Relays are like a switch. Instead of something physical like a finger turning the switch on they rely on a signal. Most relays are mechanical, meaning there’s a spring and an electromagnet opening and closing the circuit. This bank of 4 relays are solid state. They work just like a mechanical relay but with no moving parts. This has a number of advantages:
- Quicker time to turn on.
- Lower power consumption when on.
- Quieter operation. No clicking sound.
- Highly resistant to shock and vibration.
- Do not generate sparks or arcs.
- Not effected by magnetic fields.
- Insensitive to positioning.
- Longer lifetime. No mechanical wear.
These relays accept a 5V signal and can turn a circuit on or off with a voltage of up to 240VAC at 2A. The control circuit is isolated completely from the higher voltage load. This is done by the use of a photocoupler. In a photocoupler a light source, like an LED, is used and that light is sensed with a photo-transistor. The presence of light on the photo-transistor is what turns the circuit on or off. There’s an optical gap between the triggering signal and larger load.
A MOSFET transistor can perform a similar function to a solid state relay and it can be the right choice in some situations. It’s also possible to switch a higher voltage load using a lower voltage signal using a MOSFET. Compared to a solid state relay a MOSFET has a few disadvantages:
- Not able to switch an AC load.
- Not optically separated control and load circuits.
- Doesn’t block current in both directions.
2 x 4-Channel Solid State Relay Module
There are 4 relays on this module capable of controlling loads independently. The board needs 5V connected to it as well as a 5V signal to control each relay.
A row of pin headers and terminal blocks can be used to accept a signal from a 5V source like Arduino. When the signal is high the relay will turn on.
On the other side of the board 2 terminals for each relay put the relay in series with a circuit, allowing it to turn the circuit on or off.