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.

AC and DC SSRs are not generally interchangeable. A TRIAC-based AC SSR normally cannot switch DC off after it has been triggered because DC current does not cross zero. A MOSFET- or transistor-based DC SSR may not be suitable for AC unless it was specifically designed for bidirectional operation. This allows small computer chips to be able to do the task, such as the PIC16F688 or other chip.

The control input of an SSR usually operates from low-voltage DC supplied by a controller. The controller must provide sufficient voltage and current to activate the SSR input. Removing the control signal normally turns the output off, although an AC TRIAC remains conducting until the load current falls below its holding current, usually near the next AC zero crossing.

SSR Design

Most SSRs are designed similar to the diagram below:

Many commercially manufactured SSRs are available for common applications and are often the preferred choice when building mains-voltage systems because they have already been designed, tested, and packaged for safe operation.

The low power, low voltage control signal emanates from the computer and turns on an LED inside the optocoupler. The optocoupler provides galvanic isolation between its input and output. However, the safety of the completed SSR also depends on the component ratings, PCB creepage and clearance distances, wiring, connectors, fusing, enclosure, and construction quality. An optocoupler alone does not make a mains-voltage circuit safe. The LED illuminates the optically coupled output device 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 is then triggered into conduction, allowing current to flow through the load. 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.

Popular optocouplers are the MOC3023 or MOC3023M for AC SSRs or the K847PH and PC817 for DC SSRs. 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, which is ideal for putting a heat sink across multiple TRIACS to help keep them cool. 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. Component selection depends on the load voltage, load current, control voltage, required isolation, switching method, and thermal design. Devices such as the MOC3023 are random-phase optotriac drivers intended to trigger an external TRIAC in AC applications. Devices such as the PC817 and K847 are general-purpose phototransistor optocouplers that may be used in some DC control circuits. The output switching device must be selected from its datasheet for the actual load; small-signal transistors such as the 2N2222 are suitable only for relatively small loads, while larger DC loads normally require an appropriately rated MOSFET. A popular MOSFET is the FQPF13N06L.

The DirkCheapSSR uses the VO2223A, an optically isolated power phototriac that combines the optocoupler and AC switching device in one package. The manufacturer specifies a maximum load current of 1A RMS under defined operating conditions. The allowable current in an actual SSR assembly may be lower depending on ambient temperature, PCB layout, ventilation, duty cycle, load type, and enclosure. Consult the current device datasheet rather than treating 1A as a universal continuous-load rating.

Considerations for Use

"How many lights can I put on a channel?" is a common question, but the TRIAC's headline current rating alone does not provide the answer. The allowable load depends on the TRIAC's on-state voltage, power dissipation, junction-temperature limit, ambient temperature, heat sink, PCB construction, enclosure, duty cycle, and the starting or inrush current of the load.

Determine the steady-state and inrush current of the connected load, then consult the TRIAC manufacturer's datasheet and perform the required thermal calculations. The channel must also remain within the ratings of the fuse, PCB traces, connectors, wiring, outlets, and branch circuit. Applying an arbitrary percentage of the TRIAC's current rating is not a substitute for this analysis.

Another consideration is the total capacity of the SSR itself. If you put 3amps on each channel of a 4-channel SSR, the total draw of that one SSR will be 12 amps -- and that's the maximum recommended draw on an home's entire 15A circuit! You'd also have to verify that the circuitry on the SSR's circuitboard can support that much electricity, since many SSRs have fuses that limit the SSR's total to 5, 7 or 10 amps. :Light string manufacturers usually specify that you should not connect more than 3 (or 5, 8, 12, etc.) strings together end-to-end. There's a reason for this. The strings themselves are designed with a relatively small gauge wire and when you plug too many together, the total draw on the wire is too great, which can heat up the wire, melt the insulation and result in a fire. Be sure to consult the tags that are normally attached to the strings to determine what the safety limit is and don't go past it. "One more string won't hurt" is not the right attitude to take; it could be exactly one more string that causes a fire.

Use wire and cordage that are listed and appropriately rated for the voltage, current, temperature, environment, and application. The designation SPT-1 alone does not establish one universal current capacity; verify the conductor gauge and the manufacturer's rating for the specific cord being used.

The point of all the above is that the TRIAC in the SSR is just one part of the whole pie -- you always need to take the total into consideration when it comes to deciding how many lights you can put where. There's a lot more to this than simply plugging in another string of lights wherever you want.

Making Your Own

Mains-Voltage Safety Warning

Constructing an SSR that switches household AC power presents risks of electric shock, fire, equipment damage, and death. A working schematic is not necessarily a safe design. Mains-voltage construction requires appropriate creepage and clearance distances, listed components, overcurrent protection, strain relief, touch-safe terminals, an appropriate flame-resistant enclosure, protective grounding where required, and workmanship suitable for the environment.

Do not construct or use a mains-voltage SSR unless you understand these requirements and are qualified to perform the work safely. Never work on the circuit while it is energized. Local electrical codes and inspection requirements may apply.

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Although a basic SSR uses relatively few electronic components, designing and constructing one that is both reliable and safe requires careful attention to component selection, PCB layout, wiring, isolation, thermal design, and enclosure construction. The four important parts are the optocoupler, a TRIAC and two resistors. The other parts you might add for convenience are various connectors for taking the control signal from the computer and perhaps other plug connectors for mains power and connecting the output to the lights. Common problems associated with "my SSR stopped working" are poor soldering, poor physical connections for the control signal from the computer or the connections to power and the lights. Electrical-wise, usual problems are short circuits in the power output wire/connectors to lights or too much current draw, either one of which will overheat and burn out the TRIAC very quickly -- almost instantly, in fact.

Prototype construction is common during development, but mains-voltage circuits should ultimately be built on a PCB or construction method that provides appropriate creepage, clearance, mechanical strength, and insulation for the operating voltage.

The resistor values must be calculated for the specific optocoupler, TRIAC, controller voltage, and operating conditions. For an optocoupler input, begin with the controller's output voltage, the optocoupler LED's forward voltage, and the trigger current specified in the manufacturer's datasheet. Verify that the resulting current is sufficient under worst-case temperature and tolerance conditions but remains within both the optocoupler and controller-output ratings. Also calculate the resistor's power dissipation and select an appropriate wattage with adequate margin.

The TRIAC gate circuit must likewise be designed from the optotriac and TRIAC datasheets. Gate current, polarity, quadrant operation, load type, line voltage, snubber requirements, and component tolerances can all affect reliable triggering. A resistor value that works with one combination of parts should not be assumed suitable for another.

Whether a heat sink is required must be determined from the switching device's power dissipation and thermal ratings. Even loads below 1A may require thermal consideration in a sealed enclosure or at high ambient temperature. Test the completed assembly under its maximum expected load and environmental conditions, but do not rely on temperature testing alone in place of proper calculations.

There are many, many SSR circuits on the Internet using many different component parts. Many use various MOCxxxx chips as the optocoupler component and different resistor values. Take some time to study them!