Our Mains AC domestic power may be full of ups and downs or
fluctuations, which may be quite fatal to our sophisticated appliances like TV
sets, DVD players, compotes etc. A very simple circuit presented when installed
in the house electrical may help in restricting the issue to a great extent.
Here we will learn two designs of over and under voltage
circuits, the first based on transistors and the other one using an op amp.
the said protections can be built using just a couple of transistors and a few
other passive components.
arrangement where T1 and T2 are fixed as an inverter configuration, meaning T2
responds oppositely to T1.
Please refer the circuit diagram.
voltage itself is fed to the base of T1 via preset P1.
that the tripping thresholds can be determined precisely and the circuit
understands when to execute the control actions. P1 is set for detecting high
remains switched OFF and this allows the required biasing voltage to pass
through P2 and reach T2, keeping it switched ON.
load receives the required AC voltage.
limit, the sensing sample voltage at the base of T1 also rises above the set
threshold, T1 immediately conducts and grounds the base of T2.
switching OFF of T2 and also the relay and the corresponding load. The system
thus restricts the dangerous voltage from reaching the load and safeguards it as
expected from it.
switched OFF and at this situation T2 also stops conducting due to the settings
of P2, which is set so that T2 stops conducting when the Mains input goes below
a certain unsafe level.
to the load and prompting the required safety measures.
threshold is too wide, meaning the circuit triggers only for voltage levels
above 260V and below 200V, or above 130V and below 100 V for 120 V normal supply
looking for absolutely accurate tripping points and controls which can be optimized
as per ones personal preference.
be required to be included instead of transistors.
operations can also be done using an op amp, with precise results.