Mains power alarm

Our refrigerator must run at +5C at all times. But one day, the security relay of the house flipped and cut the power off. The power outage lasted 6 hours, and the temperature within the refrigerator rose to +15C. To avoid this in the future, I decided to build a mains power alarm, so I can take action if the power relay flips again.

Figure 1. Mains Power Alarm.

The PSU is connected to the mains and generates 12 V which energizes the relay coil. The relay is in position NO. If 12 V is lost, the relay changes to NC and activates the buzzer. SW1 can silence the buzzer.

The PSU in figure 1 is an AC adapter which is plugged into a 230 V AC wall outlet. The secondary of the AC adapter delivers 12 V DC. The green 12V lamp is on when 12 V DC (and mains) is present. Relay1 is active and breaks the buzzer circuit. The relay is a 12 V type. Any voltage transcients from the relay coil are bridged by D1 and will not damage the PSU.

 

The 12 V buzzer sounds when the coil of Relay1 is powered off during a mains power failure.   

Figure 2. PSU and metal box.

 The metal box in figure 2 measures 125 mm x 80 mm x 50 mm. The blue relay is taped to the wall.  

Figure 3. Front view.

Figure 3 remarks. Labels are in the Danish language. 

ALARM TIL means "Alarm is on".
LYSNET OK means "Mains Ok".

73 from OZ1BXM Lars
Homepage: oz1bxm.dk