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HowTo: How to build a triac switching unit (SSR)  (Robert Stark)

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Step 1 : Basic information

The triac switching unit described here is the same basic design used in many solid state relays (SSR). There are many different designs but I'll only describe two here.

Step 2 : Design variations

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".

Step 3 : Building a prototype

I prefer to use etched printed circuit boards for all my designs. I've managed over the years to collect all the necessary components to etch my own boards (design software, tank, chemicals, drill press, etc...) but perfboard is fine if you are used to using it. I like the etched boards because once you have a good design it's a snap to make a bunch of them.

Here is the PCB layout for a 4 channel board using the schematic diagram on the back of the TRIAC package from Radio Shed. This view is looking at the board from the component side as if you had x-ray vision. If you look closely you can see the parts placement in purple (very light... sorry). To make the boards, I use a product called "Press-N-Peel" for transferring the layout from the computer to the copper clad board. When you print the layout on the Press-n-peel, then iron it onto the board, it comes out reversed, i.e. the correct orientation for the copper side of the board.

Step 4 : Building a prototype (cont)

After you soak the copper clad board in acid for a while, clean it off, drill some holes, and mount some parts, this is what you wind up with. The triacs are mounted with long leads so they can be bent and moved a little depending on how or if you want to attach it to a heatsink.

Step 5 : Building a prototype (cont)

Here's a view of the backside of the board.

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