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The Form C has one common and both an NO and an NC.
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The relay, as an antiquated signaling device, has since
become the best means of providing switching or isolating of peripheral
devices.
A relay is an electromechanical device that is energized by
either an AC or DC input. Because fire and burglar controls operate on either
12VDC or 24VDC, any inputs will be DC. An electrical input (DC) is applied to a
coil, which is wrapped around an iron core to create a magnetic field within
the solenoid. That magnetic field attracts the armature toward the coil causing
either a “make” or “break” between contact points.
A relay is available in a single pole up to a four- or a
six-pole, although the double pole, double throw (DPDT) is the most applicable
relay used in the fire and burglary industry.
The contact ratings can be from a few mA up to 5A. Coil
inputs can be 6VDC, 12VDC or 24VDC with a current requirement of approximately
30mA. The more poles, the greater the pull-in current requirement.
Most
importantly, the coil inputs (DC), the coil and the solenoid are completely
isolated. That means there is no continuity between them and the contacts,
which is why the term “dry contacts” is used as opposed to the switched input
of a solid-state device.
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| The common relay board is approximately
a
2-inch by 2-inch circuit board with wide circuit tracks that connect the
pin-outs from the relay to the terminal strips. |
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The isolation of the contacts is where the flexibility of
the relay comes through. By using an applied voltage or a loss of voltage to
the coil of a relay, an isolated, high-current-rated, reliable, mechanical
switch is available.
Whether fire, burglary or access control, a relay allows
lights to be turned off or on, doors to be locked or unlocked, strobes to be
activated, elevators in conjunction with heavy-duty relays to be turned off and
on, and more.
The use of relays to switch or isolate devices also enhances
the reliability of that control. The standard outputs and programmable outputs
of a control box, reader or other device used to drive the coil of a relay
requires a low current draw on that power source.
The contacts of the relay, when switching a separate AC or
DC power source to a remote device, protects the control circuit board tracks
and solid-state devices from the effects of high current, transients and demand
surges or collapses.
There are approximately a half-dozen manufacturers of add-on
relay boards. The common relay board is approximately a 2-inch by 2-inch
circuit board with wide circuit tracks that connect the pin-outs from the relay
to the terminal strips.
The contact rating in volts and amps is printed on the relay
surface. Markings for the coil input (+, -) and the C, NO, NC connections are
adjacent to the corresponding screw terminal.
Typical ratings for a relay are: 1x107 mechanical operations
and 5x105 electrical. The high mechanical operations are due to the low current
requirements and simple pivot design of the armature. The high electrical
switching operation is due to the gold flash contact plating and the
wiping-action of the contact points.
The electrical absorption of a coil and the time for the
mechanics of a relay to operate is relatively slow when compared with the speed
of a solid state device so that a transient can come and go before a relay
knows it or can be damaged by it.
Finally, an add-on relay board is inexpensive and rugged
enough to withstand the rigors of a glove compartment or tool box. —
Contributed by Steve Arnold, president, Advanced Signaling Co. Inc., Arlington,
Texas.
