Why the relay coil at both ends to parallel a diode
To ensure that the current flow only from one end to determine the direction of the electromagnetic field
At this time, as long as we connect the diode at both ends of the coil, it can make it produce a loop (power equivalent to the end of the coil at both ends of the short line), so that the energy stored in the coil finished. This diode acts as a freewheel here, and we usually call it a freewheeling diode. (Attached: the voltage at both ends of the capacitor can not be sudden changes in the current at both ends of the inductor)
In general, when we connect the circuit, the relay coil and switch are connected separately (ie, connected to different branches). We can simply use a transistor to control the coil conduction (of course, the other can). When the coil is energized, the relay switch is closed and the circuit is in operation. When the coil is de-energized, the switch disconnect circuit does not work. But this time there is a problem, the coil can store energy (the coil will prevent the current mutation, that is, upstairs to mention the electromagnetic induction, that is, the current can only slowly increase and decrease), if then let the coil power, It will produce a large voltage at both ends, so that it may damage the coil, connected components breakdown
