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How to Design an Uninterruptible Power Supply (UPS) Circuit

How to Design an Uninterruptible Power Supply (UPS) Circuit

In this brief tutorial we learn how to design a customized UPS circuit at home using ordinary components such as a few NAND ICs and a some relays.

What is an UPS

UPS stands for uninterruptible power supply, these units are designed with an intention to provide power to the connected load without slightest bit of interruption during power failures or fluctuations or even brown outs.

An UPS becomes useful for PCs and other such equipment which involve critical data handling and cannot afford 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.

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.

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.

inverter circuit for the UPS construction


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.

UPS automatic relay changeover

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.

That's it, this concludes our tutorial for designing a personal UPS circuit which can be easily done by any new hobbyist by following the above detailed guide.



About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

27 thoughts on “How to Design an Uninterruptible Power Supply (UPS) Circuit”

  1. Hi sir.can i know the simulation of offline uninterruptible power supply(UPS) on the MATLAB Simulink and how can i know the system can works as backup power supply when the main supply is powered off from the simulation? Then, what can i do for the improvement harmonic distortion on UPS as the the system is already designed to mitigate distortion? I hope ur can help me. Thanks in advanced !

    • Hi Mara, you can verify the design by testing it practically and you will find it working with 100% results (if done correctly). I do not have any simulation results for my circuits.

  2. Dear sir,

    The above circuit works with 12 V DC battery, i assume. What should be the current rating of the battery. It is mentioned that refrigerator will work. What is the maximum load(s) that can be connected with the above circuit. If i want to increase the loads, say double the load what modifcation is to be done? Thanks and regards.

    • Dear Premila,

      You can upgrade the inverter wattage as per your requirement. You just have to use a transformer that matches the specs of the load, similarly use a battery as per the transformer and the load…and the mosfets which can handle the trafo and battery current confortably, only these 3 parameters should be matched for ensuring the required output wattage.

  3. Hi, Mr Swagatam, I do really like to ask you these question that seem to be bothering for a long time. It all about modifying a 650va UPS (introducing two way power source e.g both battery and solar panels), besides, I need to know the suitable nos(watt ratting) for panels. Thank you

  4. Hi, mr.swagatam, I need your help, I have transformer from old ups, I want to built inverter 12vcd to 220vac used this transformer, but the transformer not have center tap, how can I used,, thanks for help..

  5. Genial amigo swagatam. Siga así con su trabajo que es exelente. Cuando lo tenga terminado le enviaré fotos o un video del funcionamiento del inversor. Otro gran abrazo

  6. as this friend Swagatam. I am eager to build this investor to prove it in a refrigerator. I would appreciate greatly place the list of components. a big hug from Argentina

  7. Hello, again

    Where do I find the list of materials investor.

    Officer with a refrigerator? and also with induction motors? thank you very much and sorry to bother you.

    Thank you

    • Hello Jesus, yes the above inverter circuit can be used with refrigerator and induction motors…I'll try to update the parts list soon in the aboev article

  8. Hola nuevamente exelente trabajo. Donde encuentro la lista de materiales del inversor. Y otra pregunta. Funcionaria para un refrigerador? Gracias

  9. Thanks, I already have both of these ideas posted in this website, please use the search box at top right and search for.."humidity", and "incubator" you will be taken to the articles of your choice….

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