Detector circuit

A detector circuit for use with a dc power supply circuit for detecting the loss of dc power to a dc load and for detecting a low dc voltage condition of a standby storage device employed to power the dc load following a loss of dc power to the dc load, said dc power supply circuit being operative to supply dc voltages to the dc load and to the standby storage device for utilization by the dc load and for establishing a dc voltage across the standby storage device, said detector circuit comprising:

• transfer circuit means coupled to the standby storage device and to the dc load and adapted to receive dc voltages supplied to the dc load by the dc power supply circuit and the standby storage device, said transfer circuit means having a first operating state during which it couples the standby storage device and the dc voltage thereacross to the dc load and a second operating state during which it uncouples the standby storage device and the dc voltage thereacross from the dc load;
• first voltage monitoring circuit means including power failure detector circuit means coupled to a predetermined point in the dc power supply circuit and to the transfer circuit means and having a first terminal at a predetermined reference potential and a second terminal, said power failure detector circuit means being operative to examine a dc voltage at the predetermined point in said dc power supply circuit and operative when the dc voltage examined thereby has a value less than a predetermined value indicating a loss of power to the dc load to place the second terminal at a potential above the predetermined reference potential of the first terminal;
• said transfer circuit means being operative in response to the placement of the second terminal of the power failure detector circuit means above the reference potential of the first terminal to receive the dc voltage then present at the dc load and, in response to said dc voltage, to operate in its first state and thereby couple the standby storage device and the dc voltage thereacross to the dc load, said transfer circuit means being thereafter operative to receive the dc voltage coupled from the standby storage device to the dc load and, in response thereto, to continue to operate in its first state and thereby maintain the standby storage device and the dc voltage thereacross coupled to the dc load;
• and
• second voltage monitoring circuit means coupled to the dc load and to the transfer circuit means and arranged to examine the dc voltage coupled from the standby storage device to the dc load following the aforesaid operation of the first voltage monitoring circuit means and transfer circuit means, said second voltage monitoring circuit means being operative when the dc voltage monitored thereby has a value less than a predetermined value to cause the transfer circuit means to operate in its second state whereby the standby storage device and the dc voltage thereacross are uncoupled from the dc load.