High And Low Voltage Cut Off With Time Delay
The power line fluctuations and cut-offs cause 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.
The under/over voltage protection circuit with time delay
presented here is a low cost and reliable circuit for protecting
such equipments 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 is hi/low fluctuations beyond sets limits 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. If the power down time (time for which
the voltage is beyond limits) is less than the delay time, the
power resumes after the delay: If it is equal or more, then the
power resumes directly.
This circuit has been designed, built and evaluated by me to use
as a protector for my home refrigerator. This is designed around
readily available semi-conductor devices such as standard bipolar
medium power NPN transistor (D313/SL100/C1061), an 8-pin type 741
op-amp and NE555 timer IC. Its salient feature is that no relay
hunting is employed. This draw back is commonly found in the
proctors available in the market.
The complete circuit is consisting of various stages. They are: -
Dual rail power supply, Reference voltage source, Voltage
comparators for hi/low cut offs, Time delay stage and Relay driver
stage. Lets now look at the step-by-step design details.
1. Dual rail power supply.
This is a conventional type of power supply as shown in
Figure 1. The power is applied through the step-down
transformer (230/12-0-12V/500mA). The DC proportional to
the charging input voltage is obtained from bridge
rectifier. Two electrolytics are there to bypass any spikes
present. Bridge is capable of handling currents up to 1
Output is given by: -
V(out) = 0.71 X V (secondary)
= 0.71 X 24V
= 17.04 V
(This equation is similar for the negative rail as well)
2. Low voltage cut off op-amp
Figure 2 shows the use of very common and easily available op-amp
741 as a comparator. The op-amp is available in TO-5 and DIP type
In this ckt the zener diode D1 and
itís associated resistor R1 are connected to the non-inverting
terminal (+ve) of 741 to give the suitable reference voltage. The
DC voltage from the sensor is given to the inverting (-ve)
terminal through pre-set R2.This is used to set the input level.
When the sensor input is less than Zener voltage the output from
the Op-amp remains high and when it is greater than Zener voltage
the output goes low. When the sensing voltage is equal to Zener
voltage the output of the op-amp is approximately zero.
This phenomenon is used as a decision for switching the relay and
to give cutoff in a low voltage situation.
3.High voltage cut off op-amp
Here the op-amp is used as a inverted amplifier. See Figure
3.Zener and resistor network gives reference voltage to the
inverting terminal (-ve) of op-amp. Sensing voltage derived
through the 10 K pre-set is given to the non- inverting (+ve)
terminal and this sets the high level cut.
When the input DC from the sensor is less than Zener voltage the
output of the op-amp is low and vice-versa. When the input DC
voltage is equal to the zener voltage, the op-amps output is
4. Time delay
Iíve selected the 555 timer due to following reasons.
1. Timing from microseconds through hours.
2. Ability to operate from wide range of supply voltages.
3. High temperature stability.
4. Easily Available.
5. Its triggering circuit is quite sensitive.
This is basically a monostable. The external timing capacitor C2
is held initially discharged by the timer. The circuit triggers
upon receiving a pulse to its pin 2 when the level reaches 1/3
Vcc. Once triggered., the circuit will remain in that state until
the set time is elapsed or power to the circuit cuts off. The
delayed period in seconds is 1.1 C2.R1 where R1 is in megohms and
C2 is in microfarads. In practice, R1 should not exceed 20 M. If
you use an electrolytic capacitor for C2, select a unit for low
leakage. The time delay may have to be adjusted by varying R1 to
compensate for the wide tolerance of electrolytics.
The output from the voltage level detectors cannot directly drive
the relay and hence the relay driver is used.
In this a relay (12V <500 ohms) is
connected to the collector of NPN transistor. The out put voltage
from the comparator is applied to the base of NPN transistor
through a resistance R1. When the output from the comparator is
low the transistor is in OFF state and the relay is in
de-energized state. Similarly when the output from the comparator
goes high the transistor switches ON and the flow of current from
the collector to emitter of transistor energizes the relay.
Generally in a relay
driver circuit, parallel to the relay coil, a diode or a
capacitor is used. This is to eliminate the back e.m.f
generated by the relay coil when currents are suddenly
broken. Capacitor C1 is connected in parallel to the coil,
which filters out the back emf but it, slows down the
working of relay.
A better method is to connect
two diodes (as shown in the figure 5) that stop the relay Ė
transistor junction swinging more than 600mV above the positive
rail or below the zero-volt rail. During normal operation the
diodes are reverse biased and have no effect on the performance of
circuit. But when back emf is induced, the diodes conduct heavily
and absorb all transient voltages. However, I have employed the
6. The Complete Circuit
Under normal operating conditions i.e. when
the input voltage is between maximum and minimum limit the
output from the both the comparators are low. The
transistor Q1 is OFF and the relay is in de-energized (pole
connected to N/C pin) state and the output is obtained.
When the input voltage is below or above the limits set by
the pre-sets R8 or R9, the output of the Op-Amps goes
either low or high and diodes D1 or D2 would be forward
biased depending on the situation. Transistor Q1 switches
ON and the flow of current from collector to emitter
energizes the relay and the output is cutoff.
A small amount of hystersis has been added via feed back
resistors R10 & R11 so that the relay turns on when the
level falls to a particular value but does not turn again
until it raises a substantial amount above this value.
Other wise the relay contacts will frequently turn on/off
and produce chattering.
1) I used a piece of varoboard, which has copper strips on
one side to mount the components, and housed the entire
circuit and the transformer in a discarded ATX PC power
2) An autotransformer has been used to set the limits. Set
the output of the autotransformer to 250V AC and connect it
to the primary of transformer T1 (see Figure 1). Then
adjust the pre-set R9 such that relay just energizes. This
is the high limit. Next set the output of the
autotransformer to 200V AC and adjust the pre-set R8 such
that the relay energizes. Please note that these are my
preferred limits but you may select any range from say 170
to 270V AC.
3) A neon with a suitable resistor could be connected
between the AC supply lines as an ON indicator.
Alternatively, LED with a current limiting resistor could
be connected between the relay coil so when the relay is
energized LED will indicate the situation.
take the greatest of care in handling AC mains supply
while constructing this project. If you have no
knowledge of mains wiring or unfamiliar with household
mains supply, PLEASE DO NOT ATTEMPT CONSTRUCTION. I
take no responsibility in any personal injury or loss
of life or properties suffered by any person while
undertaking the construction of this project or using
the end product by following my instructions.
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