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Description: A solid state relay (SSR) is just what it sounds like; an IC that acts like a mechanical relay. They allow you to control high-voltage AC loads from lower voltage DC control circuitry. Solid state relays,  have several advantages over mechanical relays. One such advantage is that they can be switched by a much lower voltage and at a much lower current than most mechanical relays. Also, because there's no moving contacts, solid state relays can be switched much faster and for much longer periods without wearing out.

They accomplish this by using infrared light as the 'contact,' a solid-state relay is really just an IR LED and a phototriac sealed up into a little box. Thanks to the fact that the two sides of the relay are photo-coupled, you can rely on the same type of electrical isolation as in mechanical relays. These particular solid state relays can switch 400VAC and 8A.

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Comments 41 comments

  • It’s worth noting it’s 400V peak, not RMS. If you’re living in the 230V part of the world, you should definitely go for a 600V version (S208T02, S202S02F). 400V peak is only ~280V RMS which leaves a quite low safety margin for continuous usage under 230V. By the way, these are listed as obsolete by Digi-Key - they list the S202S02F instead. Just saying.
    To the guys looking for a way to make a light dimmer: you won’t make a proper dimmer by just getting a non-zero-crossing version of this one here - you’ll have to drive it with a zero-crossing sensing circuit. Google the U2008 IC instead. It’s a cool phase-control IC. It’s not as “plug-and-play” as this one here as it requires some external circuitry like the optocoupler on the input and an external power triac to trigger (it doesn’t carry the current by itself). But with proper few external elements you can control your mains load with a PWM output - how cool is that! Maybe there is some completely integrated solution like this one here. Somewhere. But I’m not aware of that.
    A a footnote, Sparkfun should pay a little more attention to the customers from 230V parts of the world. You ship globally and have customers all around the world (me included) so the information on the mains voltage should scream at you in bold red font in the relevant product descriptions. Not like in case of soldering stations you carry, where the voltage info is just barely mentioned somewhere within the description. These days, when surfing for stuff, sometimes you don’t even bother to check where in the world the seller is located and OTOH you sometimes tend to take the “right” voltage for granted so it’s really important for you to pay a little more attention to that.

    • I don’t know where you get your math from, but it’s definitely not the same one the rest of the world uses. Let’s say I have 230V rms, the peak voltage is 230 multiplied by the square root of 2, which is approximately 1.414. So 230 * 1.414 is between 325 and 326 volts, well within the range of this solid state relay (with almost 75V safety margin…)

  • Can these be used in parallel to increase the maximum current allowed?

  • Relay noob here,

    1) On page 9 of the datasheet (showing the standard circuit), a protection diode is on the DC controller side. Why is this necessary? Is not the optical isolation sufficient protection?

    2) Any decent tutorials on snubber circuits?

    3) How do I figure the required size of heatsink? On pg6, fig2, do I read up my ambient temp to the desired A, then find the closest curve above it? Do these “Al plates” described need cooling fins, or is just a smooth lunk of metal sufficient?

    • I’m also curious about that diode. Why is that in the sample circuit?

      • It is there most likely in cases where the relay is being triggered by a microcontroller, as when the microcontrollers effectively turn “off” a load, they actually send signal 0 which to the microncontroller is ground - 0 v. So it simply reverses polarity in the 5V circuit, this could cause damage to the solid state relay, so the diode prevents this from being a problem since current can only flow in the correct direction with the diode in place.

  • A couple of comments to help people use these:
    a) As others have said, because it’s a triac-based device, it can not be used on a DC load. AC load only!
    (You can also get MOSFET-based easy to use “solid-state relay” devices for DC use.)
    b) The input side is basically just a LED. Forward voltage drop is 1.2 V at 20 mA. (It’s an IR LED.) So, basically, if you’ve got an Arduino or whatever, you just drive it like it’s a LED. You hook the cathode up to ground, and hook the anode up to an Arduino output with a resistor in series - about 220 R or 330 R or so, maybe 470 R or 1 k if that’s all you’ve got laying around. And it works!
    c) If you’re wiring up 120 V or 240 V AC line voltage, remember to be careful and sensible and stay safe.

    • I should have read this earlier concerning hooking it up like an LED, now I burned up 2 relays without knowing why…

  • When these are used with higher power devices, a heat sink is recommended. The hole size on the TO-220 sink (https://www.sparkfun.com/products/121) looks compatible. Are there any problems using those, that I’m not seeing?

  • Hey, I was thinking of starting a Christmas project in which several light strands are timed to music. At one point I would like them to blink very fast; is there any danger to switching on and off 120V AC at high speeds (like 10 times per second)? Thanks.

