What is an UPS
UPS which stands for uninterruptible power supply are inverters designed to
provide a seamless AC mains power to a connected load without a slightest bit of
interruption, regardless of sudden power failures or fluctuation or even a brown-
out.
An UPS becomes useful for PCs and other such equipment which involve critical
data handling and cannot afford mains power interruption during a vital data
processing operation.
For these equipment UPS becomes very handy due to its instantaneous power
back-up to the load, and for providing the user with ample time to save computer's
crucial data, until actual mains power is restored.
This means that an UPS must be extremely quick with its changeover from mains
to inverter (back up mode) and vice versa during a possible mains power
malfunction.
In this article we learn how to make a simple UPS with all the bare minimum
features, ensuring that it conforms with the above fundamentals and provides the
user with a good quality uninterrupted power throughout the course of its
operations.
UPS Stages
A basic UPS circuit will have the following fundamental stages:
1) An inverter circuit
2) A Battery
3) A battery charger circuit
4) A changeover circuit stage using relays or other devices such as triacs or SSRs.
Now let's learn how the above circuit stages may be built and integrated together
for implementing a reasonably decent UPS system
Block Diagram
The mentioned functional stages of an uninterruptible power supply unit
could be understood in detail through the following block diagram:
Here we can see that the main UPS changeover function is carried out by a
couple of DPDT relay stages.
Both the DPDT relays are powered from a 12 V AC to DC power supply or
adapter.
The left side DPDT relay can be seen controlling the battery charger. The
battery charger gets powered when AC mains is available through the upper
relay contacts, and supplies the charging input to the battery via the lower
relay contacts. When AC mains fails, the relay contacts changeover to the N/C
contacts. The upper relay contacts switches OFF power to the battery charger,
while the lower contacts now connects the battery with the inverter to initiate
the inverter mode operation.
The right side relay contacts are used for changing over from grid AC mains to
the inverter AC mains, and vice versa.
A Practical UPS Design
In the following discussion we will try to understand and design a practical
UPS circuit.
1) The Inverter.
Since an UPS has to deal with crucial and sensitive electronic appliances, the
involved inverter stage must be reasonably advanced with its waveform, in
other words an ordinary square wave inverter may not be recommended for
an UPS, and therefore for our design we make sure that this condition is aptly
taken care of.
Although I have posted many inverter circuits in this website, including
sophisticated PWM sinewave types, here we select a completely new design
just to make the article more interesting, and add a new inverter circuit in the
list
The UPS design utilizes just a single IC 4093 and yet is able to execute a good
PWM modified sine wave functions at the output.
Parts List
• N1---N3 NAND gates from IC 4093
• Mosfets = IRF540
• Transformer = 9-0-9V / 10 amps / 220V or 120V
• R3/R4 = 220k pot
• C1/C2 = 0.1uF/50V
• All resistors are 1K 1/4 watt
Inverter Circuit Operation
The IC 4093 consists of 4 Schmidt type NAND gates, these gates are appropriately
configured and arranged in the above shown inverter circuit, for implementing the
required specifications.
One of the gates N1 is rigged as an oscillator to produce 200 Hz, while another
gate N2 is wired as the second oscillator for generating 50Hz pulses.
The output from N1 is used for driving the attached mosfets at the rate of 200Hz
while the the gate N2 along with the additional gates N3/N4, switches the mosfets
alternately at the rate of 50Hz.
This is to ensure that the mosfets are never allowed to conduct simultaneously
from the output of N1.
The outputs from N3, N4 break the 200Hz from N1 into alternate blocks of pulses
which are processed by the transformer to produce a PWM AC at the intended
220V.
This concludes the inverter stage for our UPS making tutorial.
The next stage explains the changeover relay circuit, and how the above inverter
needs to be wired with the changeover relays for facilitating the automatic inverter
back up and battery charging operations during mains failure, and vice versa.
Relay Changeover Stage and Battery Charger Circuit
The image below shows how the transformer section of the inverter circuit may be
configured with a few relays for implementing the automatic changeover for the
proposed UPS design.
The figure also shows a simple automatic battery charger circuit using the IC 741
on the left side of the diagram.
First let's learn how the changeover relays are wired and then we can proceed
with the battery charger explanation.
In all there are 3 sets of relays which are used in this stage:
1) 2 nos of SPDT relays in the form of RL1 and RL2
2) One DPDT relay as RL3a and RL3b.
RL1 is attached with the battery charger circuit and it controls the high/low
cut charge level cut-off for the battery and determines when the battery needs
is ready to be used for the inverter and when it needs to be removed.
The SPDT RL2 and the DPDT (RL3a and RL3b) are used for the instant
changeover actions during a power failure and restoration. RL2 contacts are
used for connecting or disconnecting the center tap of the transformer with
the battery depending on the mains availability or absence.
RL3a and RLb which are the two sets of contacts of the DPDT relay become
responsible for switching the load across the inverter mains or the grid mains
during power outages or restoration periods.
The coils of RL2 and DPDT RL3a/RL3b are joined with a 14V power
supply such that these relays quickly activate and deactivate depending on the
input mains status and do the necessary changeover actions. This 14V supply
is also used as the source for charging the inverter battery while the mains
power is available.
The coil of the RL1 can be seen connected with the opamp circuit which
controls the battery charging of the battery and ensures the supply to the
battery from the 14V source is cut-off as soon as it reaches the same value.
It also makes sure that while the battery is in the inverter mode and is
consumed by the load, its lower discharge level never goes below 11V, and it
cuts off the battery from the inverter when it reaches around this level. Both
these operations are executed by the relay RL1 in response to the opamp
commands.
The setting-up procedure for the above UPS battery charger circuit can be
learned from this article which discuses how to make a low high cut off battery
charger using IC 741
Now it simply needs to integrate all the above stages together for executing a
decent looking small UPS, which could be used for providing an
uninterruptible power to your PC or any other similar gadget.
Applications of UPS systems include medical facilities, life supporting
systems, data storage and computer systems, emergency equipment,
telecommunications, industrial processing, and on-line management
systems. The static switch provides redundancy of the power source in the
case of UPS malfunction or overloading
