问 Transistors and ICs (chips) must be protected from the brief high voltage 'spike' produced when the relay coil is switched off. The diagram shows how a signal diode (eg 1N4148) is connected across the relay coil to provide this protection. Note that the diode is connected 'backwards' so that it will normally not conduct. Conduction only occurs when the relay coil is switched off, at this moment current tries to continue flowing through the coil and it is hARMlessly diverted through the diode. Without the diode no current could flow and the coil would produce a damaging high voltage 'spike' in its attempt to keep the current flowing.
答 1: 我不能理解是  the damaging high voltage 'spike' 它的级性是怎样的?我的理解是在coil上端是正,这样二极管就不能导通了.
  谢谢~ 答 2: 失败中的失败~   突然理解了~呵呵~ 答 3: 二极管是为了防止线圈感应的反向高压吧！