Switches are needed to turn electrical power on and off. In essence, a switch is a component that can control electrical conductivity and produce a very high resistance (when in off state) as well as a very low resistance (when in on state).
The simple mechanical switch is just one example for this, and there are many different technologies and strategies.
Mechanical Switches And Buttons
In a mechanical switch, two pieces of wire are either connected or moved away from each other, using air as insulation: the electrical circuit is closed or opened.
Switches…
Mechanical switches can either be normally open (NO) or normally closed (NC), and when the switch is operated, it permanently switches from one state to the other and stays there.
…and Buttons
Buttons (more correctly push buttons, or momentary push buttons) have a much simpler mechanical construction: the connection is closed only while you push the button, and once you release it, the electrical connection is opened again.
Latched buttons look like momentary buttons but have clever mechanical constructions that keep them in either state after being pushed. Electrically, they behave like a switch.
Electrical Switches
Often, switches need to be electrically controlled from the distance. There are a number of good reasons for this:
- Power: mechanical switches quickly become bulky when they are designed to switch large loads. Such switches do not fit nicely into an operating panel. It is also not very practical having to use expensive and heavy wires to connect the load to a switch that may be far away from the actual load.
- Safety: electric power can be dangerous both with high voltage and high currents. It is not a good idea to deliver dangerously high energy in close vicinity to an operator.
Relais
The classic approach is to use an electro-magnetic relais: when a small current is applied to a coil, a magnetic field is produced and physically moves a piece of metal so that a separate heavy-duty switch is operated.
Relais use two electrically separated circuits: the control circuit that operates the coil can use safe low voltages and needs just a few milliamperes of current. The load is switched via a separate circuit that uses an iron latch that can be moved by the magnetic field.
A reed relais is a special miniaturized type of relais with no coil (external magnet required). In home alarm systems it is i.e. used to detect the opening of a window: a permanent magnet is mounted on the moving side of a window frame, and the reed relais is fixed to the other side.
Transistor
A transistor is a semiconductor switch with no moving parts. It can turn DC circuits on and off. It does not separate control and load circuits from each other, though.
Thyristor
A thyristor is a semiconductor switch with no moving parts, much similar to a transistor. It is designed for AC though and can be used to turn AC power on and off.
Optocoupler
An optocoupler is a semiconductor designed to electronically separate the control from the load circuit, much similar to a mechanical relais.
An optocoupler uses a built-in LED instead of a magnetic field: when the operator applies power to the optocoupler, the internal LED starts to light up. Adjacent to it, a photo-sensitive transistor picks up the light. This current lowers the transistor resistance and closes the second and completely electrically separated circuit.
Solid State Relais (SSR)
Mechanical relais are robust but have certain disadvantages: due to their moving parts they can fail over time. Their coil produces a magnetic field that can interfere with other electronic parts and produce unwanted large spikes of high voltage in reverse direction when turned off (due to the collapse of the magnetic field). The coils in a magnetic relais require relatively much electrical power to produce a magnetic field strong enough to operate the switch, and this power consumption is required all the time during which the relais is closed. Finally, since a relais internally uses a classic switch, you can hear the relais when it is operated, and this noise can be loud and irritating.
Solid State
With the advent of readily available and powerful semiconductors, the functionality of a mechanical relais can be achieved with solid-state technology (without moving parts and without magnetic fields) in a much more efficient way.
Solid State Relais typically use optocouplers to safely separate the control signal from the load circuitry. The switching of the load is then taken care of by powerful transistors or thyristors, depending on the use case.
Types of SSR
Here are the common types of SSR:
- DA: DC controls AC Most common: a low DC current controls an AC load. Internally, this type uses thyristors to switch the AC load.
- DD: DC controls DC Same as DA, except this SSR controls a DC load and uses transistors for switching.
- AA: AC controls AC In DIY rather uncommon: an AC control signal controls an AC load.
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(content created Apr 29, 2024)