This $30 Solar Setup Heats a 30 X 40 Workshop for Five Hours or More Every Sunny Winter Day
(Page 9 of 9)
November/December 1977
By the Mother Earth News editors
Pretty neat, huh? Except, of course, for the fact that a high voltage thermostat can set you back about $12 to $15. However, as you might have expected, there is a way for the dedicated scrounger to get the same job done for considerably less.
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Go back to that junked gas furnace you've been scavenging parts from and pull out its thermostat. Yes, this is a low voltage thermostat, which means that it can't be wired directly into your blower's circuit the way a high voltage thermostat can (the load would burn it out). But that's no problem either. With just a little tinkering, we can make this one work too.
You'll need the step-down transformer that you can take from that old trusty-rusty gas furnace that has served you so well for so long. You'll also need one of the 12-volt relays that
Radio Shack and other electronics stores sell for anywhere from $3 to $5. The relay should have a 12-volt coil and contacts rated for 120 volts at 5 amps minimum. And try to get one with a coil designed for alternating current. A relay with the more sensitive direct current coil (which is what we used, since that's what we already had) won't work unless you add a diode and capacitor as shown in Diagram 2.
And since that's about as complicated as we can possibly design our circuit, let's hop on down to Diagram 2 and learn how to make this final, scrounged-up system work.
What we have is a circuit in which a low voltage thermostat connects and disconnects a low voltage transformer to and from the coil in a high voltage relay. And as that relay opens and shuts, it—in turn—connects and disconnects your solar heater's blower to and from the 110-volt electricity that makes it run. If the relay came wired with an AC coil, all well and good. You're home free. If it has a DC coil, however, you'll have to add the diode and capacitor shown in Diagram 2.
Which brings us to one final bit of electronic know-how that you should have. A capacitor large enough for this job (100 microfarads or so) will probably be electrolytic and, thus polarized. (That is, it will have me terminal that is positive + and one that is negative -). If you hook such a capacitor up "backwards" you'll burn it out and you must therefore take care to connect it properly.
But that's not difficult either, since there's such an easy way to determine the polarity of any low voltage source. With your transformer plugged in and the diode connected, simply stick the bared ends of the copper wire about 1/4" to 1/2" apart in a chunk of raw potato. Leave them there for a half hour - Dur ing that period an electrochemical reaction will take place, turning the potato dark blue around the wire connected to the positive pole. Connect the + side of the capacitor to that leg of the thermostat and the - side to the other one. —D.W.
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