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Push Buttons/Selector Switches - Working Principle
Push buttons and selector switches are essential pilot devices used to control electrical circuits, particularly for starting, stopping, or selecting operational modes in industrial and consumer applications. While both serve as user interfaces for machinery, their operating principles differ based on how they engage electrical contacts
1. Push Button Switch: Working Principle
A push button switch is an electromechanical device that, upon being physically pressed, moves internal contacts to either break (OFF) or make (ON) an electrical circuit
- Key Components: Actuator (button), Spring, Moving/Fixed Contacts, Terminals.
- Operating Principle:
- Resting State: A spring holds the internal contacts in a default position—either closed or open.
- Actuation (Pressed): When the button is pressed, it moves the plunger, overcoming the spring tension and changing the contact state.
- Returning (Released): Once released, the spring forces the mechanism back to its original state.
- Types by Operation:
- Momentary (Self-resetting): Works only while pressed (e.g., doorbell).
- Maintained (Latching): Stays in the new position until pressed again (e.g., ON/OFF power button).
- Contact Configurations:
- Normally Open (NO): Circuit is OFF by default; pressing closes the circuit.
- Normally Closed (NC): Circuit is ON by default; pressing opens the circuit.
2. Selector Switch: Working Principle
A selector switch is a device with a manually rotated handle or knob that activates electrical contacts to select between two or more different circuit conditions.
- Key Components: Operator (knob), Cam/Selector Mechanism, Contact Blocks (NO/NC), Terminal Blocks.
- Operating Principle:
- Rotation: The user rotates the knob to a specific position (e.g., Left, Center, Right).
- Cam Action: The turning action moves an internal cam, which engages or disengages specific contact blocks based on the position.
- Maintained/Momentary: Selector switches can be maintained (stay in position) or return to center automatically (momentary).
- Common Use Cases: Selecting "Hand-Off-Auto" modes, switching between high/low speeds, or choosing between two different power sources.
Comparison Summary
| Feature |
Push Button Switch |
Selector Switch |
| Actuation |
Pushing (In/Out) |
Rotating (Left/Right) |
| Primary Function |
Immediate, short-term actions |
Selecting/maintaining modes |
| Default Action |
Usually springs back (momentary) |
Usually stays in position (maintained) |
| Example Use |
Start/Stop button, Reset, Emergency Stop |
Auto/Manual, Forward/Reverse, Speeds |
Key Takeaways
- Push Buttons are for "pulse" or short-term, instant actions.
- Selector Switches are for selecting persistent states (e.g., leaving a machine in "Auto" mode).
- Safety (E-Stop): Emergency stop buttons are specialized push buttons designed to instantly cut power to machinery when pressed.
For further technical details, you can explore the Schneider Electric Blog or Omron’s Switch Guide.
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