Typical circuit of ICL7673 uninterruptible power supply

The typical UPS circuit composed of ICL 7673 is shown in Figure 3.5.4. In the picture, the main power supply is +5V, and the backup power supply is 3.6V nickel-cadmium battery pack E. During normal operation, the +5V power supply also trickles charges the nickel-cadmium battery through the resistor R and the silicon diode VD, and the output voltage Vo is also +5V. When the power grid fails, E continues to supply power to the CMOS system, and the output voltage becomes +3.6V, which ensures that the data in RAM will not be lost. The function of VD is to prevent E from injecting current into the main power circuit after power failure. A 1N4001 diode is optional. The R value depends on the trickle charging current. When the charging current is 10mA, take R=100 ohms. If a larger output current needs to be provided, the current can be expanded through an external PNP power transistor (such as 3 AD 6, 3 AD 30).

The circuit is shown in Figure 3.5.5. Under normal conditions, Pscr=0+VT1 is turned on, and the main power supply supplies power to the instrument (or system) through VT1. When the power grid fails, Sscr=0, VT2 is turned on, VT1 is turned off, and the backup power supply E continues to supply power through VT1.