Types of SSRs

Understanding

A relay is a device controls one electrical circuit when a control is sent to it from another electrical circuit. In this way, a low-voltage signal can be used to control power for a much larger circuit. Many relays are mechanical devices and work by making two metal contacts touch one another, usually by the use of an electromagnet: when the electromagnet is energized, it pulls on a metal lever causing contacts to touch and electricity to flow; when power is cut from the electromagnet, a spring separates the contacts and the electricity stops flowing.

An 'SSR' is a solid-state version of the mechanical switch but essentially does the same thing using electronics instead of magnetism, metal levers and springs. As such, an SSR, or 'solid state relay' is much quieter and can operate much more quickly than a mechanical device.

An SSR can control either AC or DC electricity, but the control signal itself that turns it on or off is almost always low-voltage DC. This allows small computer chips to be able to do the task, such as the PIC16F688 or other chip. Typically, these chips utilize 5vdc signaling, which is usually enough to trigger an SSR to turn on or off when the signal current stops.

SSR Design

Most SSRs are designed similar to the diagram below:

The low power, low voltage control signal emanates from the computer and turns on an LED inside the optocoupler. The optocoupler provides a measure of electrical safety to the computer by physically isolating the low power side of the circuit from the high power side. The optocoupler's internal LED shines on a light-sensitive plate inside the optocoupler which then allows a small bit of electricty to flow through the that side of the optocoupler to the "gate" of the TRIAC (or other high power control device). The TRIAC's gate then "opens" and allows power to flow through the TRIAC to the lights and the lights turn on. This is the basic design of an SSR for AC current.

DC SSRs work in much the same way although instead of a TRIAC to control the powerful DC current, they usually use a MOSFET, which is a special type of transistor. Low power DC SSRs sometimes substitute a power transistor in place of the MOSFET, but the concept is essentially the same as above: a low voltage signal activates an optocoupler which lets higher voltage/current flow through a power control device which then turns on the lights.

To limit the electrical current coming from the computer into the optocoupler to within the optocoupler's tolerance, a resistor is often used. In like fashion, a resistor is often used to limit the current that flows to the TRIAC's gate from the optocoupler. The values of these two resistors are often 680 and 180 ohms respectively, but these values aren't universal -- the values depend on the components selected and the current present in the circuit.

Commonly used optocouplers are the MOC3023 or MOC3023M; popular TRIACS are BTA06 or BTA08 types. BTA implies that the metal tab on the back side of the TRIAC is isolated and not electrically connected. BTB TRIACS can also work, but the BTB type has a "hot" metal tab that carries electrical current and cannot be safely used with a common heat sink across all TRIACS. Therefore, the BTA type is much preferred.