ACTIVATE_BACK_UP_POWER_SUPPLY

Published:2009/6/24 2:24:00 Author:May | From:SeekIC

A circuit,which can be built around two TI TL7702 chips, monitors a 15-V power supply and activates a relay to turn on a back-up supply if the voltage drops below±14.1 V. With the back-up in place, the malfunctioning supply can be replaced without any down time. The TL7702 power-supply voltage supervisor chips are intended for use as reset controllers in microprocessor systems, but they work well in their modified form for this application.One chip detects the positive supply (U1),and the other is used for the negative (U2). A pair of output-relay drive transistors,Q1 and Q2 form a wired OR circuit so that relay K1 is activated by the positive or negative voltalge detector that switches U1 or U2 on.The supervisor chips have a direct connection to the input comparator so that the trigger level is set by a resistor-divider network (R1 and R2) at the sense-input pin.These chip also have an internal, stable, reference-voltage source set at +2.53V,typical.The positive-sensed voltage drops enough to activate the comparator, its output goes low,switches the internal gate,and triggers the siliconcontrolled rectifier (SCR).The output comparator then forces the two output transistors (pin 6) turn on the relay driver,Q1.The negative detector is preceded by halt of the dual op amp LM358N (U3a)-an inverting amplifier with a gain of－0.178.R5 connects to the －15 Vdc being sended. The output of U3a is usually set at 2.67 V, higher than the +2.53-V referenCe voltage. Therefore, no switching occurs. If the －15-Vdc voltage decreases, U2 switihes Q2 and activates K1 in the same manner as described for the U1-Q1 positive detector. R3, R4, and R8 through R10 serve as current-ltmtttng reststois.



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