How to Make a Mini Homemade Uninterruptible Power Supply (UPS) Circuit

Uninterruptible power supply units are always considered to be having complicated circuitry and are therefore are costly, difficult to procure or build. A simple idea presented here can be built at home using most ordinary components to produce reasonable outputs. It may be used to power not only the usual electrical appliances but also sophisticated gadgets like computers. Its inverter circuit utilizes a modified sine wave design.

UPDATE (must read)

How to design your own customized UPS – Full Tutorial

An uninterruptible power supply with elaborate features may not be critically required for the operation of even the sophisticated gadgets. A compromised design of an UPS system presented here may well suffice the needs. It also includes a built-in universal smart battery charger.

What’s the difference between an uninterruptible power supply (UPS) and an inverter? Well, broadly speaking both are intended to perform the fundamental function of converting battery voltage to AC which may be used to operate the various electrical gadgets in the absence of our domestic AC power.

However, in most cases an inverter may not be equipped with many automatic functions and safety measures normally associated with an UPS. Moreover, inverters mostly don’t carry a built in battery charger whereas all UPSs have a built in automatic battery charger with them to facilitate instant charging of the concerned battery when mains AC is present and revert the battery power in inverter mode the moment input power fails. Also UPSs are all designed to produce an AC having a sine waveform or at least a modified square wave resembling quite like its sine wave counterpart. This perhaps becomes the most important feature with UPSs.

With so many features in hand, there’s no doubt these amazing devices ought to become expensive and therefore many of us in the middle class category are unable to lay their hands on them.

I have tried to make a UPS design though not comparable with the professional ones but once built, definitely will be able to replace mains failures quite reliably and also since the output is a modified square wave, is suitable for operating all sophisticated electronic gadgets, even computers.

Understanding the circuit diagram

The figure alongside shows a simple modified square inverter design, which is easily understandable, yet incorporates crucial features.

The IC SN74LVC1G132 has a single NAND gate (Schmitt Trigger) encapsulated in a small package. It basically forms the heart of the oscillator stage and requires just a single capacitor and a resistor for the required oscillations. The value of these two passive components determines the frequency of the oscillator. Here it’s dimensioned to around 250 Hz.

The above frequency is applied to the next stage consisting of a single Johnson’s decade counter/divider IC 4017. The IC is configured so that its outputs produce and repeat a set of five sequential logic high outputs. Since the input Is a square wave the outputs are also generated as square waves.

Parts list

R4,R5 = 220 Ohms
T0 = BC557B
T3,T4=BDY29 or 2N3055 or TIP35
IC1= SN74LVC1G132 or a single gate from IC4093
Battery = 12V/25AH

Uninterruptible Power Supply – Battery Charger Section

The base leads of two sets of Darlington paired high gain, hi-power transistors are configured to the IC such that it receives and conducts to the alternate outputs.

The transistors conduct (in tandem) in response to these switching and a corresponding high current alternating potential is pulled through the two halves of the connected transformer windings.

Since the base voltages to the transistors from the IC are skipped alternately, the resultant square impulse from the transformer carries only half the average value compared to the other ordinary inverters. This dimensioned RMS average value of the generated square waves very much resembles the average value of the mains AC that is normally available at our home power sockets and thus becomes suitable and favorable to most sophisticated electronic gadgets.

The present uninterruptible power supply design is fully automatic and will revert to the inverter mode the moment input power fails. This is done through a couple of relays RL1 and RL2; RL2 has a dual set of contacts for reversing both the output lines.

As explained above an UPS should also incorporate a built-in universal smart battery charger which also should be voltage and current controlled.

The next figure which is an integral part of the system shows a smart little automatic battery charger circuit. The circuit is not only voltage controlled but is also includes an over current protection configuration.

Transistor T1 and T2 basically form an accurate voltage sensor and never allows the charging voltage upper limit to exceed the set limit. This limit is fixed by setting the preset P1 appropriately.

