OPERATING & DEMONSTRATING YOUR FIRE ALARM BOX
By Dave Windle
WARNING!!!! NEVER CONNECT A BATTERY OR ANY OTHER POWER SOURCE DIRECTLY TO YOUR FIRE ALARM BOX! EVEN A MOMENTARY HIGH CURRENT MAY BURN OUT THE COILS!
HOW A FIRE ALARM BOX OPERATES
Fire alarm boxes operate on a specific current – 100 milliamps DC – not a specific voltage. They are wired in series, just like many Christmas tree lights, so that when any one box breaks the circuit by being pulled for an alarm, the entire circuit opens.
Inside the mechanism of your box a set of contacts ride against the code wheel – a metal wheel with points around its edge. These points duplicate the number on the front of your box. For example, box 451 will have 4 points, a space, 5 points, more space, then 1 point and a final space back to the beginning. At the normal stopped position of the code wheel, the contacts are being forced together. When the box is pulled, the wheel begins the rotate and the contacts open and close operated by the code wheel, sending the box number.
DEMONSTRATING A FIRE ALARM BOX WITHOUT POWER
If you do not want to power your alarm box, but want to show how the contacts operate against the code wheel, you must bypass the non-interfering feature of the box. To do this, locate the two electric coils in the mechanism (they are side-by-side and about 2 inches long by 1 inch diameter). At one end of the coils is a pivoted metal arm that touches the end of the coils when they are energized. Tape this are against the coils. The contacts will now ride on the code wheel and the box will stop after four revolutions of the wheel.
CONNECTIONS TO DIFFERENT MODEL FIRE ALARM BOXES
|HORNI or SAFA box||Use the two wires leading out of the bottom of the inner box which houses the mechanism and bell.|
|HARRINGTON box||Use the black and white wires leading from the inner box (the green wire is a Ground and is not necessary for your hookup)|
|GAMEWELL box||Use the two outside terminals on the porcelain connection block in the inner box (the center terminal is for ground and is not needed)|
WIRING AND POWERING THE ALARM BOX
Because the current must be limited to about 100 milliamps DC to protect your alarm box, it is necessary to place a RESISTOR between the battery or DC power supply you intend to use and the box. These are available from local electronics stores.
To calculate the value resistor you need, decide what power source you will use, so you will know the voltage you will be using. This must be a DC source such as a battery or DC power supply from an electronics store. Dry cell or lantern batteries work fine and it is recommended that the source be no more that 12 volts for safety.
To determine the resistor value (in OHMS) multiply the voltage you are using by 10 to get overall resistance. Then subtract 20 from this value and the result is the value resistor you need.
EXAMPLE: You will use a 12 volt battery. Multiply 12 by 10 and the result is 120 ohms, the overall resistance for 100 milliamps current from a 12 volt source. But the alarm box itself has about 20 ohms resistance, so subtract this from 120. The result is 100 ohms. Buy a 100 ohm resistor and place it in the line between the battery and the box.
NOTE: Resistors not only have a value in ohms, but also a power rating in watts. For power sources of 6 volts or less, the resistor can have a ˝ watt power rating. For 6 to 12 volts, the resistor needs a 1 watt power rating. You can always use a higher power rating than called for, but not a lower rating.
Because an alarm box uses a closed circuit, if you leave a battery connected to your box (through a resistor, of course) it will run down. Therefore, put a switch between the battery and alarm box, or connect it only when you want to demonstrate it.
Also be aware that resistors are available only in particular ohm values. If the value you calculate for your power source turns out to be one not available at your local electronics store, buy the closest value available. For example, if you calculate a resistor value of 50 ohms is needed, a 47 ohm or 56 ohm resistor will work fine.
DEMONSTRATING THE ALARM BOX
With power connected to the box, pulling the hook and starting the box will cause it to make four revolutions of the code wheel and then stop. No other indication other than hearing the mechanism turning will take place unless you open the door of both the alarm box and its inner box. Then the bell in the inner box will tap the box number as the code wheel turns. The bell works only with the inner box door open because it was intended as a means for the fire alarm office to communicate with a person using the telegraph key inside the fire alarm box, such as a Chief’s aide calling for more apparatus to a fire.
To demonstrate a full fire alarm system you would need a gong or paper tape punch register designed for a standard 100 milliamp fire alarm circuit. Typical alarm bells available from electronics stores will not work. If you add a gong or punch register, you must recalculate the resistor you are using between the power source and the box because the resistance of the gong and/or register must also be subtracted from the overall resistance calculation.
To accurately set the current at 100 milliamps when adding gongs or registers in the circuit, use a meter such as a VOM (Volt-Ohm-Milliamp meter). Connect the meter, set for reading milliamps, in the line between the power source and box, with the gong or register also in the circuit. If the current is less than 100 milliamps, use a little smaller value resistor. If it is more than 100 milliamps, a little larger resistor.
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