In this lesson, we will be discussing about Low Voltage system with load Isolator using LM358, AT89C2051 micro-controller.
The power line fluctuations and cut-offs
or power failure causes damages to electrical appliances connected to the line.
It is more serious in the case of domestic appliances like fridge and air
conditioners. If a fridge is operated on low voltage, excessive current flows
through the motor, which heats up, and get damaged. Low voltage monitor is
designed to protect electrical appliances from under voltage issues. The under
voltage monitor circuit with alarm system and isolator presented in this work
is a low cost and reliable circuit for protecting such equipment's from damages.
Whenever the power line is switched on it gets connected to the appliance only
after a delay of a fixed time. If there low fluctuations beyond sets limit the
appliance get disconnected. The system tries to connect the power back after
the specific time delay, the delay being counted from the time of
disconnection.
Block
Diagram of Protection System [1]
Working AC supply is stepped down to 12
V by using a step down transformer. The AC supply is converted to DC supply
through bridge rectifier. The supply is then filtered by capacitors connected
across rectifier to reduce harmonics. Then the unregulated supply is then fed
to voltage regulator whose output is given to the comparators IC LM358 and
relay as supply. The unregulated supply from bridge rectifier is fed to preset
1 and preset 2 as input. The preset 1 and preset 2 are potentiometer ckt.1 and
potentiometer ckt.2 respectively connected to comparators IC LM358 as command
or input. The Preset conditions can be adjusted. Further, the comparators are
connected to the relay and load is connected to relay. Whenever there is under
voltage the comparators analyze the preset conditions and give the signal to
the micro-controller and the controller will drive the relay and relay trips
and the load get switched off. Thus protects the electrical appliance.
Under
Voltage Protection
1.
When the line voltage is below 180V, the
voltage at the inverting terminal (pin 6) of operational amplifier N2 is less
than the voltage at the non-inverting terminal (6V). Thus the output of
operational amplifier N2 goes high and it energizes the relay through
transistor T1. The AC supply is disconnected and electrical appliances turn
off.
2.
Thus the appliances are protected
against under-voltage. IC1 is wired for a regulated 12V supply.
3.
The relay energizes in two conditions:
first, if the voltage at pin 3 of IC2 is above 6.8V, and second, if the voltage
at pin 6 of IC2 is below 6V.
4.
under-voltage levels can be adjusted
using presets VR1.
No comments:
Post a Comment