Transistor T3 and T4 together keep an “eye” over the rising current intake by the battery and never allows it to reach levels which may be considered dangerous to battery life. In case the current starts drifting beyond the set level, the voltage across R6 crosses over – 0.6 volts, enough to trigger T3, which in turn chokes the base voltage of T4, thus restricting any further rise in the drawn current. The value of R6 may be found using the formula:

R = 0.6 / I, where I is the charging current rate.

Transistor T5 performs the function of a voltage monitor and switches (deactivates) the relays into action, the moment mains AC fails.

Parts list


RL1=12V/400 OHM, SPDT
RL2=12V/400 OHM, SPDT, D1—D4=1N5408
C2 = 1uF/25V

208 Replies to “How to Make a Mini Homemade Uninterruptible Power Supply (UPS) Circuit”

  1. Swagatam Post author


    It will depend directly on the wattage of the transformer and the AH of the battery used.

    Suppose if you use a 12-0-12v/20 amp transformer and a 100 AH 12V battery, then the ups would be able to support 300 watts for 3 hours.

  2. network advise

    hi swagatam
    i have to rewound the old computer ups transformer for this circuit. how i calculate how many turns & which guage i can use.old computer ups transformer core area is 32*40 plese suggest me. thanks

  3. Anonymous

    hi sir,,,
    sir plz can u tell me or upload, how to make ultra sonic wave mosquito repeller circuit in effectively working condition,etc etc,,

    haris khalil

  4. Anonymous

    hello sir, hope u will b fine,,,

    i hv some questions in my mind,

    1: can this above circuit be used for 2 household ceilling fans of 220 volts at a time without any sort of modification ?

    2: how can i increase the capacity of above circuit for bear/running required load of my house?

    3: sir, what is the charging current of your above battery charger circuit ?

    4: Is there any formula for calculating charging current for different amperes batteries, OR in other words, how can i find that how many amperes is required for charging batteries of different amps ?

    5: do you have any helpfull artical for understanding relays in above circuit or other various circuits of relays ?


    1. Swagatam Post author

      Hello Wasim,

      The power of the above inverter can be increased or decreased by simply modifying the transformer and the battery current.

      Roughly, if you multiply the input winding voltage (half winding) with the its current, that would provide you the wattage capacity of the inverter.

      Charging current should be ideally 1/10th of the battery AH, so you can use a transformer with this much current rating.

      You may refer the following article for understanding relays in details:

  5. zakaria B.

    thanks for the reply swagatam….
    i found that the Average Rectified Forward Current of the bridge diodes 3A (for the charger) is that enough "i think it needs more current i am afraid it is going to melt down, do i have to use another high current diodes like 6A2….

    1. Swagatam Post author

      Helo Zakaria,

      If you are using a battery that's above 40 AH, then you might require 6A diodes, so it will depend on how much current you want to draw from the charger.

  6. Anonymous

    This is a very helpful read. I appreciate your time with the knowledge you have provided me with.

    Thanks again!


  7. lalji

    hi, swagatam,
    i tried to run this ckt on multisim , but it wont workd, i replace 4050 by 2sc945, is it ok?
    and plz tell me the range of battery that i can use with this ckt?

  8. Alfred Dawoud

    Hello Swagatam, my project is to make two AC 120 Volt into UPS circuit one from solar energy inverter 120V Sine wave and the other from 120 Volt normal to connect 360 watt and specially computer to don't the data when the energy outage so please help me ..thanks

    1. Swagatam Post author

      Sorry Alfred, I don't have PCB designs for the above circuits.

      for 30 amp load you will have to use a transformer rated at 30 amps.

  9. Anonymous

    HI swagatam I want made ups of 6v 4.5amp battery and I also want dc output only please reply what is change required.

  10. Anonymous

    Hi swagatam I want to build ups from 6v 4.5amp battery and I also want dc output only. Please reply me what is change require in above circuit.

  11. Anonymous

    Hi swagatam I buy a transistor (2n6107) from a shop in Mumbai and that retailer told me that it was from Motorola company and m8824 code was mentioned on it. When I find Motorola semiconductor manufacturers logo from internet it was different. Please help me how to identify transistor manufacturers logo and other semiconductors. I hope you will reply.

