Mar 102013
 

switches

push_button_switch

 

 

 

 

 

 

 

Push buttons, switches, commutators. Without this simple device a lot of machines would not operate. In the field of technical solutions there is a great variety of buttons, switches, rotary switches, toggle switches, slide switches, rocker type switches, etc.

Razor On&Off Switch

ON-OFF switch

 

 

 

on-off-switch

 

 

 

 

 

 

 

Some of them emit light after they have been turned on, others have only one stable state ор pass into an active state through a switch. Others are quite small and are mounted on printed-circuit boards (micro switches).

micro button

micro switches

Push-button switches control executive mechanisms, such as contactors and relays through commutation.

all_normal_astro_switches_by_shockwavetherobot-d4vt4u0 jpg

They are mounted on the control panel, most often in a hole and are secured by a nut on the rear side of the panel. Their electrical connection is via soldering. Switches, depending on the number of contacts and the way they switch, have different ways of marking. The simplest one is by a toggle switch (ON-OFF). Its symbol is SPST. The main symbols and indications are shown in the diagram below.

SPST-diagram jpg


There are, of course, a lot of other options and variations of the contact configuration.

spdt jpg

more-switches-relays jpg

More types of switches and their schematic indications


Another important feature of any device equipped with a contact system is the system’s resistance. You should always remember that contacts are the weakest part of the circuit. The contact resistance is affected by dirt, air humidity, pressure force applied on the contacts, material of the contacts, roughnesses on contact areas (the areas where the contacts get in touch), etc.

roughnesses


Influence of roughnesses on the contact configuration’s conductivity


 

contacts

Image of electric power lines when there are roughnesses on contacts.

 

 

 

 

 

 

 

 

In the course of time and switching contacts get oxidized. Every time we open or close the contact this leads to the occurrence of a micro electric arc. The arc facilitates oxidation processes which increase the contacts’ resistance and help for the formation of a stronger and earlier occurring arc. The arc also generates high-frequency electronic interference and changes in the sinusoid of current and voltage.

Contact_arc

Diagram of the formation of a contact electric arc, when contact surfaces come near to each other during switching.

Increased resistance leads to contact heating and energy losses in the system. Gradually soot appears between the switching areas and if nothing is done, it can lead to welding of the contact system and failure. In order to prevent these undesirable processes, we need to undertake a number of special measures while designing the contact bodies: in order to reduce the resistance – the contacting surface is greater than the cross section of the conductive cable core, in order to prevent oxidation, the contacts are covered with a layer of nickel or silver, which are difficult to oxidize and have high conductivity, thus reducing the inevitable voltage drop. The clamping force is large enough to increase the effective cross-section of the conductive surface. Push-buttons with silver contacts like this one have low contact resistance and long electrical durability.

 

 

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