I've designed (and mostly implemented, now) a system for turning on and off 24VAC solenoids. In order to achieve this, I used optocouplers with TRIAC output (Sharp PR26MF1xNSZ series). The datasheet for this device describes it as a "Solid State Relay", "an integration of an infrared emitting diode (IRED), a Phototriac Detector and a main output Triac."
My interest in such things has lead me to look around for other options for this part. I happened upon Vishay Semiconductor's offerings for similar parts, and Vishay divides this type of component into two distinct categories: solid state relays and optocouplers with TRIAC output.
I looked at Vishay's datasheets for two representative devices, and I discovered that while the optocoupler seems to be quite similar to the Sharp part, the solid state relay part doesn't use a TRIAC for output. Instead, it seems to use a pair of MOSFETs as the output stage.
What are the advantages and disadvantages of these two different types of parts? What would be the benefit of using MOSFETs as the output stage instead of a TRIAC? Is there a real distinction between "solid state relay" and "optocoupler with TRIAC output", or is "solid state relay" a term used for any device that can be used to switch AC or DC?
Best Answer
The SSR is a low voltage DC device, but may also be used for low voltage AC, while the triac device is for AC usage only, typically mains voltage.
A triac has a
PNPN-structure, which means that there will always be a voltage drop across it, while FETs are resistive devices, and for those an \$R_{ON}\$ is specified, in this case 0.25\$\Omega\$ maximum.The voltage drop over the triac makes opto-triacs less convenient for low voltages, where they may drop out too much of the available voltage. For instance a 3V drop on a 24V supply means that you lose 13%. So it looks like in your case the "SSR" is a better choice. You'll have to look at the maximum current, though. The datasheet says 2A, but on the Absolute Maximum Ratings graph it shows 1A. My guess is that this should be read as a normalized value, and that the actual maximum is indeed 2A, to be derated at higher temperatures.