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.

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. Popular power transistors for DC SSRs are the 2N2222A, PN2222ABU and KSC2328AYBU. A popular MOSFET is the FQPF13N06L.

Considerations for Use

"How many lights can I put on a channel?" is a common question asked by newbies. This is really a simple mathematics problem, really. If you follow the guideline that you shouldn't put more current draw on a TRIAC than more than half its rated maximum, a BTA06 TRIAC should be pretty safe if you never drew more than 3 amps on it. If a single light string draws .39A for example, 3amps divided by .39amps per string = 7.69 strings, so rounding down, you could put 7 strings on that channel's TRIAC. Why is the guideline only half of the rating? Because sudden on-rush current usually is greater than a steady current so for a moment, sudden on-rush current could conceivably be between 5-6 amps. A secondary reason is that the more current you put through an electrical component, the hotter it will get. Even at 3 amps, you're likely to need a heat sink on the TRIAC to help keep it from being destroyed by heat, and you certainly don't want to create a fire -- the ultimate result from too much heat!

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.

Other considerations are the gauges of wire that you use to supply power to the SSR as well as the wire that goes to the lights from the SSR. SPT-1 wire is very popular for wiring DIY displays but SPT-1 is limited to a maximum draw of 7amps.

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.

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.