Fig. 1: Power supply circuit with resume indicator The low voltage protector circuit described here keeps your AC motors and other electrical devices from getting damaged by low voltage when they turn on. When power comes back on, it stays in standby mode without giving power to the load. The load can only be turned on by hand. This keeps the device from getting broken if it is "ON" when power comes back on. The unregulated power supply comes from a step-down transformer that goes from 12V to 12V at 300 mA and two rectifying diodes, D2 and D3. Capacitor C3 gets rid of the ripples in the rectified DC. Along with the power supply for power resumption, there is an audio/video indicator (a piezobuzzer and an LED3). When the power is turned on, capacitor C4 charges through the piezobuzzer and LED3, making both of them work. The piezobuzzer beeps, and for a few seconds, LED3 lights up. When capacitor C4 is fully charged, the LED's cathode goes high. This stops any more current from going through the buzzer. When the power is turned off, capacitor C4 drains through resistor R9.

Fig. 2: Circuit of low voltage protector The voltage comparator IC CA3140 (IC1) is used in the circuit to find changes in the voltage of the unregulated power supply caused by AC mains. The mains voltage changes in both the primary and secondary windings of the transformer. IC1 picks up on these changes to turn on or turn off the relay. To make transistor T1 work, Zener diode ZD1 sets a reference voltage of 3V. ZD1's breaking point is changed by the setting VR1. When the voltage is normal, the zener diode ZD1 breaks down and the transistor T1 is biased in the forward direction. Capacitor C1 gives the device a few seconds of time to start up before any changes can affect it. When transistor T1 is on, the inverting input of IC1 (pin 2) is low. But IC1 doesn't have a high output because its power comes from the way SCR1 works (BT169). To turn on the relay, you have to do something by hand. When the push-to-on switch S1 is pressed, SCR1 fires and sends power to IC1's pin 7 through the switch. Since the voltage at the non-inverting input (pin 3) of IC1 is half of the supply voltage, its output goes high, and the

relay (RL1) turns on. When IC1's output is high and the relay is turned on, LED2 lights up. When the line voltage drops below 180V, the secondary voltage of the transformer also drops, say to below 12V. When this happens, ZD1 stops conducting and the collector of T1 goes high. The output of IC1 is low because its inverting input (pin 2) has a high voltage. The power to the device is cut off when the relay loses its charge.

Building and testing Put the circuit on a PCB that can be used for many things and put it in a suitable cabinet. Use a 12V relay on a PCB. On the front of the case, make holes for the LEDs and switch S1. Connect the motor (the load) to the AC power source through the relay's common and normally open (N/O) contacts. After putting the circuit together and making sure it works, turn it on and wait a few minutes. As capacitor C1 fills up, LED1 will get brighter and brighter. To turn on the relay, press S1. Change VR1 so that LED1 is fully on. This will make it easy for the relay to latch.