All electric lock circuits use some form of wire as a conduc- tor. Wire is normally made of a copper core surrounded by a plastic or rubber insulator. The thickness of the copper core determines the amount of current the wire can carry without
FIGURE 27—Since both coils in a transformer are wound on a single core, the magnetic field in the primary winding is induced on the secondary winding.
becoming hot. The thicker the conductor, the greater the amount of current the wire can carry. Wire size is rated by a number system, the American Wire Gage system, or AWG. Figure 28 displays the common AWG sizes.
One unusual aspect of the AWG system is the larger the number used to identify the wire, the smaller the wire’s cur- rent-carrying capability. With most electric lock circuits, you’ll be using AWG 18 or 16 gage wire. The coils of electric locks draw less than one amp, allowing you to use the higher number wire that has a lower current-carrying capability. The insulation that surrounds the copper conductor is nor- mally made of plastic or rubber. Special coatings can also be made of silicon rubber, teflon, and other synthetic compounds. As mentioned, the type of insulation and the spacing between the wire and metallic surfaces determine the maximum volt- age that the wire can safely carry without the wire arcing to a
FIGURE 28—The AWG wire gage system of rating a conductor is shown here.
grounded object. Typically, in electric lock circuits, you’ll be us- ing thermoplastic single- or two-conductor cable. Thermoplas- tic cable carries the markings TFFN for thermoplastic teflon. Electric lock manufacturers have cable available in spools of various lengths for use with their systems.
The individual wires or cables used must be properly stapled or otherwise held in place at various points in the installation. Typically, the wires run from the power supply to a remote switch or push button, and then to the locking device.
If you use staples, make sure to use insulated staples as shown in Figure 29. These staples can be hammered into wooden beams to hold the wires or cables securely to the beams. The staples shouldn’t be hammered too far into the beam, pinching the wire. Instead, hammer them in just enough so that the wire is held loosely in the staple’s insulator.
Some installations require the use of conduit. Here, you have two choices—plastic or metal conduit. Plastic conduit is very simple to cut, glue, and install. Metal conduit requires many additional tools, such as cutters and benders and should be avoided.
Some AC electrified lock manufacturers use an electrical spike suppressor across the coil of the lock. This device is called a metal-oxide varistor, or MOV. This is the same type of device that’s used on power strips for computers and other home entertainment equipment. In operation, a MOV doesn’t conduct and is invisible to the electric lock circuit. However, when the coil of the lock is de-energized, the magnetic field around the coil acts as a form of generator. A large voltage
FIGURE 29—An insulated staple can be used to hold cables or wires to wooden beams.
spike, up to four times the voltage rating of the coil, can be developed for an extremely short period of time. This spike can damage switch contacts and be passed back into the power lines influencing computers, television sets, or whatever. When placed across the coil of the electric lock, the MOV dampens this spike, preventing damage to the electric system and equipment. Figure 30 displays a typical MOV, its electric circuit symbol, and how you should connect it across an electric lock coil.
If the electric lock coil is powered by a DC power supply, the same type of spike can occur as the magnetic field around the coil collapses. A MOV can be used across the coil to dampen this spike. However, a semiconductor device known as a di- ode is normally used. A diode is a form of rectifier device that allows the flow of current in one direction while preventing the flow in the opposite direction. A typical diode, its symbol, and a circuit diagram are shown in Figure 31.
FIGURE 31—A diode can be used in a DC circuit to prevent circuit damage. A diode is shown in (a) with its circuit symbol
in (b) and connection in (c).
FIGURE 30—A typical MOV is shown in (a) with its circuit symbol in (b). In (c), you can see how the MOV is connected across the coil’s leads.
A diode looks like a small barrel with two leads exiting from each end. These leads attach to the diode’s anode and cathode. The cathode is always on the band end of the diode. The trick to installing a diode across the coil is to place it with the anode on the negative lead of the coil and the cathode on the positive lead of the coil. This configuration prevents the diode from conducting when the button is depressed, energizing the coil. The diode conducts only when the button is released, absorb- ing the spike. The easiest method of properly placing the diode is to use color-coded wires for connection to the coil. If, for ex- ample, you use red wire for the positive supply and white wire for the negative supply, you can easily attach the anode to the white wire and the cathode to the red wire. If the diode is installed incorrectly, it will provide a short circuit which would cause the fuse to blow in the transformer’s primary circuit.
A current limiter finds wide use in mortise or cylindrical elec- trified locksets that are energized for long periods of time. By limiting current to the lock’s coil, the coil’s temperature is greatly reduced, prolonging the coil’s lifespan. Sometimes current limiters are built into full-wave rectifier modules. The final components we’ll look at in this section are the actual switches used to energize or de-energize the coils. These switches come in a wide variety of styles, from simple toggle switches to keyed-style switches. Normally, the switches have a single set of contacts to be used in the circuit. However, in special applica- tions, the switches may be ganged together to provide multiple contact arrangements. Figure 32 displays two types of switches.