Control Unit Of Automatic Pneumatic Lift - Online Article

The control unit basically comprise of a relay, timer circuit, half wave rectifier, full wave rectifier, push button and solenoid actuated valve.

The half wave rectifier is used to provide half wave DC pulse to the solenoid actuated valves and the full wave rectifier is used to provide full wave DC pulse to the timer circuit. The timer circuit is used to generate a periodic pulse for the relay and the relay is used to provide this periodic pulse to the solenoid actuated valves. The push button is used to as the triggering device for the relay. The solenoid actuated valves are used for opening and closing the passage of air from the pressure vessel to the working unit.



Half-Wave Rectifiers

When a single rectifier unit is placed in series with the load across an ac supply, it converts alternating voltage into unidirectional pulsating voltage, using one half cycles of the applied voltage, the other half cycles being suppressed because it conducts only in one direction. Unless there is an inductance or battery in the circuit, the current will be zero, therefore, for half the time. This is called half-wave rectification. Diode is an electronic device consisting of two elements known as cathode and anode. Since in a diode electrons can flow in one direction only i.e. from cathode to anode so the diode provides the unilateral conduction necessary for rectification. This is true for diodes of all types-vacuum, gas-filled, crystal or semiconductor, metallic (copper oxide and selenium types) diodes. Semiconductor diodes, because of their inherent advantages are usually used as a rectifying device. However, for very high voltages, vacuum diodes may be employed.


The half-wave rectifier circuit using a semiconductor diode with a load resistance RL but no smoothing filter is given in figure. The diode is connected in series with the secondary of the transformer and the load resistance RL, the primary of the transformer is being connected to the ac supply mains.



The A.C. voltage across the secondary winding changes polarities after every half cycle. During the positive half-cycles of the input ac voltage i.e. when upper end of the secondary winding is positive w.r.t. its lower end, the diode is forward biased and therefore conducts current. If the forward resistance of the diode is assumed to be zero (in practice, however a small resistance exists) the input voltage during the positive half-cycles is directly applied to the load resistance RL, making its upper end positive w.r.t. its lower end. The waveforms of the output current and output voltage are of the same shape as that of the input ac voltage. During the negative half cycles of the input ac voltage i.e. when the lower end of the secondary winding is positive w.r.t. its upper end, the diode is reverse biased and so does not conduct. Thus during the negative half cycles of the input ac voltage across the load remains zero if the reverse current, being very small in magnitude, is neglected. Thus for the negative half cycles no power is delivered to the load. Thus the output voltage developed across load resistance RL (VL) is a series of positive half cycles of alternating voltage, with intervening very small constant negative voltage levels. It is obvious from the figure that the output is not a steady dc, but only a pulsating dc wave. Since only half-cycles of the input wave are used, it is called a half-wave rectifier.

Full-Wave Rectifier

In half-wave rectifier only one half cycles of the input are utilized but in full-wave rectifiers both half cycles of the input are utilized. Alternate half cycles are inverted to give unidirectional load current. There are two types of full-wave rectifier circuits namely:-

  1. Centre-Tap rectifier.
  2. Bridge rectifier.

Centre-Tap Full-Wave Rectifier

In such a rectifier, the ac input is applied through a transformer, the anodes of the two diodes D1 and D2 (having similar characteristics) are connected to the opposite ends of the centre tapped secondary winding and two cathodes are connected to each other and are connected also through the load resistance RL and back to the transformer.


When the top of the transformer secondary winding is positive, say during the first half-cycle of the supply, the anode of the diode D1 is positive w.r.t. cathode, and anode of the diode D2 is negative w.r.t. cathode. Thus only diode D1 conducts, being forward biased and current flows from cathode to anode of diode D1, through load resistance RL and top half the transformer secondary making cathode end of load resistance RL positive. During the second half-cycle of the input voltage the polarity is reversed, making the bottom of the secondary winding positive w.r.t. centre tap and thus diode D2 is forward biased and diode D1 is reverse biased. Consequently during this half-cycle of the input only the diode D2 conducts and current flows through the load resistance RL and bottom of the transformer secondary, making the cathode end of the load resistance RL positive. Thus the direction of flow of current through the load resistance RL remains the same during both halves of the input supply voltage. Thus the circuit acts as a full-wave rectifier.

Timer Circuit

One of the most versatile linear ICs is the 555 timer which was first introduced in early 1970 by Signetic Corporation giving the name as SE/NE 555 timer. The 555 is a monolithic timing circuit that can produce accurate and highly stable time delays or oscillation. Like general purpose op-amps, it is very much reliable, easy to use and cheaper in cost. It has variety applications including monostable and astable multivibrators, dc-dc converters, digital logic probes, wave form generators, analog frequency meters and tachometers, temperature and control devices, voltage regulators etc. The timer basically operates in one of the two modes either as a monostable (one shot) multivibrator or as an astable (free running) multivibrator.

The SE 555 is designed for the operating temperature range of -55˚C to 125˚C while the NE operates over a temperature range of 0˚C to 70˚C. The important feature of 555 timer are : it operates from a wide range of power supplies ( +5 V to +18 V supply voltage), sinking or sourcing 200mA of load current; proper selection of only a few external components allows timing intervals of several minutes or frequencies as high a several kHz; it has a high current output; the output can drive TTL; it has a temperature stability of 50 parts per million (ppm) per degree Celsius change in temperature, or equivalently 0.005 %/ ˚C, it has an adjustable duty cycle; the maximum power dissipation per package is 600mW and its trigger and reset inputs are logic compatible.


