Voltage across alarm pair

[Spark123]

So your saying - with your test meter on volts you get a reading ACROSS the alarm pair ? I`d be very worried if I was you

you`ll only get a reading with reference to the 12v or 0v terminal - think about it - if there was 12v ACROSS the alarm relay the ffng fuse would blow everytime the sensor closed wouldnt it

Yes, you will get a reading using a digital volt meter when there is an open circuit as the high impedance of the DVM will not allow enough current to flow in the circuit to pull the voltage down. When the circuit is closed the components allow a small current to flow in the circuit but not a short circuit.
2 little experiments for you to try:
DMM set to volts i.e. open circuit condition:

DMM set to mA i.e. causing a closed circuit condition:

 

[Barsteward]

The point of Dry contacts is,

volt free.

DVMs can be misleading

 

[Spark123]

Nope, DVMs can help you fault find a system if you can read and understand what they are telling you. How I understand it - the switch in the PIR may be volt free, the voltage is supplied by the panel allowing a small current to flow through the circuit through the PIR dry contact switch when it is closed. When the switch is open, no current can flow hence the larger voltage measurable across the panel (a negligible amount of current will flow through the DVM when it is connected).

Using ohms law, it would appear there is roughly 25k of resistance in the circuit hence the reason why when it is a closed circuit there isn't a short.

 

[Barsteward]

Around 10K on s/c and 33k on o/c
But you shouldn't have 12v across them

 

[Spark123]

Why not? If there is no path for current to flow then you are going to measure the supply voltage (or a midges less given the DVM impedance.)
How does the resistance change depending on wether the system is O/C or S/C, are the terminals bridging a resistor?

 

[joe-90]

There is normally a 22 ohm resistor across the contacts. If you make a mistake wiring it up and put the supply leads into the N/C - it won't blow a fuse.

 

[saxondale]

very strange - i took exactly the same picture yesterday but couldnt get it to upload showing there are no volts across the alarm terminals

BUT THERE AGAIN I HADNT GOT MY LEADS ACROSS THE AUX TERMINALS LIKE YOU HAVE THERE

good try but were not that stupid

 

[saxondale]

22ohms? ROFL

BTW sparks - what were you trying to do in the bottom piccy? meter set on AC but leads as current ? you can get a volt reading from your left fingers if you try

 

[saxondale]

right time to stop this foolishness once and for all

YOU DO NOT HAVE 13V ACROSS THE ALARM PAIR

you have a positive or a negative WITH REFERENCE TO GROUND not in isolation across the pair of terminals

you have a small current flowing through the circuit WITH REFERENCE TO THE PANEL CHASSIS (GROUND) it is this change in current the panel detects to trigger the system

put your meter BETWEEN the 12V termial and the circuit and you`ll get a reading but the difference between OC and CC is minimal - far to varied from manf to manf to be of any use in fault diagnostics, the only time (and it is the only time ) you use volts to assertain an open curcuit is when you checking the tamper return and the circuit IS LIVE

QUESTION FOR JOE - IF ITS 13V NORMALLY WHATS THE READING IF THE TAMPER SWITCH IS OPEN? ( give you a clue - it isnt zero)

QUESTION FOR SPARKS - YOU ALWAYS MEASURE CURRENT ACROSS A CIRCUIT ? (of course you dont you measure it in series)

just for once listen to someone who does this for a living

 

[joe-90]

right time to stop this foolishness once and for all

YOU DO NOT HAVE 13V ACROSS THE ALARM PAIR

you have a positive or a negative WITH REFERENCE TO GROUND not in isolation across the pair of terminals

you have a small current flowing through the circuit WITH REFERENCE TO THE PANEL CHASSIS (GROUND) it is this change in current the panel detects to trigger the system

put your meter BETWEEN the 12V termial and the circuit and you`ll get a reading but the difference between OC and CC is minimal - far to varied from manf to manf to be of any use in fault diagnostics, the only time (and it is the only time ) you use volts to assertain an open curcuit is when you checking the tamper return and the circuit IS LIVE

QUESTION FOR JOE - IF ITS 13V NORMALLY WHATS THE READING IF THE TAMPER SWITCH IS OPEN? ( give you a clue - it isnt zero)

QUESTION FOR SPARKS - YOU ALWAYS MEASURE CURRENT ACROSS A CIRCUIT ? (of course you dont you measure it in series)

just for once listen to someone who does this for a living

If what's '13 volts' normally? Do you mean nominally?

 

[Spark123]

very strange - i took exactly the same picture yesterday but couldnt get it to upload showing there are no volts across the alarm terminals

BUT THERE AGAIN I HADNT GOT MY LEADS ACROSS THE AUX TERMINALS LIKE YOU HAVE THERE

good try but were not that stupid

Nope, look again. The leads are connected across the alarm pair for zone 1. The aux supply is to the left hand side. How to upload a pic is in forum information.

Edit: I will upload a pic of the PCB later.

 

[Spark123]

22ohms? ROFL

BTW sparks - what were you trying to do in the bottom piccy? meter set on AC but leads as current ? you can get a volt reading from your left fingers if you try

Look again, it is set to the amps DC scale and the leads are plugged into the mA current positions. I work with instrumentation and electronics and am capable of setting a multimeter

QUESTION FOR SPARKS - YOU ALWAYS MEASURE CURRENT ACROSS A CIRCUIT ? (of course you dont you measure it in series)

When you want a closed circuit, connecting the ammeter across the open terminals is connecting the ammeter in series i.e. the ammeter is completing the circuit between the two terminals.

 

[Europlex]

very strange - i took exactly the same picture yesterday but couldnt get it to upload showing there are no volts across the alarm terminals

BUT THERE AGAIN I HADNT GOT MY LEADS ACROSS THE AUX TERMINALS LIKE YOU HAVE THERE

good try but were not that stupid

Nope, look again. The leads are connected across the alarm pair for zone 1. The aux supply is to the left hand side. How to upload a pic is in forum information.

Edit: I will upload a pic of the PCB later.

Don't bother, you'll never convince him.

He obviously does not know his control panels either, that's a Texecom Excel so it's obvious where the meter is connected.

and, by the way, the main AUX supply is to the far right, the one you mention should be used for the RKPs

 

[plugwash]

There should be a voltage accross an open circuit alarm pair but that voltage comes through a high resistance. when the sensor is closed the voltage will obviously drop down to zero and a current will flow through the sensor. You can't design an input circuit to read a switch (and an alarm sensor is really just a fancy switch) without having some voltage accross the switch when it is open.

What that voltage is will depend on the panels designer but it will be there.

 

[Spark123]

It is a Veritas R8 panel, the AUX supply is to the left of the zones along with the switch and tamper terminals, Z1 which I had the meter on above (the one with the commissioning link removed). The keypad connections are to the right - you can just see the T and R terminals.