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HowTo: How to build a triac switching unit (SSR)
( Robert Stark)
Step 2 : Design variations
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| | Fig 1 shows a the schematic for our circuit. I'm using 4 amp triacs not for any specific reason, I just got a good price on them. You can put any type of triac in it. The larger the current rating on the triac the cooler it will run at lower currents. A board that runs warm with 4 amp triacs would probably run cool with 8 amp units. Figuring that a typical 100 mini bulb strand operating at 115v uses about 1/3 amp, there is lots of overhead here to play with. C6 or any other type of large bulb, however, will draw more current so watch out. If you plan to load up and operate this SSR at anything over 1/2 the triac's current rating, I highly recommend using a heat sink.
Here's how the circuit works: The optoisolator protects your computer from the evil nastiness of the power line. It has an LED that is optically coupled to a triac inside the package. In our application we are supplying the LED side of the MOC3010 with +5 volts. This may be a little new for some folks who are used to grounding everything and supplying power where needed. Here we're doing just the opposite. We're supplying power everywhere and grounding where needed. When we get to the controller description, it'll make perfect sense. When pin 2 of the 3010 is connected to ground, the optoisolated triac inside is "turned on" and current is allowed to flow between pins 6 and 4. This applies current to the gate of the triac, thus turning on the "relay".
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