Other Types of Interlock Switches.- Transistors can be used as switches in solid-state circuits. More information is presented on these switching devices later in this chapter.

Relays

A relay is simply an electromagnetically operated switch. Relays are designed to open or close a circuit when the current through its coil is applied and removed, or varied in magnitude. The main parts of a relay are a coil wound on an iron core and an armature that operates a set of contacts. A simple relay and circuit are shown in figure 5-8.

Looking at figure 5-8, if you close switch S1, current flows through the coil, energizing the electromagnet and drawing the armature upward. The action of the armature closes the contacts and power is applied to the load. More contacts can be added to the armature so that other functions may be accomplished,

The operating speed of a relay is determined by the time between the closing of the coil circuit and the closing of the relay contacts. In small specially

Figure 5-8.-SimpIe relay circuit.

designed relays, like the ones in the 5"/54 gun mount control circuits, the operating speed may be as low as 1 millisecond. The operating speed of a relay may be increased by any technique that reduces eddy currents in the core. Making the core of laminations is one method of reducing eddy currents and thus increasing the operating speed of a relay.

Another method is to place a resistor in series with the relay coil and increase the operating voltage. These actions will increase the speed of closing because, at the instant power is applied to the relay, all the voltage will appear across the coil and the magnetic field will build up faster. The speed of relay operations can be reduced by placing a heavy copper sleeve over the core of the coil. The copper sleeve has the effect of a shorted turn. Current flow in the sleeve opposes the field in the coil as it builds up or collapses, thus delaying the operation of the relay.

The type of material used for contacts depends on the amount of current to be handled. Large power relays usually have copper contacts and use a wiping action to make sure of a good connection. Small relays may use silver or some silver alloy, while in some applications tungsten or some very hard material may be used that will prevent contact burning or oxidation. In general, relays that open and close with a fast positive action cause much less trouble than those that operate slowly. Relays that malfunction or fail completely should be replaced. Relays should not be repaired.

CONTROL RELAYS.- Control relays are used in control circuits to perform switching operations automatically and in the proper sequence.

Figure 5-9.-Miniature "canned" relay.

Miniature-Type Control Relays.- Miniature relays (fig. 5-9), often called "canned" relays, are two-position hermetically housed relays with plug-in pins that mate with a receptacle. Wiring from the receptacle terminal arrangement completes electrical circuits as appropriate. Basic internal components of the miniature relay are a spring-return armature, a coil, and six sets of contacts. (Each set consists of a common contact, a normally open contact, and a normally closed contact.) When the relay energizes, the normally closed contacts (HB) open and the normally open contacts (HF) close.

Time-Delay Relays.- Time-delay relays (fig. 5-10) impose controlled time lapses into electric circuits. The relays consist of pneumatic timing units, coils and switch housings, and wiring terminals. When the coil energizes, the coil plunger acts on the diaphragm in the pneumatic timing unit. Because the relay contacts cannot make (or break) until the air escapes from the diaphragm through an adjustable orifice, the size of the orifice determines the delaying interval. The gun mounts use two types of time-delay relays: slow-make instantaneous-break (SMIB) and instantaneous-make slow-break (IMSB).