Over & Under Voltage Protection Circuit
Introduction
A OV-UV Protection
circuit/ Circuit breaker is an automatically operated electrical switch
designed to protect an electrical circuit from damage caused by excess current,
typically resulting from an overload or short circuit. Its basic function is to
interrupt current flow after a fault is detected. Unlike a fuse, which operates
once and then must be replaced, a circuit breaker can be reset (either manually
or automatically) to resume normal operation. Circuit breakers are made in
varying sizes, from small devices that protect low-current circuits or
individual household appliance, up to large switchgear designed to protect high
voltage circuits feeding an entire city. The generic function of a circuit
breaker, RCD or a fuse, as an automatic means of removing power from a faulty
system is often abbreviated to ADS (Automatic Disconnection of Supply).
< Wikipedia >
Terminology: You Must Know
1. Voltage Divider
A voltage divider (also known as a
potential divider) is a passive linear circuit that produces an output voltage
(Vout) that is a
fraction of its input voltage (Vin). In other words if you have a voltage supply of 10 Volt but you want
only 6 Volt as excitation of your circuit, then the simplest solution is to use
a voltage divider circuit(only two resistors).
How to do? => Choose a random
value for R1 let’s say 1kΩ then to find R2’s value use this formula
Vout = Vin(R2/(R1+R2))
Hence R2 is 1.5kΩ and your circuit
should look like
2
.
Comparator
A comparator circuit compares two
voltages and outputs either a VDD (the voltage at the plus side) or a 0 (the voltage at the negative side)
to indicate which is larger. Comparators are used to check whether an input has
reached some reference value. In most cases some comparator IC’s like LM339 is
used for this purpose, but op-amps may be used as an alternative. In this
exercise we are using a comparator IC LM339 which works as explained in diagram
below.
3
.
Relay Driver Circuit
A typical digital logic output pin
can only supply tens of mA of current. Larger devices like Light Bulbs, Motors,
Fan, TV, Refrigerators might even need several amps. But a Relay requires only
a few mA to operate and can be used as a switch for such high current seeking
devices. To switch ON and OFF relay a Transistor based driving circuit is
needed as shown in figure below. When Base current is not present Transistor is
in cut-off region (switch off) but when a sufficient amount of current is
provided on the Base terminal Transistor comes under saturation region (switch
ON)
4
.
Zener Diode
A Zener diode allows current to flow
from its anode to its cathode like a normal semiconductor diode, but it also
permits current to flow in the reverse direction when its "Zener
voltage" is reached.
Zener diodes are widely used in
electronic equipment of all kinds and are one of the basic building blocks of
electronic circuits. They are used to generate low power stabilized supply
rails from a higher voltage and to provide reference voltages for circuits,
especially stabilized power supplies. They are also used to protect circuits
from over-voltage and provides a constant value across it as shown in figure.
What are we
designing?
We are going to design a protection circuit
for electrical appliances say a Refrigerator. This circuit will protect
(cut-off) your refrigerator if the Line Voltage (AC Voltage coming to your
electric plug) exceeds 250V or reduce below 180V. The block diagram below
showing different units of a circuit breaker.
We will design this circuit in reverse order (Last block
will be designed first) to maintain the accuracy.
1.
Designing
Relay Driving Circuit:
Designing of Relay Driving Circuit is discussed is previous
exercise which involves following steps
è Measurement of Relay Coil
Resistance (Rcoil)
è
Find Current required by coil/
collector current for transistor (Ic)
è
Selection of transistor and
current gain (β)
è
Calculate Base Current(IB)
è
Choose Base voltage as 3.3V (TTL VOH(min)) and find value of Base
Resistance (RB)
Values for our
design are
Rcoil=71.9Ω, Ic=69.541mA, βmin=35 , IB=1.986mA , RB=1.359 KΩ
NOTE: Value
of RB for your circuit may be different because of different Coil resistance
Now test this circuit in all respects and be sure that it is
working properly.
2.
Design of Comparing
Circuit:
For this
design we are using LM339 Quad-Comparator IC. Now to set two reference voltages
for comparator inputs we will use Zener diodes as voltage regulator. This
manual is using a 4.7V Zener (Corresponding to 180V AC) and a 5.6V Zener (corresponding
to 250V AC) to set reference voltages as shown in figure. (You can use any
value of Zener diode but make sure both should be different)
Selection of Rs
The
current through the resistor Rs is given by (KVL and Ohm's law):
I= (Vin - Vz)/Rs
This is
the maximum current through the Zener diode and so, RS is chosen to limit the current
through the diode to some value such that the maximum power dissipated by the
diode is below the maximum power rating.