    1. Swagatam Post author

      A motorola part is not necessary, other brands would also work as good.

      I am not very sure about the initial letters…you may refer to the datasheet of motorola for knowing the exact component codes and equivalents.

  12. Anonymous

    hello, I wanna know why its necessary to use 4017? meaning why do we need the sequence to be formed?

  13. Anonymous

    can u please explain as to what would be the output of the transistors and what is applied to the transistors?

  14. Anonymous

    like u hv connected from the primary side a middle point to the ic.. y have u done dat?? would be really helpful if u could reply … thanks :!!

    1. Swagatam Post author

      The battery positive is connected to primary center tap, this battery voltage should also go to IC3 input, so it's connected there.

  15. Anonymous

    ser swagatam..i will put a fuse it okay?if i use 6 amps here,,what will be the rating of the fuse? thanks

    1. Anonymous

      okay ser..thank you so about 12 amps transformer here? is that okay ser? for inverter circuit only..

  16. Ram kumar

    Sir if i am using 60 Ah battery and a 300 W tranaformer what will be the maximum discharge current flowing to the transformer secondary????

    Will it affect the incorporating ICs and components badly if the discharging current goes much high??????

  17. Anonymous

    Can i use TIP 36 instead of TIP35?
    What iz about TIP122?

    In an easy manner is it possible to replace the output driving stage transistors T1 – T4 by two IRF540N mosfets keeping the power output not changed

  18. Ram kumar

    SIR I THINK ALL THE PROBLEMS OCCURED TO ME WAS DUE TO THE SERIOUS 'shoot through problem' CAUSED BY ERRANEOUS SIGNAL FEDING OF THE GATES.OF THE MOSFETS( MAY BE AT THE SAME TIME ) that had occured due to the variation of duty cycle of the signal from 50/50. I think my calculations for R & C values in the oscillating stage of 4047 is wrong. I've used R=22.7 K adjusted by a linear 47K pot and C was selected to be 0.1 uf(104) in accordance with the formula f=1/(8.8RC) . But for this i had set f to 50 . So that was the mistake. Wasn't it??????

    Sir let me ask one more doubt to u that even i am getting 50/50 duty cycle with approx. 50 Hz signal, will it undergoe any shoot through again????
    Also can i use a MOVRD240/20 varistor at the secondary terminal in parallel with a 0.1uf/600 V and 2.2 uf/200V capacitors to avoid damage to the appliances from higher voltage

  19. Anonymous

    Dear sir, In this schematic which are those components that determines the frequency as well as the duty cycle of the wave. How it can be adjusted?'???

  20. Swagatam Post author

    Ram,a 4047 IC will never produce shoot throughs, it's internally protected from this.
    select any standard 4047 circuit from any online source and select the components as per that circuit, instead of calculating it yourself.

    MoV is not necessary in a 4047 circuit.

    1. Anonymous

      Dear brothef,
      Let me know whether a 0.1 uf can be used instead of a 0.095uf to generate 250 Hz. If it needs accurate precision for generating the more accurate 50 Hz signal how could that capacitor value be implemented using available 0.1 uf or near value capacitors????

  21. Alex martin

    Dear sir,
    How can we calculate the maximum power that can be delivered by this circuit given the specifications of battery as well as transformer????'?

    Any formula is there?

  22. Anonymous

    What is the formula to get 250 Hz at the output of NAND gate IC sir'??
    Can i use CD4011 which is a CMOS IC composing of 4 independent two input NAND gates instead of 4093???? Reply as early as possible Sir…

    1. zeeshan abdullah

      hello, if i use 12v,7ah bettery, then what will the changes occure in above transformer rating e.t.c……………………………………..,

  23. Ram

    Sir, i am not getting any ac output from this circuit. I have used IC CD4011 ( quad two input NAND GATES) instead of CD4093 also each transistor pairs at output were replaced by IRF540N mosfets .
    0.095uf capacitor was accurately set up by paralling capacitors 0.047uf*2 and 0.001uf(102)


    1. Swagatam Post author

      Ram,you are doing as per your own wish and will,so I cannot suggest you anything.
      you were worried about mosfets getting burnt with 4047 IC circuit so i suggested you to use transistors in your 4047 circuit by referring the above circuit.
      Now you are making the above circuit with mosfets… inviting new problems.
      Stick with your original 4047 circuit and use transistors at its output just as done for the above circuit.

