In North America, power is usually provisioned to a premises as either split phase or three-phase power. In split phase, there are two hot lines and a neutral line, which is connected to an earthed ground at the service entrance. The two hot lines are each 120VAC RMS relative to the neutral, and 180 degrees out of phase with each other. This allows 120 volt circuits to be provisioned with one of the two hot lines, the neutral and the ground, or 240 volt circuits to be provisioned with both of the two hot lines and a ground (and optionally the neutral as well). Split phase service is the norm for residences and small businesses. Larger businesses get three phase power, which is exactly the same, except that there are three hot lines, each 120 degrees apart in phase. Any two hot lines will yield a 208 volt circuit. Most devices that require 240 volts will work acceptably well with 208 volts.
Sometimes you need to temporarily power a 240 volt device. We have this issue because we occasionally visit our parent's house with our electric vehicle, and desire to charge it. While we can charge with a single 120 volt outlet, it takes far longer than if we can obtain 240 volt power. This device allows us to do exactly that.
I must pause at this point and raise the very serious safety concerns that this project brings with it. In general, you cannot expect to find two outlets on different circuits (which must also happen to be from opposite phases) right next to each other on the wall. That means you're likely going to be using extension cords to do this. You must use heavy duty models for this purpose. You must not exceed the rating of the lightest-duty component in the entire system. Failure to heed this warning will very likely start an electrical fire. In addition, the concept of attempting to combine two separate circuits back into one is fraught with all sorts of dangerous traps. Every portion of this design was chosen to ameliorate a particular safety hazard. Do not attempt to construct this device until and unless you understand what the design is attempting to achieve and do not attempt to make any changes to the design without fully understanding the ramifications.
In general, doing this is actually a really bad idea. I can't recommend anyone attempt to do this without thinking it through. The only reason I am documenting this project is in the hopes that anyone who does try to achieve this won't fall into some of the very dangerous traps that are endemic to the concept.
Follow along with the schematic as you read this design description. Note that the schematic does not completely represent how the circuit is actually constructed, as the high-current path must be made with large gauge wire while the relay coils and pilot lamps can use smaller wire.
There are three sections of the design, each centered around a DPST relay. Two of the relays have 120VAC coils and the third has a 208/240VAC coil. Each of the first two is connected to a 120V plug, such that the hot and neutral directly activate the coil. The hot line of each plug goes into one pole of the relay. The two relays form a sort of "X" figure. The idea is that each hot line must be switched on by both relays. If either relay turns off, then both hot lines will be disconnected from what follows. So one of the hot lines is switched by the first relay, and the output of that switch goes to the opposite relay where it is switched again. The other relay has the exact same setup - its hot line is switched by one of its own poles, and then fed through the unused pole on the opposite relay. Along the way, the two primary relays have neon indicators across their coils. This allows you to see at a glance if the device is functioning and if not, why not.
The purpose of these two relays is to insure that nothing is connected to the output unless both source circuits are energized. One of the hazards of this concept is that if you yank one of the 120 volt plugs of a naively designed...Read more »