If max
Power Rating =500mW(datasheet), Vz=5.6V then Iz=89.28mA and IIO=50nA(max) then from above equation Rs=(12-5.6)/(89.28m-50n). Hence Rs is nearly 72Ω but for safety purpose
we are using 1KΩ instead.
Working of Comparator Blockà Negative input of C1 and positive
input of C2 are referenced as 5.6V and 4.7V respectively. Hence if voltage at
positive input of C1 is greater than 5.6V than voltage at Vout will be higher.
It will also higher if voltage at negative input of C2 will be lesser than 4.7V
and LOW otherwise.
Coupling 1st and 2nd Block
Working : As
mentioned above whenever the voltage at comparator side reach beyond 5.6V
or below 4.7V, Vout becomes high and provide a sufficient amount of current to
base of transistor which energize the relay and circuit connected in its output
start functioning. But when the input of comparator is in the range of 4.7V to
5.6V Vout remains 0V, no current flows through Base and hence relay will remain
off.
Need of D1 & D2
The working of our circuit is such
that output of either one of the two comparator will high or both zero. So in
case C2’s output is high D1 will not allow the current IB to enter into C1. D2 also perform
the similar task in case output of C1 is high.
3. Voltage Step down from 230V to 12V
Now we need some samples from voltage supply of our house
(which is also the input for your Refrigerator) and we have to convert this
voltage to a small value Vc such that
Vc = 5.61V when VAC(supply) =250V
Vc = 4.69V when VAC(supply) =180V
Why?
So that when the supply
goes beyond 250V, output of Block 1 will be 5.61V which will be the input of
Comparators and output of C1 will high. In case input is 4.69V (for AC 180V),
output of C2 will high and turn on the relay which will switch off your
Refrigerator.
So for
this purpose we need two voltage divider circuit as shown in figure. To
calculate the values of R1, R2, R3 and R4 follow these steps
i.
Assemble
this circuit without connecting R1, R2, R3 and R4 in the circuit.
ii.
Choose
an appropriate value of Capacitor for filter circuit.
iii.
Increase
the input AC voltage to 250V and measure the output voltage (Vo1) across
capacitor. Now to divide this Vo to get VB = 5.61V
a. Select R2’s value as 5.6kΩ.
b. Calculate R1 as
VB = Vo1(R2/(R1+R2))
c. For this circuit it is calculated as
R1=19kΩ
iv.
Decrease
the input voltage to 180V and measure the voltage (Vo2) across capacitor. And
follow the same procedure to calculate the value of R3 and R4.
Coupling 1st, 2nd and 3rd Block
· --> Connect
VA to ‘–‘pin of
comparator 2 (C2).
· --> Connect
VB to ‘+’ pin of
comparator 1 (C1).
Still there
are two additional DC power supplies of 6V (for Relay) and 12V (for comparator
IC) are needed to operate this circuit. But the final product should not
require such additional supplies, right?? So now we will work for removing
these.
·
Connect a 7812 Voltage Regulator IC across the
capacitor to provide 12V DC to Comparator IC (as shown in figure).
·
To provide Voltage to relay (6V) again we need a
voltage regulator circuit
i.
We make use of 6V Zener diode for this purpose.
ii.
Connect a 100 Ω resistor in series with Zener to
provide sufficient amount of current to relay coil (needs at least 70mA which
is Ic).
Now test this circuit by varying
the line voltage from 100V-260V and observe the results.
How My Circuit should function?
·
If
your Line voltage is in the range of 180V-250V, output of both comparator C1
and C2 will be 0V, no Base current will flow in the transistor so collector
current will flow. Because Ic = 0A, relay will not energized and hence light
bulb connected in output circuit will remain ON.
·
As
soon as line voltage decreases below 180V, VA (Output of Block 3) becomes nearly 4.69V (or
<4.7V) and hence the output of comparator C2 goes high and a sufficient
amount of current flow in the base of transistor which drive collector current
to energize relay. Now relay disables output circuit and the electric appliance
connected in output goes OFF.
·
When
line voltage reaches beyond 250V, VB (Output of Block 3) reaches 5.61V (or >5.6V)
because of which output of comparator C1 goes high and a Base current IB flows to energize the relay and
disable the output circuit to protect electric appliance (Bulb/Refrigerator)
connected in that circuit.
References