Low AC Voltage Detector Amanda Potter and Chris Greacen We recently received a letter from John Boles inquiring as to whether we have published a "brownout alarm" in any of our recent issues. He wrote that he had a friend who lives in a remote area who occasionally gets low AC voltage from the grid at his home. He would like to build a circuit that would disconnect his major appliances (refrigerator and freezer) when this happens to prevent future failure. We built this homebrew circuit in response to his letter. The circuit sounds an alarm when the voltage gets too low. The circuit could be modified by replacing the buzzer with a relay which would turn off major appliances. Convert AC Voltage to DC Voltage Our first step was to convert the AC voltage into a constant DC voltage. We did this with a full-wave rectifier which charges a big capacitor through a 2.2 kê resistor. The output of the bridge rectifier, Vr, is shown in figure 1. This voltage isn't very useful though; it must be smoothed out to generate a constant DC voltage. The RC combination does this. The capacitor is chosen so that RC >> 1 / f (where f is the ripple frequency, 120 Hz in our case). This relation ensures small ripple by making the time constant for discharging much longer than the time between recharging. INSERT FIGURE 1 Fig. 1 - Voltage at the output of the rectifier and voltage at the output of the capacitor. The output of the capacitor, Vc, also shown in figure 1, is not Vrms; it is the average voltage. See Chris Greacen's article on RMS voltage for a description of Vrms. For a sine wave, Vrms is 1.1 times larger than Vavg. See sidebar if you're interested in how this was calculated. Most multimeters use this type of circuit to estimate Vrms; the voltage is then scaled up by a factor of 1.1. This scaling factor changes for non- sinusoidal waves, such as the waveform of modified sine wave inverters. This is the reason non true RMS multimeters give false readings for modified sine wave inverters. Detect The Voltage Level An LM723 was used to detect the voltage level. As it is wired in our circuit, pin 9 of the LM723 will go high when the voltage at pin 5 goes above 7 volts. Resistors R4, R5, and R6 form an adjustable voltage divider which reduces Vavg to a level which the LM723 can deal with. Turn The Buzzer On Transistors Q1 and Q2 ensure that the buzzer turns on when the voltage level at pin 5 of the LM723 drops below 7 Volts. When the voltage level at pin 5 is above 7 Volts, pin 9 goes high which turns on transistor Q1. Current flows from +12 VDC through resistor R8 and transistor Q1 to ground. The voltage at resistor R9 is thus at 0.6 Volts, transistor Q2 remains off and the buzzer will not sound. Diode D5 was put in the circuit to ensure that 0.6 Volts is not enough to turn transistor Q2 on. When the voltage at pin 5 drops below 7 Volts, pin 9 goes low and transistor Q1 turns off. As a result, the voltage at resistor R9 gets high enough to turn transistor Q2 on and the current flows from +12VDC through the buzzer, transistor Q2, and diode D5 to ground. This sounds the buzzer. Set the Low Voltage Point The final step was to set the potentiometer so that the buzzer would turn on at our "brownout" voltage. We chose 100 Volts rms. We made a voltage divider which converted the 120 Volts rms out of the outlet to 100V rms. We chose R1 and R2 according to the following equations. The power dissipation of the resistors is 0.25 Watts. (1) Vcutoff rms (into our circuit) = {R2 / (R1+R2)} x Vrms (out of outlet) 100 V = R2 / (R1+R2) x 120 V (2) P = V2 / (R1 + R2); P = 0.25W, V = 120V R1 + R2 = 60k ohms We then changed the potentiometer until the buzzer just barely turned on. We used a multimeter to verify that the voltage into the circuit at that point was 100 Volts. Parts List U1 - LM723 Voltage Regulator D1-D4 - 1N4003 1 A, 200 PIV Diodes D5 - 1N4001 1 A, 50 PIV Diode C1 - 1000 æF, 200 V Capacitor Q1, Q2 - 2N2222A NPN Transistors R1 - 10 kê Resistor R2 - 56 kê Resisitor R3 - 2.2 kê Resistor R4 - 47 kê Resistor R5 - 10 kê Potentiometer R6 - R9 - 1 kê Resistors 7 mA, 12 V PC-Mountable Piezo Buzzer or 130 mA coil current, 10A at 125 VAC rated Relay