      In the above circuit 4011 will work but with two or three gates together.
      4093 will allow only a single gate to be used

  24. Anonymous

    How the gates can be arranged in 4011. In series????
    I have another doubt also that
    Whether it is necessary to take the center -ve wire when using three batteries in parallel and used as a separate ground????

  25. RAm

    I Am Very sorry for disturbing you again SIr.
    Actually i was in the middle of searching for a better 200 W to 250W inverter which is suitable to operate even sophisticated components. Atlast i found it in this article. Now i am not so affraid of getting the MOSFETS blown, since it is working properly when i checked the whole in last night producing a favourable ac output. SO I decided to stick on this new project since it can fullfill my needs and used the components as available with me( just changed IC4093 and output stages as said before).. I must must have to implement this to suit my needs. So expect your valuable suggestions for this. Now i want to know , how the gates in 4011 can be used to produce the same result as that of 4093 and make the entire one in working condition

  26. Ram

    Sir i have connected three NAND GATES in 4011 in series. ( output of first NAND gate connected to one of the inputs of second NAND gate and there inputs shorted. Its output further connected to the shorted input terminals of the 3rd one. Finally 0.095 uf connected b/w the shorted inputs of 1st gate and ground only. Also the 20 k resistance connected in between the first input terminals and the output of the last NAND GATE). When the testing has completed the result was fruitfull. I got ac out. But a small problem is there that i am getting only about 120v out. I could work a 3V adapter for torch with it nut not a table fan. Will you plz help me???

    1. Swagatam Post author

      OK, if you have succeeded with it then it's a great news.
      since it's a modified sine wave inverter it'll produce 50% less output if the batery/trafo voltages are rated identically.

      for a 12-0-12V trafo you will need a 24V battery for getting 220V with this circuit. you can connect 2nos 12V bats in series for getting 24V.

  27. Ram

    what much maximum power can be drawned from a 24V 20Ah combination SIR????
    Will i get more power by adding an additional 12 V 20Ah battery???? how????

  28. Harigovindh

    Hellow sir……….. is this circut features an automatic load correction??? i mean will i get constant 230 V even various loads are used at the load side utilizing differrent loading voltages???
    If it is not, how can it be made to operate such a feature???

    1. Swagatam Post author

      No this circuit does not have this feature, it will require an external circuit for implementing it…I'll try to discuss it in a new post soon.

  29. Ram

    Sir i want your help again. I think your suggestion at this time is very necessary to complete my work.
    I have done everything as per you suggestions. Put 24V instead of 12 and extract ed separate 12v for ICs from one of the batteries( 12V fed to the ICs after regulation using 7805 ). now i got better ac out which could be understood by a more brighter light indication by TESTER( my voltmeter shows only 23.5 when the range is put in 750V ac. But the led of tester becomes more brighter than the last time at which it was less bright and showed 120V in voltmeter ). But the real problem is , i can't operate a fan 6A rated by this provided all the batteries are fully charged

  30. Ram

    Will it be an error due to mismatch of value of feedback resistance R1(20k) ???
    Can a pot be used there and adjusted at run time untill fan rotates????

    1. Swagatam Post author

      yes you can use 100k pot in series with the 20k pot and adjust it for getting 50Hz.

      LED tester is useless for testing an inverter, you should use a 100watt bulb at the output of the transformer.

      check battery power by connecting a car headlight bulb to it…if the bulb lights up very brightly with voltage of the batt not dropping below 12.5v, would confirm a fully charged battery.

