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Insight - How Rotary Switch Works

Written By: 

Abhimanyu Mathur
Rotary-Switch.jpg
 
Fig. 1: Image Showing Structure of Rotary Switch
 
When it comes to selecting one mode (or state) from multiple modes (or states) to operate a device , rotary switches turn up as one of the most exemplified type of switch. Some well known applications of rotary switches include speed control in fans, speed control in conveyer belts, selecting time, frequency on the CRO, step control of AC/DC drives and electro-hydraulic valves. These switches can be thoroughly customized depending on applications.  But what makes these mechanical switches able to deliver multiple types of outputs? What structure they have to accommodate so many output stages? How are they able to avoid the size and cost trade-off? This article will explore answers to many more interesting questions.
 
Rotary-Switch-2.jpg
 
Fig. 2: Detailed Structure of Rotary Switch made of Wooden Ply and Metal
 
The image above shows a conventional rotary switch made up of wooden ply and metal. The top of the switch has a knob which works as an actuator and is rotated to put the switch into different working states. . Also seen in the image above is a metallic ball which is held by two thin rectangular plates placed on the lower side of knob. The projections at the bottom are the metal contacts through which the switch is electrically connected to the circuit. Wires are generally soldered on these contacts.
 
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Fig. 3: Bottom Side view of Rotary Switch

Knob & Bearing

The knob extends till the bottom and is strongly punched at the end so that the different components can be held at their respective positions.
Technically, the wooden plies shown in the image above are called decks or sections. The metal plate which forms the bearing also has extensions which hold the knob and the wooden ply together.
The structure of the knob can be understood more thoroughly when it is removed from the wooden ply assembly.
 
Rotary-Switch-4.jpg
 
Fig. 4: Knob Structure and Parts
 
Metallic Ball Movement : The bearing structure is formed with the help of two separate thin rectangular plates as shown in the image above. A small slab holds the rectangular plates to the knob. The bumps have a smooth, blunt edged structure so that the metallic ball can move with an adequate amount of force without creating extra friction that reduces the efficiency of the bearing structure. The thin metallic plates posses’ spring like qualities which enable them to move back and forth as the metallic ball moves over the bumpy surface and keep it firmly placed on the cut sections between them.
The knob extends to the bottom of the rotary switch as it has to move the make and break contact of the switch too. The elongated part of the knob works as dowel as it holds the wooden ply and the movable rotary contact of the switch.
 
Rotary-Switch-5.jpg
 
Fig. 5: Image Showing Mechanics of Grooves and Rectangular Metal Plates
 
The upper part is a cylindrical structure having a pattern of grooves for easy mounting of the knob region over a surface.
The metallic ball moves along the bumped path when the knob is rotated, thus producing a “click” sound every time. This click sound serves as an indicator whenever the knob is rotated.

Contacts

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Fig. 6: Image Showing Metal Contacts Embedded in Wooden Ply Structure
 
Stationary and Movable Contacts : Image above shows the larger wooden ply structure which holds the metal contacts of the switch. The contacts have a small circular cut section through which they can be soldered to the circuit or can be wired.  The common pole is in the shape of a semi circular disc which has an extension through which it can be connected to the circuit.The movable contact of the switch is placed over of this disc as shown in the image below:
 
Rotary-Switch-7.jpg
 
Fig. 7: Image Showing the Movable Contact Switch Arrangement
 
The movable contact is protected by the smaller wooden ply. The movable contact has its one end on the common pole disc and other end keeps on making and breaking contact with the output ports as the knob is rotated. The number of switches that the movable contact can cover signifies total number of switching states for which the rotary switch has been configured for.
 
Rotary-Switch-8.jpg
 
Fig. 8: Movable Contact Switch Parts
 

The image above shows the movable contact of the rotary switch. It has a protrusion which makes and breaks contacts with ports of the switch. A circular section in middle is cut to attach it to the rotary knob. Punched screws are applied to attach the rotary knob to the wooden ply. 

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