CIRCUIT DESIGN
Design layout
An electrical circuit shows the interconnection of circuit elements with the power source. A circuit is divided into sub circuits or current paths each one being an elementary circuit consisting of one or more switching element and one consuming element. The power supply is drawn as two parallel horizontal lines.Refer the following figure
The various current paths are numbered. The contacts of a relay are not drawn next to their operating coil, but in the path where they perform their function. A list of path numbers under the path with the relay coil indicates where its contacts can be found. On the other hand ,the contacts and the coils have the same code.
Circuit design
7.2 Basic circuits
Multiplying a contact
To exercise the multiplying of a single NO push button contact using a relay we assume that by pressing pb1 two functions are activated. That is to light a lamp and to simultaneously move the cylinder A out. When these two functions are combined one single contact can energize both elements in parallel. However when at least one of the two functions is also activated from another switch(as solenoid A in fig),pb1 must have two NO contacts. If such a switch is not available(For example because it is reed or other miniature contact a relay with two or more contacts can provide the two separate contacts as shown in figure. In the fig the cylinder is out stroking when either pb1 or pb2 is pressed. The lamp lights only when pb1 is pressed
The holding circuit
In the figure shown OFF push button is normally closed. When pressed the relay coil is cut off from the supply line and the relay is deenergized.Once the relay is made on by pressing ON button it has no control in deenergising the relay.
Contact inversion
A cylinder switch has normally only one NO contact. If a NC contact is required for a circuit, a relay can do the inversion. We show this in the following example. In the figure(second fig of basic circuit) , Cylinder A returns immediately when pb1 is released, even if it is not fully outstroked.The circuit now will be expanded with two basic circuits: holding and inversion. To press pb1 shortly starts A and it returns automatically after reaching its full end of stroke. That is cylinder A will fully extend before it retracts.
In the figure above the switch a1 is drawn as a normally closed contact. A cylinder switch has only one make contact (NO). Fig b shows, how a relay is used to provide the required NC contact. The pneumatic circuit is the same as in the second figure of basic circuits.
Timing circuits
With the help of two timer relays, a cylinder can make a repeating forward and backward movement, with an adjustable time to remaining each end position. Both solenoids are energized via one timer. The following figure shows two circuits.
The cylinder C is in rest position initially. As start button is pressed timer is energized and the waiting time is set on that timer .Its NO contact energizes the solenoid C+ after the time delay. When the cylinder leaves the rest position it is deenergised, the timing is reset and solenoid C+ deenergised.At the end of the stroke C1 energizes C- and cylinder returns.
Flashing lamp A similar function can be done without a cylinder and its two limit switches using only the timers and a relay. The following figure shows a possible solution.
When the bitable start switch is closed the timer T1 is energized and the lamp glows on. After the set time, the contact in line 3 closes and activates the the relay and timer T2.This opens the NC relay contact in line 1 and the lamp goes out.T1 is deenergised and its contact on line 3 opens again. The relay holds itself via a NC contact of timer T2.When the time set on T2 runs out ,its contact in line 4 breaks and the relay and T2 are deenergised.The lamp glows on again and so on.
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