  31. Ram kumar

    Hai sir, Ram again.
    As per your suggestion i used 24 V battery bank for the purpose of 230V generation. Now i need a charger suitable for the battery. SO i have created a logic turn over switch for the battery bank which allows the batteries to connect in series when power is needed during power failure and connect in parallel when charging mode is reached. I have drawn a simple schematics to make it practical using only a rated relay. I have posted the diagram in the link given below. Please study it and point out mistakes if any sir.

    May i ask you onething that ,can the 24V battery be charged with a 15-0-15 transformer by extracting 30V from the terminal leads????

    i have made such a schematic intented to modify for parallel charging of higher rated batteries which are usually connected in serious under load…n

    1. Swagatam Post author

      Hi Ram,
      It looks correct to me, however will need practical verification for confirmation.

      yes 30V from 15-0-15 can be used for charging a 24V baterry

  32. Anonymous

    Sir in one of the comments in this article, i saw the formula for getting 250 Hz signal by the schmitt trigger arrangement as f= 1/0.8RC.
    How it is possible for 'f' to hold a value of 250 by chosing R as 20k and C as 0.095uf. Is it a calculation mistake????
    I am getting 658 as the value for 'f' on substituting these

    1. Swagatam Post author

      20k is not the correct value.
      and C will be in farads not in uF, so you should first convert uF into F before substituting the values in the formula.

  33. superman

    Sir what are the values of frequencies coming from pin2 and pin7 of IC4017 in this figure??? Can i use these values to make an assumption to ensure that the exact 50Hz ac is coming out of transformer primary????

    1. Swagatam Post author

      Superman, it should be 50Hz on either pins, yes surely if this is confirmed, the output from the trafo would be also confirmed with the same.

  34. Arun P Dev

    Can you please suggest a PWM based inverter circuit that uses the PWM chip ic SG3524 as the heart of entire operation ??
    The output of the inverter shouldn't be fluctuated with varying loads and the inverter should be capable of operating atleast a 250 W load.
    One of the most necessary requirement that i missed to say is that, this inverter should be capable of driving even sophisticated components such as laptops

    1. taps

      thank you Swagatam for that info. I just have few more questions about this project:
      1. what is the replacement for SN74LVC1G132?
      2. did you simulate the circuit on breadboard or in proteus?
      3. what is the power rating of the resistors?
      4. do you have a pcb layout on this?
      5. is this project uses pulse width modulation?

    2. Swagatam Post author

      1)you can use a single nand gate from the IC 4093 or a 555 IC astable will also work.
      2)I simulated it in my mind not with any any software.
      3)all resistors are 1/4 watt except R6
      4) it's a crude modified sine wave, but does not incorporate a pwm circuit.

    3. Swagatam Post author

      the backup time will depend on the battery AH rating, higher value will produce higher backups and vice versa….. it has no connection with transformer ampere rating.

  35. Robe

    Hey…your design is great.I wanted to know how you calculated the capacitor and resistor values to get a frequency of 250Hz.and how can i change the values to get a 50Hz frequency?

    1. Swagatam Post author

      Thanks Robe,
      Actually it has to be 250Hz at pin14 of the IC for achieving 50Hz across the transistor bases, since the sequencing of the output pins of 4017 are not continuous rather skipped in between for getting the proposed modified output.

  36. Arun Dev

    How much power will i get from my 24 V 40 Ah battery bank and a 230/11.5-0-11.5V power transformer ?????

    Battery bank formed by the series combination of two 12 V 40 Ah battery banks, each 12V 40 Ah battery bank is a parallel configuration of two 12V 20 Ah sealed lead accid batteries, means i have formed a 24 V 40 Ah battery bank with suitable configurations of four 12V 20 Ah batteries.
    Transformer has 5 input windings and 4 output windings. The maximum voltage at secondary terminal is 15 V

  37. Erns Balana

    sir is there any replacement or alternative that i gave in the above comment, the electronic components?because i have same problem in DPDT relay..i really appreciate all your works and very nice masterpiece..thank you for this.

