Page 116 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

Power Supply Failure and Alarm Circuit:
UPS starts supplying the power to the system when the main power supply i.e., 230 V AC fails. Red LED and an
alarm are used to indicate the power supply failure. A simple circuit for this purpose is shown in the figure.

The alarm may sound continuously or at time intervals of 10 to 20s. The above circuit is based on AC 230 V, 50
Hz operated relay. When the main supply is present, the common terminal of the relay rests connected with NO
(Normally Open) terminal, thus rendering the LED’s Circuit disconnected.
When the main supply fails, the relay gets connected with NC (Normally Closed) terminal. Thus, the battery circuit
becomes complete, the red LED starts to glow and the buzzer (or alarm) starts to produce sound. As and when
the main supply is restored, the relay gets energized and peaks the circuit of LED and buzzer.
Single Phase UPS:

Single phase is commonly called “residential voltage” because it is widely available in homes. For example, the
microwave oven, the coffee machine, your PC in your household can be single phase devices. In different regions
there is a similarity for single phase connections: It requires two wires (one voltage wire and one neutral wire) for
completing the circuit. The figure below shows the current flow in single phase AC power.

Single-phase UPS have a single input and output source for the electrical equipment. With just one sinewave
voltage, it only requires two wires to complete the circuit, one conductor and one neutral.
Single-phase uninterruptible power supplies typically cover requirements up to 20 kVA and are used for smaller
installations such as rack-mounted servers, telecoms or computer systems, and network switches, along with any
device that runs directly from a standard three-pin plug

101

CITS : E & H - Electronics Mechanic - Lesson 56 - 63