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im looking to control some 120v ac circuits with my arduino and i was looking in to the ssr and triacs,  from my understanding a ssr is a opto isolator and a triac in one package right?   is the opto isolator nessesary or  can the arduino drive a triac directly.   this will be just for on/off, no dimming
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I'd absolutely use opto-isolation, either as a discrete component or as part of an SSR. Many SSRs are opto-isolated, but not all. SSRs come in AC and DC versions, the latter often employing MOSFETs. If I went with an SSR, I'd be paying close attention to the datasheets. An isolator similar to something from the MOC30xx family and a discrete TRIAC is also perfectly reasonable.
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Get an SSR specified for controlling AC, with a current rating high enough for the job. If the load is inductive (e.g. a motor), make sure the SSR is specified as suitable for inductive loads. Coupling an opto isolator to a triac yourself isn't easy because you need a power supply for the triac gate drive.

You absolutely must use isolation between the Arduino and a triac, otherwise there is the danger that the Arduino will become live.
« Last Edit: December 31, 2011, 05:16:36 am by dc42 » Logged

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Coupling an opto isolator to a triac yourself isn't easy because you need a power supply for the triac gate drive.

???



* moc30xx.JPG (44.89 KB, 992x688 - viewed 21 times.)
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OK, I was thinking about ordinary opto isolators, I didn't realise that opto triacs were inexpensive and readily available. Thanks!
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Yep, they're handy critters. That particular datasheet is for the random-phase variety, there are also types with zero-crossing detectors built in.
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There are many ways to skin a cat, morover loads of cats to be skinned....
If you scrap box only has SCR's insert a bridge rectifier in the load circuit with AC terminals emulating MT1 and MT2 of the triac example, and place the SCR across the DC side Anode to Bridge + and Cathode to Bridge -. A diac used for the gate trigger can be fashioned out od 2 transistors, a 2N3904 and a 2N3906

* SCR.pdf (13.17 KB - downloaded 10 times.)
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And before anyone gets the urge to say the transistors are in backwards, they are not! The circuit is using the VRBEB which is typically 7 VDC to emulate a diac. The transistors have a hfe of around 5 when using the E-B junction to serve as C-B junction. A 2N3904 or 2N3906 makes a good 1/4 watt zener in a pinch when you can accomodate for a variation in zener voltage.
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