| 摘要 |
<p>PURPOSE:To avoid the power loss of a back-up power supply by using an NPN transistor TR to completely turn off a PNP TR when a main power supply is switched off. CONSTITUTION:When a main power supply is kept turned on, a Zener current flows to a Zener diode 5 via a current limiting resistance 4 only when the voltage level of the main power supply is higher than the Zener voltage of the diode 5. Under such conditions, no voltage drop occurs substantially for the main power supply to a memory 1 when an NPN TR2 is turned on. Thus the power is supplied to the memory 1 from the main power supply. While the TR2 and a PNP TR3 are completely turned off in case the voltage of the main power supply is reduced down to the level of the Zener voltage of the diode 5. When the main power supply is switched off, the memory 1 is separated from the main power supply. Thus no reverse current is produced to the main power supply from a back-up power supply. As a result, the power loss of the back-up power supply can be avoided.</p> |