    1. Swagatam Post author

      Please refer to the link which I gave in your other comment….it consists of the list of the all the equivalents…

      TIP122 has no direct replacement

  38. Erns Balana

    sir, i need you help..i made the ckt. above..and im having a problem in the circuit in the the the inverter ckt. the output voltage in the transformer is only 150v instead of 230v..why?..please help me sir

    1. Swagatam Post author

      Hi Hapu,
      the easiest way to test it is by loading the output of the inverter gradually until the voltage begins dropping. The total value of the load wattage during that instant will provide the wattage capacity of the inverter.

  39. Arun Dev

    Sir, it z very interesting to implement your ideas…. Thank you so much sir for providing us such various ideas.
    Now i am in the middle of making a small 150 W UPS system …. I have a 15 V 5A Old Thoshiba laptop charger, four no.s of 12 V 20Ah sealed lead accid batteries taken from an old electric scooter, and some necessary components to make out the UPS……
    The main intention behind this is : i want to charge my mobile phone, laptop, even a torch during power failure.. The inverter should also support a table fan….so a maximum rating of 150 W is necessary….
    Since i want to operate such sophisticated components , could u plz suggest me the most appropriate version satisfying all ma needs..n
    Also i want to know that, if the UPS is based on PWM, what battery voltage to be selected…12 or 24 ??…
    I have only a 15 V charger with me… So if i have to use 24 V confih. (series combination of two parallel configuurations each 12V 20 Ah batteriea = 24 V 40 Ah ), how it be possible to break all these config. and forming a complete parallel config. ( 12 V 60 Ah ) for charging7 ????

    1. Swagatam Post author

      Thanks Arun!
      Here fan is the only gadget that will require 220V rest of the items (cell phone, laptop, torch) can be actually charged directly from the battery, so an inverter won't be necessary for those items.
      anyway if you want to charge them from the same inverter you may do so and it won't be necessary to employ a sine wave inverter since all these items work on DC and any AC whatsoever will be ultimately converted to DC, so even a square wave inverter would work for the application.

      you can try the desin that's shown in the above article or you can try the one that's shown in the following link

      both can handle upto 200 watts

      for charging a 12V 20ah battery you would require a 14V/3amp battery, so your smps will need to be 14V at least, 12V will not work.

  40. Arun Dev

    Sir what much wattage to be expected by two 12V 20Ah batteries in series ?…. The Transformer is rated at 12V,300W

    sir i failed in executing the below design for my need

    Again i have decided to go on with the current UPS design…. Now as a whole saying i got AC out…. But a 100 W bulb didn't give a single glow on connecting it at the output of the transformer… On measuring the amount of current passing to the centre secondary winding of the inverter transformer , i came to know that only 0.77A current is flowing, thereby it seemed to produce a power of only 18.48 W ( 24.5 * 0.77 ) at the output.. That is why the bulb didn't glow…

    SO Tell me sir, how the amount of current ( that to be drawn from the battery ) could be increased to a minimum level of 7A, so that inverter output reaches a minimum of 175W….

    I have used 2 no.s of 20Ah batteries in series……
    Eventhough the expected wattage couldn't be achieved by this config., the current drawn from the 40 Ah config ( Series config. of two battery banks, each banks are the parallel config. of separate batteries ) was also seen to be so low, so that the 100 W bulb couldn't start glowing…


    1. Swagatam Post author

      Arun, What is the rating of the transformer that you have used? If it's a 5amp transformer it should weigh about 4 to 5kgs, if it's much lighter than this would indicate a lower rated transformer which won't produce the intended results.

      the voltage rating of the trafo must be lower than the battery voltage, for 24V it must be 18-0-18V

      check the gate voltages, the peak voltage should be 12V since it's supply is 12V and the average voltage should be 6V since it's operating at 50% duty cycle.

      please check all this and correct them accordingly.

    2. Swagatam Post author

      12-0-12V will also do, 300watt must weight at least 12kgs.

      initially keep the PWM diodes disconnected and check the results.

