The one I'm looking at is 685 Watts, and I may want two fans. A knob is fine. I would need an override to force on or off - separate from controller. I do have tools for soldering, testing, etc, but I haven't done much circuit board work -- happy to try it.
Folks, does this really have anything to do with Single-Board Computers ? If not, then perhaps move it to a more appropriate area ? element14jamie might be able to help with that if necessary
For these sort of projects around my house, if my n isn't too great, what I do first is build a box to contain my (solid-state-relay) SSR. I get an electronics box that is sand-cast rather than made from sheet metal. I like Hammond Industries of Indiana because they make sturdy stuff. I would get a good-quality SSR from a major manufacturer, rated for at least 20 Amperes. Try to get one that features zero-cross switching and inductive load capability. This goes in the box. You can get power inlet (get one with a fuse) and outlet modules that go on the box. You will need to cut/file rounded-cornered rectangular holes to mount them, but that is easier than messing with strain-relief pliers in low quantities. You have three lines from the power co. Hot, Return and Neutral. Hot is a black wire associated with the thinner blade. This is what you switch with the relay. Return, the white wire associated with the the thicker blade, goes straight through the box. Ground, green wire, central, roundish blade goes straight through, but requires a robust electrical attach to the chassis. Be sure to use a SS star-washer, to scrape through the oxide layer on the box. Don't even use a crimp connector, screw the solid wire onto the inside of the box. This is the one connection that mustn't fail. Use a self-tapping screw from the cost-killer's electrical aisle intended for this purpose. Use beefy wires for the high-current path. Green gnd is supposed to be the thickest of all, just preponderant. Use a slightly bigger box than necessary. This is good thermally, easier on our fabricator, and provisions space for a terminal block should we decide to use one for any protection stuff we may decide to add later.
Skinny wires can connect to the control side of the SSR. I use a mini stereo jack that has three connectors. I use tip for +in and first ring for -in. I float the outermost ring, but this automatically attaches the shield of my cable to chassis gnd. I think I am supposed to put a 'not audio' label on the input. I get a chassis-mount led set-up, toss in a ballast R, and shunt my SSR input with it. This gives me an indicator light that is useful when I want to test a system with out hooking up the end-effectors.
This is not the cheapest way to go but it is the best way. Now you have a fungible relay-box that can be used for other projects. This also allows us to make the choice of control circuit later, while getting significant work done sooner.
I would look for an SSR that features dual SCR rather than TRIAC output. Except for rubber feet, I don't modify the box, just the lid. I use calipers and a drawing program program to make footprints of where everything goes. Then I print a sticker on a laser printer. I check everything twice before I undergo the irreversible step of drilling holes. If you can use roll-form taps rather than store-bought ones, you will have far less little shavings running around your box. You might get away with slightly, slightly smaller holes than the tap manufactures recommend. They are thinkin' Fe, you are using Si Al.
Thank you! As an old school programmer (Fortran, C, Visual Basic, ...) I thought someone might have an SBC solution for this application. Looks like this "analog computer" is the way to go. Thanks again, Don.
I forgot to add that the SS Relay box should be labeled with something like "Warning -- Turns on Randomly." If you build this box and can demonstrate that it can turn on, and more uncertainly, turn off the fans, we can control them with either an analog or digital device. But, if all you want is the simple operation you delineate, say, turn the fans when the outside ambient is 5C or so cooler than the house, back off when the differential falls, say to 2C, simple, cheap and highly reliable op-amps will work. If you decide you want to data-log later, we can monitor the op amps with a Von-Neumann type computer board, or supplant them completely with one.
There is something elegant about avoiding such a computer in this application. In 50's Sci-Fi they may have refereed to this overall circuit as an example of a homeostat.
