topics on alarms

Switches

Current and voltage rating
1. Current- or power-switching ratings that current or power that may be switched in a purely resistive circuit.
2. Voltage rating is the maximum voltage that can be carried by the switch in a purely resistive circuit.
3. Current-carrying rating is the maximum current that can flow through the closed contacts. This is normally higher than the current-switching rating.
Contact configuration
1. Make, break or changeover action.
2. If the changeover action required is 'make before break' or 'break before make'.
3. The number of poles in the switch (this is the number of separate circuits that can be switched simultaneously).
4. The number of ways or throws (the number of positions to which each pole may be switched).
5. Type of switch action - momentary action which resets the switch when the operating force is removed.
6. Latching action in which the switch remains in the operated position even when the operating force is removed; to reset the switch a reset force must be applied.
Examples
1. Single pole, single throw, single-pole on/off. Abbreviated to SPST.
2. Single pole, double throw, single-pole changeover. Abbreviated to SPDT
3. Double pole, single throw, double-pole on/off. Abbreviated to DPST.
4. Double pole, double throw, double-pole changeover. Abbreviated to DPDT.
Method of switch operation
Switch operation may be by push button, lever or rotate. The action of a switch involves
making or breaking of an electrical current, the action of the switch when breaking direct
and alternating currents is different. The electrical arc created or drawn when contacts carrying a d.c. current are parted will vaporise the contact metal and maintain an arc
over a considerable distance. Thus for d.c. switching a rapid break and wide contact
separation is required. In an alternating current the voltage falls to zero twice per cycle,
and the arc drawn on switching a.c. tends to be extinguished.

An inductive load may cause current and voltage surges as the circuit is broken and the magnetic flux collapses. With capacitive load there is a high initial current surge as the circuit is made. With both inductive and capacitive load the switches should be de-rated by about 20 per cent otherwise the switch life can be considerably reduced.
Switches, which are suitable for a.c. and d.c. systems, are given two ratings; for example, 250 V a.c.6A or 30 V d.c.10A.
Limit switches
In pneumatic systems, as in many others, it is essential to know that one operation is
complete before the next starts. Switches can be used for this purpose, indicating that a
aguard is closed, a component is in place, a cylinder has fully extended and so on. There
are two basic types of switch:

1. Mechanical contacting.
2. Proximity or non-contacting.
Mechanical limit switches
Basic construction of a micro-switch (2.701 Kb)
These are operated by a moving part striking the actuating mechanism of the limit switch. There are many types of actuators (plunger, roller, lever, etc.). The majority of limit switches have a pair of normally open contacts and a pair of normally closed contacts, but different numbers of contacts and different configurations are available.

When selecting a limit switch for a particular application the following points must be considered: Contact rating, a.c. or d.c.; current life expectancy of the contacts; contact bounce and how it can affect any control circuit; physical size of the switch; movement required to operate switch; effect of over-run; effect of environment on switch; and tile protection needed.

Micro-switches are a particular type of limit switch. They are smaller ill size and have only three contacts, a common contact (C), a normally open contact (NO) and a normally closed contact (NC), as in figure shown above. When the switch is operated the contacts change over, the NO contact is made and the NC contact is broken.
Non-contacting switches
A number of different types are available, the most common being,
¿ magnetic
¿ inductive
¿ capacitive
¿ photoelectric
Magnetic or reed switches consist of a pair of reeds with silver- or gold-plated contacts sealed in a glass envelope, which is filled with an inert gas. When a magnet or magnetic field is brought close to the reeds it causes the reeds to become magnetised and either attract or repel each other, so changing the contacts over. Proximity switches are available, built into pneumatic cylinders; reed switches are positioned on the cylinder barrel and the piston is fitted with magnets.



Solenoids


Construction of a solenoid
Typical construction of a solenoid (2.769 Kb)
The principle of operation of both types of solenoid is similar. An electrical coil wound on a former, laminated for a.c. coils, surrounds an armature, as shown in the above figure.
Principle of operation
Force/stroke diagram for a d.c. solenoid (1.802 Kb)
When the coil is energised, an electromagnetic field forms, which pulls the laminated steel armature into the coil. The pull exerted on the armature depends upon the electrical power applied to the coil and the stroke of the armature. The diagram below shows a typical force-stroke curve for a solenoid.

The force reduces rapidly as the voltage reduces. The shorter the stroke the greater the pull available. The solenoid characteristics in this diagram show that a force of 40 N is available at a stroke of 10 mm. When the armature is at the end of tile stroke the pull will be considerably greater than 40 N; this will cause 'hammering' unless some form of cushioning is used.

The maximum permissible frequency of operation of a solenoid depends upon tile stroke and force required; the less the force or stroke the higher the maximum operating frequency. Next figure shows the relationship between current flowing through a coil and stroke, assuming a constant load.

The current flowing in the d.c. coil is almost constant, independent of stroke. However, the initial current in the a.c. coil is very high. This is known as the in-rush current, and is up to 10 times the current flowing when the armature is at the end of its travel, i.e. at zero stroke. An a.c. coil must always be allowed to complete its stroke, otherwise the large current flowing will burn out the coil.
Applications
Current/stroke diagram for a.c. and d.c. solenoids (1.715 Kb)
Solenoids have two main applications in electro-pneumatic systems:

1. To operate valves, i.e. solenoid valves
2. To operate electrical relays.
Electrical relays
Relay construction (3.239 Kb)
These are electrically operated switches which may be single or multiple. A diagrammatic sketch of a relay is shown in figure below.

Relays are available with either a.c. or d.c. coils to operate at voltages up to about 240 V d.c., 440 V a.c. The voltage of the relay contact switch does not influence the operating voltage of the coil. The solenoid coil may be 12 V d.c. and be switching 120 V a.c. or more.

