Douglas Krantz - Technical Writer - Describing How It Works
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The End of Line Resistor Pulls the Voltage Down?

Electrons are pushed and pulled through the wires and resistors by the power supply. The Voltage of the power supply is shared by the two resistors. In other words, the power supply voltage is split and divided between the resistors; the voltage of one resistor is added to voltage of the other resistor to equal the voltage of the power supply.
The same electrons (electrical current) are pushed and pulled by the voltage through both resistors.


By Douglas Krantz

Sometimes when troubleshooting, the technician has to think inside the box.

For instance, take End-of-Line Resistors. The installer thinks of the end-of-line resistor as a device that gets added to the end of zone wires. The technician often thinks of the end-of-line resistor as something that allows a small supervision current to pass through all the zone wiring.

The voltage on the zone terminals goes to full voltage when there is no current through the end of line resistor.
When there is no current through the zone, like when a wire breaks, no current goes through the internal resistor. The internal resistor then pulls the voltage up to the power supply voltage.


Another way of thinking about the end-of-line resistor, though, is to see it as a pull-down resistor.

Pull-Up Resistor and Pull-Down Resistor

On a sensing input for many electronic circuits, a pull-up resistor or a pull-down resistor keeps the voltage from just "floating"; the voltage is either "pulled-up" to the power supply voltage or "pulled-down" to zero. Without this resistor, the voltage, seemingly by itself and at inappropriate times, can easily drift up or down. For a fire alarm panel, this would be the cause of intermittent troubles and false alarms.

On a fire alarm panel, to prevent this "floating input voltage" from affecting the supervision of the input and output zones, inside the circuitry is a pull-up resistor. When nothing is connected to the zone screw terminals, the voltage is pulled up to the power supply voltage.

The voltage on the zone terminals is pulled down to normal supervision when the end of line resistor is conducting current.
The end-of-line resistor pulls some current, so the voltage is pulled down a ways because the current is causing the internal resistor to take up some of the power supply voltage.


Wired as Class A or Class B, out at the end of the Initiating Device Circuit (input circuit or IDC), and at the end of the Notification Appliance Circuit (output circuit or NAC), is a pull-down resistor (End-of-Line Resistor).

When this loop with its pull-down resistor is connected to the zone terminals of the panel, the voltage is divided between the pull-up resistor and the pull-down resistor. As seen on the zone screw terminals, this is the "normal" voltage (or supervision voltage).

Detection

Similar to the usual voltmeter carried by the technician, inside the circuitry of the fire alarm panel is a voltage detector. The voltage detector measures the screw terminal voltage of the input or output zone (loop).

If it's an input loop (Initiating Device Circuit or IDC):

•  Normal - The voltage detector sees the normal supervision voltage: the voltage is divided correctly between the internal pull up-resistor and the external pull-down resistor.

•  Trouble - The voltage detector sees high voltage: a wire is broken, a connection is loose, a device is missing. There is no pull-down resistor so the voltage is pulled up to the power supply voltage.

•  Alarm - The voltage detector sees low voltage: a device has pulled further down on the voltage.

A smoke detector has its own pull-down resistor that, when the detector goes into alarm, pulls the Initiating Device Circuit voltage further down, and the voltage measurement device detects an alarm.

The voltage on the zone terminals is pulled down to zero when there is a wire to wire short on the zone.
The short draws as much current as the internal resistor will allow. This means that the internal pull up resistor has the full power supply voltage while the zone terminals have zero voltage.


A pull station, a heat detector, or a waterflow switch shorts out the Initiating Device Circuit, pulling the voltage down to zero, and the voltage measurement device detects an alarm.

If it's an output loop (Notification Appliance Circuit or NAC):

•  Normal - The voltage detector sees the normal supervision voltage: the voltage is divided correctly between the internal pull up-resistor and the external pull-down resistor.

•  Trouble - The voltage detector sees high voltage: a wire is broken, a connection is loose, a device is missing. There is no pull-down resistor so the voltage is pulled up to the power supply voltage.

•  Short - The voltage detector sees low voltage: something extra is pulling down the voltage. To protect itself, if there's an alarm, the panel won't connect power to the circuit.

When trying to troubleshoot problems with the fire alarm system, because of all the different ways that a fire alarm system can have troubles, thinking about an End-of-Line Resistor as a Pull-Down Resistor, in combination with the Internal Pull-Up Resistor, will sometimes help.
Based on his electronics training, and his understanding of Life Safety, Douglas Krantz has compiled his knowledge of Conventional Fire Alarm Systems into the book Make It Work - Conventional Fire Alarms. The book covers the basics of the Conventional Fire Alarm System, and shows how Life Safety and internal supervision affects the fire alarm system.

Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

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