Circuit Description

Astable and monostable multivibrators are used for timing operations in many electronic systems. The popular eight-pin 555 timer IC can act either as a "square" wave oscillator (i.e. an astable) or as a single pulse generator (i.e. a monostable).

Keeping in time is very important for every human being, that is why many instruments are being used to make the work in discipline order and in time. The timer as the term is used in digital electronics, is an electronic circuit that once triggered, produces an output pulse for a predetermined time or after a predetermined time and then resets when desired. A simple timer would involve momentarily pressing a button (reset switch) to 'on' the device to count the delay time and a variable control to set the required time. After that time interval, the connected device either an alarm circuit or indicating device will 'on' for that presented time.

In this circuit, monostable timer is used for timing operation in electronic systems. This is an electronic switching circuit, which energizes the alarm (oscillator) circuit after a certain period of time. This can be adjusted for few seconds to several minutes. When the circuit is activated and bleed off through timer control, then VR-1 at low, the discharge occurs at a fast rate and vice-versa. The added resistance brings a slow delay of energy to increase time. The circuit has been divided in two part, timer circuit and oscillator circuit.

Monostable Timer

The connections are given in Figure. In this case only one external resistor, VR-1, is required and (Pin No. 6) is joined to discharge (Pin No. 7). One rectangular output pulse is produced when the circuit is triggered by the falling (negative-going) edge of an external pulse applied to trigger (Pin No. 2). It then returns to its one stable state ('low' output) to await the next trigger pulse. The trigger pulse can be obtained for example, by switching R1 from (i.e. +Vcc) to key (i.e.-Vcc) and back to R1, the operation being completed in a time which must be less than the output pulse time.

We occur at Pin No. 3 of IC. This energies the oscillator circuit which given an audio tone as alarm. It can rest by just giving a finger touch to key point. 6 & 7 pins connected each other, which will rest the alarm for further period of time. One can increase the time period by add more value to 100 mfd. and VR-1.

Solenoid - Actuated Control Valve

A very common way to actuate a spool valve is by using a solenoid. When the electric coil (solenoid) is energized, it creates a magnetic force that pulls the armature into the coil. This causes the armature to push on the push rod to move the spool of the valve.

A cutaway view of an actual solenoid-actuated directional control valve. This valve has a flow capacity of 12 gpm and a maximum operating pressure 0f 3500 psi. It has a wet armature solenoid, which means that the plunger or armature of the solenoid moves in a tube that is open to the tank cavity of the valve. The fluid around the armature serves to cool it and cushion its stroke without appreciably affecting response time. There are no seals around this armature to wear or restrict its movement. This allows all the power developed by the solenoid to be transmitted to the valve spool without having to overcome seal friction. Impact loads, which frequently cause premature solenoid failure, are eliminated with this construction. This valve has a solenoid at each end of the spool. Specifically, it is a solenoid-actuated, four-way, three-position, spring-centered directional control valve and is represented by graphical symbol. The symbol used to represent the solenoid at both ends of the spool.

We see a solenoid-controlled, pilot-operated directional control valve. That the pilot valve is actually mounted on top of the main valve body. The upper pilot stage spool (which is solenoid-actuated) controls the pilot pressure, which can be directed to either end of the main stage spool. This 35-gpm, 3000-psi valve is of the four ways, three-positions, spring-centered configuration and has a manual override to shift the pilot stage mechanically when trouble-shooting.

Solenoids are commonly used to actuate small spool valves. This three-way valve is specially designed to be used with small cylinders and other similar devices requiring low flow rates.

Push Button

The push button used overhear is used to provide the trigger pulse to the IC-556 timer circuit. The timer circuit has variable time period which is controlled by a variable resistor. As soon as the trigger is provided to the circuit the output is available at the output pin for the specified period of time and the valve is open for the specified period of time through a relay circuit coupled with timer circuit.

Static Relays


A static relay refers to a relay in which there is no armature or other moving element and response is developed by electronic, magnetic or other components without mechanical motion. The solid-state components used are transistors, diodes, resistors, capacitors and so on. The function of comparison and measurement are accomplished by static circuits.

A relay using combination of both static and electro-magnetic units is also called a static relay provided that the response is accomplished by static units.

In static relays, the measurement is performed by electronic, magnetic, optical or other components without mechanical motion. However, additional electro-mechanical relay units may be employed in output stage as auxiliary relays. A protective system is formed by static relays and electro-mechanical auxiliary relays.

Transistor Relays

Transistor relays are the most widely used static relays. In fact when we talk of static relays we generally mean transistor relays. Transistor which acts like a triode can overcome most of the limitations posed by the electronic valves and thus has made possible to develop the electronic relays more commonly known as static relays. The fact that a transistor can be employed both as an amplifying device and a switching device makes it suitable for achieving any functional characteristics. The transistor circuits cannot only perform the essential functions of a relay (such as comparison of inputs, summation and integrating them) but they also provide necessary flexibility to suit the various relay requirements.


In either of the circuits current of constant magnitude flows in the collector circuit only when the input ac quantities are simultaneously positive; a relay in the collector circuit will pick up when the overlap angle exceeds a certain value, i.e., when the mean dc level in the collector circuit exceeds the relay pick-up as a consequence of phase coincidence.

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