  • Hello, question: The datasheet says that the minimum trigger current, under this conditions VD=6V, RL=30Ω, is 8 mA, i’m a chemical engineer and i don’t know much about electronic circuits, so help-me out here…. i’m assuming the RL is the resistor shown on pg 9 connected to the VCC. is this correct? Can i use one lipo (3.7V) as power supply? if so, how can i calculate the RL?

    Thank you guys!

    • RL usually refers to the load, i.e. what you are powering on the A/C side. I’m not sure if that is the case here. But a 3.7V power supply should be plenty. To calculate the value for the resistor connected to Vcc (R1 in the diagram), use V=IR. Datasheet says typical voltage to turn the relay on is 1.2V, and current is 20mA. The purpose of the resistor is to set this current at around 20mA. If supply voltage is 3.7V and the relay needs 1.2V input, the voltage across the resistor should be 2.5V. 2.5V / 20mA gives a resistor value of 125ohms. I don’t think resistors come in 125ohms, but 120ohm or 130ohm should work just fine.

  • I would also love a DC version, I do fun stuff with the Rebel LEDs. The Rebel LEDs are something that must be witnessed in person to understand how bright they are, a photo doesn’t do them justice.

  • Anyone ever used these to switch 24vac HVAC lines in a thermostat?

    • The data sheet says the minumum “load supply voltage' is 80V. It also says the maximum zero crossing voltage is 35V, so at 24V it may not "see” the sero crossing and never turn on.

  • I am running a fan with one at 230V 1.7A (>10A for very few milliseconds during startup). It doesn’t get noticeably warm when used 2-3 times per hour for 2 minutes on each time. During the summer it could be on for 8-10 straight hours. Is that an acceptable use for this thing, or should I use a relay?

  • Using these with a MSGEQ7 to drive christmas lights and I’m pleasantly surprised at how well they work. Don’t even get warm with 100 bulb strands. I’m probably still comparing it to the HSR412s I’ve melted though. Teehee.

  • Any idea what the leakage current of the triac is? Since it is not a physically “open” switch, some current must leak, right?

  • Thanks for the example layout. On the top copper layer, the switch current is run through a couple pours, which is good, but then the pin connections are isolated with thermals. That would, it seems to me, “neck down” the effective copper width to carry the current to something like 80 mil (only two spokes per pour, per side connect to the pins on the relay). That’s barely enough to carry 8A with 2.5oz copper. Removing the thermals, or increasing the size of the pour to get 4 spokes per side would be a good idea.
    See images

  • I’m just throwing in a vote for carrying a DC SSR as well. This is a great one at a great price, btw, so thank you!!!

  • Anyone have an Eagle library for this yet?

    • Check the example layout we linked to. You can export the part and footprint using the standard eagle ULP.

  • Note to anyone wanting to use this. Because it uses a TRAIC for the switch, it can not be used to control DC current.

  • THESE CANNOT switch 8 Amps without a VERY large heatsink! 20x20cm! Read the derating curves on page 6 of the datasheet or you will let the smoke out. We’ve done it before at work.

  • A decent solid start relay for sale? You just made my day Sparkfun! I always have a huge amount of trouble finding SSRs. Just wish I knew what kind of amperage it could take at 120 volts.

  • I would have loved to use one of these for an AC light dimmer, but alas, it is not meant to be.
    If these start selling pretty well, would there be any chance of stocking the version without the zero cross detector?

    • I’ll second that! first thing I looked for was if the device would wait to switch at zero crossing. Really helps with noise problems that way. please stock the device version that has the zero crossing detector.

  • Am I reading the datasheet right, it takes 1.4V to switch the load?

  • It almost looks like a bridge rectifier offhand :) what with the pin labels. More importantly though, the datasheet does not specify whether the switched voltage is ac or dc. the tilde on the ic package labels it for ac, but what would the dc limits be, if it will even accept dc?

    • I thought the same thing when I saw the pic. The data sheet says the minimum load frequency is 47Hz so I assume it is AC only, but I wonder if it would switch a smaller DC load. (2 Amp?)

      • As the switching element is a triac (two thyristors back to back) it would only be able to turn on a DC load, not turn it off.

        • Worse than that, since the triggering element is zero crossing, it would not switch on until … power is removed !
          Reminds me of the write only memory or the batteryless solar lamp !

  • Need a breakout board like the Relay Control PCB.

    • Why do you need one? It’s not like an EM relay that you need diodes and such… you only need a transistor. Are you actually going to pay for a PCB that has this SSR, a transistor, and maybe screw terminals for the output? Make it on a mini breadboard!

      • I had thought about this. This SSR is nicely 0.1" spaced and very breadboard compatible. But when I’m dealing with 110VAC, don’t think a breadboard would be capable of handling a lot of power. I don’t want main voltage anywhere near the rest of circuit or my body. I don’t want connections shaking lose. I really like the idea of a PCB that is solid and stable.

    • Agreed! We’ll start spinning them today, available soon.


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