop that had
more than a minor fraction (say, 1/3rd) of that resistance,
to allow a little room for the contact resistances to rise
with oxidation. It is also a good idea to wire the loop as
a flattened one, using a pair of wires that take the same
route in both directions, to keep the loop from acting like
a magnetic loop antenna that will cause false alarms any
time a thunder storm passes nearby.

2. ### ABGuest

First of all let me once again say thanks for all the good advice I
get here. I know a lot of this stuff is so basic for many of you, but
for me it gives me just enough info to expand my horizons.

Now I have another question that's been confusing me. I have this
wireless alarm panel at home that basically waits for a signal from a
small wireless transmitter to let it know if a zone is closed (normal
condition) or open (alarm condition). Some of these transmitters have
2 terminals where you can physically wire in a multiple magnetic
window contacts (in series)...so therefore you use only 1 transmitter
to monitor, say, 3 windows.

Now many that post on alarm forums say to limit the series wired
contacts to 3 per transmitter...but other alarm guys say you can add
"as many as you want as long as you don't increase the resistance too
much". There aren't any hard and fast rules it seems. I'm wondering
how you will know how many contacts is too many...when the alarm goes
off?? What is "too much"

Basically I was wondering a little about the theory of this. Is there
is a way to measure when say 5 contacts is OK...but 6 is too many?
False alarms can get very costly with fines and all. I welcome your
Thanks,
Bob

3. ### MassiveProngGuest

That depends on the alarm circuit.

The way to test it is place a potentiometer in that location, and
increase the resistance until you get a trigger event.

Read the resistance of the pot, then read the resistance of a given
length of connection wire, and switches.

As long as you are a good deal (say 20%) below that value in
resistance, you won't cause any false triggers.

4. ### MassiveProngGuest

Oh yeah... notify your alarm monitor station that you are in a
service mode and for how long and they should ignore any triggers
during that time, and NOT be able to charge you for them. You should
be able to ask them if this procedure is allowable.