  41. Arun Dev

    Will the voltage be stable at the output…
    I mean,, will there be any problem in connecting more than one instruments at the output.
    I want to connect maximum of 4 instruments ( A Notebook charger of 13.5 W, A mobile charger of 5W, A Table fan of 80 W, )
    Will they get burned ???

    And if this connection is not possible, how to make the AC Voltage stable independent of different loads in this circuit???

    1. Swagatam Post author

      Arun according to me there won't be any problem with the mentioned gadgets since all these employ SMPS regulated power supplies except the fan which is not so vulnerable anyway..

      To make things stable the PWM feature is essential.

      By the way how did you proceed with the above circuit, what components did you use?

      Please send me the built units picture for reference.

  42. Arun Dev

    I have also an another doubt sir…

    1). What is the main difference between a normal MOSFET inverter and an H bridge MOSFET Inverte ?

    2). Will the former one have more efficiency than the other

    3). Does the H bridge means the config. in which 4 MOSFETS are used with each pair of mosfets connected in parallel ??

  43. Arun Dev

    First of all let me say thanks for all the valuable supports and suggestions that were made from your side in doing the current UPS design….
    Now i want to know few things about thz…;

    1). What will be the value of frequency shown at the gate terminals of the MOSFETs if the output stage is gonna completed using MOSFETs ? Will this be the same 50 Hz that is really expected to be, or more than that ( say few Khz, 666 Hz ) ?

    2). How does the developed ac resembles to the real AC, helpful to be used for sophisticated components ??

    1. Swagatam Post author

      Thanks Arun,

      1)gate frequencies must be around 50Hz.

      2)In the above design, the output will not resemble a real AC, you can modify it digitally for the same by employing a PWM input

  44. Arun Dev

    Could you plz tell me how to measure an ac current using an ordinary digital multimeter ???
    I have never faced such a situation of measuring the ac current, was only concentrated in measuring dc current… so at the first, trying to measure connecting the +ve lead of the meter to ac live and the negative lead to one of the plug lead of a table fan, keeping the neutral connected as usual gave a result which tends to have an up and down values 1.7 and -0.5 something at the 200 mA range of the meter

    1. Swagatam Post author

      select AC range in your digital meter (220V) and simply touch the prods across the source terminals, don't bother about the polarity since AC has an alternating polarity and can be checked anyway round.

      But please remember that in your inverter AC will be available at the output of the trafo only.

      At the gates of the mosfets it will pulsed DC, not AC, so here you will need to use the DC range for the measurements.

  45. Arun Dev

    I asked you to how to measure AC current, not the voltage…
    I want to find the power consumption of the TV when it is switched on by P= V*I

    1. Swagatam Post author

      sorry, current can be measured by putting the meter prods in series with any one of the load wires.
      Select AC range and 10Amp range in your meter….in some meters it's 10amps in some it could be 20amp, both will work.

  46. Arun Dev

    Also tell me whether it is possible to replace the 8550 transistor in the battery charger section by another pnp transistor such as BC557 and 8050 by BC547 ??????

  47. Basheer Ahmad

    Enter your comment…hi sir,
    Plz help me how we can desigan and make the auto on and off generator circuit when electrity available the generator off and when electricit has gone so the generator should be on autometicly in three slaf of trying for starting.thank u.

    1. Swagatam Post author

      Basheer, please use the search box given at the top of the page and search for "ATS" you will be able to find the required designs as per your specifications.


    Hello sir,I am Abdus, you didn't tell ,In which diagram I have to connect 1k resistor across c1,upper or below?Also tell me about the ups circuit in which single transformer can be used?

    1. Swagatam Post author

      divide the battery AH value with 10….that will be the ampere of the charging transformer.

      for a 12V battery the output from the transformer should be 14.4V approximately….not above this.

  49. Anonymous

    Hello Sir Swagatam, how can i make my old computer UPS self charge, pls if u have explain that on your page pls reply me with the links thanks

    1. Swagatam Post author

      Hi, A UPS will already have a self charging feature while mains supply is present, that's why it's called an uninterruptible power supply


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