The life expectancy of a relay depends upon the switch contact material and is usually quoted at a given current and voltage for a.c. or d.c. Relays are available with a fixed number of poles, so many normally open and so many normally closed; with some types the number of poles can be altered by adding contact blocks as needed. The relay contacts can be arranged to be normally open, normally closed, changeover, or make before break.
Solenoid valves
Solenoid operated 3-port, 2-position pilot valve (5.792 Kb)
In the earlier designs the solenoid acted directly on the valve spool moving it from one position to another, the spool being returned either by a spring or a second solenoid. The solenoid had to exert a considerable thrust over a relatively long stroke, which necessitated a high-power solenoid. When a.c. solenoids are used a very heavy initial current flows to start the solenoid moving and if, for any reason, the solenoid cannot complete its stroke, the heavy current will damage the coil. To overcome this problem it is common to use the solenoid to operate a small air pilot valve, the pilot air then being applied to move the main spool. A typical solenoid-operated pilot valve is shown in the figure.
Pilot valve
Symbolic representation of a solenoid-controlled pilot-operated direction control valve (1.97 Kb)
The pilot valve is arranged to operate the main spool valve, as shown symbolically in figure below. The air supply to the pilot stage may be internally connected from the main valve or it may need a separate supply as shown.


Switch and micro-switch

Sketches
Basic SPST, DPST, DPDT switches (3.808 Kb)
Sketches of simplest type of mechanical switches are enclosed for reference. Sketch also explains what is meant by Pole (output terminal of a switch) and throw (movement of a contact from one stationary point to another) of a switch. Note a double-throw switch has a normally open and a normally closed circuit per pole.
Ideally, closed switched contacts should have zero resistance for maximum power transfer. In reality, however, clean contact resistance is directly proportional to volume resistivity of the contact material and inversely proportional to the contact area. This resistance typically would be few milli-ohm for metal to metal contact, but due to complex mechanical, electrical, and chemical phenomena that occur during switching, detoriates the contact surface and increases contact resistance tremendously. The degree to which this phenomena occurs depends on many factors such as contact material, operating environment, contact pressure and type and magnitude of current switched. Accordingly switches are rated and must be operated within the specifications.

Electrical switches used for domestic purposes is usually designed to handle electrical loads from about 3 amp to about 25 amps. The physical dimension of switches depends on this rating. Switches to handle large current must be constructed big and robust. They are usually called circuit breakers. They also need a large force to operate them. Similarly, switches are also made small, called Micro switches. The advantage is the force required to operate them is very small and hence is commonly used in Instrumentation. The disadvantage is that it can carry less load.
Snap-acting switches
Plunger-type and lever-type snap-acting switches (5.492 Kb)

High level alarm
Simple high level alarm (2.985 Kb)
Let us consider a high level alarm as shown in the figure.
In the sketch, if the water level goes up, the float goes up, presses the micro switch down and turns on the lamp. It is a simple circuit. Required change in water level to operate the switch may be around 2 to 3 inches. The float may be of about 4" diameter. The load the Micro Switch can handle, may be 20m.a. and maximum 24v. Under such circumstances probably one can use is only low voltage low current lamps such as LEDs to draw attention and not loud 240v bells or gongs.



Relays

Construction and functions
Functional sketch of electro-mechanical relay (2.479 Kb)
An electrical relay should overcome this problem. An electrical relay is a device that receives and passes on a signal and thereby strengthening it. . An electromagnet is held on the base. Next to it is an armature which is pivoted and can swing. Attach to the armature are two electrical contacts to handle larger current. A spring pushes the armature out when the electromagnet is not energized. The electromagnet can be energized with the small 20 m.a. current. This is the signal in or input side. On the output side, we have three terminals (1) `C` for common;(2) `NO` for normally open and (3) NC for Normally closed. We usually connect the load or the 3 amp alarm bell on the output side.
The input side, or the electromagnet side, is connected to the micro switch circuit, replacing the indicator lamp.
Now what happens is this. Assume we have less water in tank and the 6V and 240v circuits are switched on. The bell does not ring, as the electromagnet is not energized. As the water level goes up (of course, you will need to fill the tank somehow) it will push the small float. This in turn will switch on the micro switch. As soon as micro switch is switched on the electromagnet is energized, the contacts are pulled together, and the bell rings.
Sketches
Typical electromagnetic relays (11.971 Kb)

Advantages
Advantages of relay: -
(1) It takes small current to operate on the input side or the electromagnet side but it can operate larger loads, that is to turn on or to turn off higher load devices.
(2) It provides for independent secondary or output side circuit. Which in turn reduces electrical shocks or short circuit hazards.



Other uses of Micro switch


Other improvements
Earlier we saw how micro-switch is used as level switch. The diagram given there was simple and functional to visualize. However in practice you will find in these gadgets, lot of other improvements to the basic design. This is to take care of the operational problems, which you may not realize at first. For example in the earlier float switch, the float arm that operates the Micro switch must be so constructed, that though the arm should be able to operate the Microswitch, no oil or water from the tank should find its way into the electrical circuit. Normally to overcome such problems, glands may be used. Still better and a permanent solution would be that the float arm actuates the Microswitch through a pair of magnets. See figure. There may be other varieties of them from different manufactures.

Microswitches are not only used in level switches using float as above, but they are also used as pressure switch, flow switch, temperature switch or the differential pressure switch with which you are already familiar in the lab. Functional sketches are provided for reference.

Functional sketch 1
Level switch (2.985 Kb)

Functional sketch 2
Pressure switch (2.613 Kb)

Functional sketch 3
Flow switch (2.732 Kb)