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VOLTAGE_MONITOR

2015-03-02 10:08  
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VOLTAGE_MONITOR
Developed for use in systems having multiple DC bias voltages, to prevent damage when one supply voltage goes down while other5 remain normal. Control circuit includes its own independent AC/DC supply that ensures protection even when equipment containing RAMs and MOS devices is turned off. Failure of AC supply for monitor shuts down entire system. Can be applied to any number of supplies by adding resistive dividers, Schmitt triggers, diodes, and latches as required. Closing system power switch activates solid-state relay for applying AC line voltage to main power supplies. Half of 556 dual timer and 74LS174 hex D latch inhibit voltage monitor until all supplies have stabilized, about 500 ms later. Other half of 556 then clocks 74LS175 power-monitor latch. System then operates normally as long as all D inputs to monitor latch stay at logic 0. If one supply fails, logic 1 appears at is latch input and next clock pulse initiates shutdown of sys-tem, LED identifies supply that has failed.-J. E. Draut, Voltage Monitor Protects Against Power-Supply Failures, EDN Magazine, Nov. 20, 1977, p 239-240.