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Automatic Micro UPS Circuit

Automatic Micro UPS Circuit

The following article discusses a simple automatic micro UPS circuit which can be used with modems for acquiring uninterrupted power from a DC source, and battery during mains power failures. The circuit also incorporates an automatic over charge cut off, and a low battery indication feature. The circuit was requested by Mr. Kapil Goel.

Technical Specifications

Hi Swagatam, How are you, and was really happy to read your blog as I was scrolling through circuit sites for my requirement.If you can help me out for that, I have a requirement:

This is exactly what my requirement is https://www.mini-box.com/picoUPS-100-12V-DC-micro-UPS-system-battery-backup-systemI have a 12volt operated device, it consumes approx 35 watts right now I power it up using a 12volt adapter, but when main power fails its get rebooted..

I wanted to use 12volt 2200mh Li-ion battery pack so that whenever there’s a power cut it will automatically shift to battery Also, the circuit should have over charge protection, and low battery indicatorAt last I am not asking this circuit for free, as I am ready to pay for it. Many thanks in advance

Regards, Kapil Goel

How it Works

The design was actually presented in one of my earlier posts also, however it does not include an automatic over charge cut off feature.The present design has similar functions, but has an added protection feature in the form of an automatic battery over charge cut off and also an under voltage indicator.

The proposed circuit diagram of an automatic micro UPS may be understood with the following points:

The input supply is acquired from any standard AC/DC adapter rated anywhere within 15 and 19V DC, current at anything above 1.5 amps.

The above supply is regulated via a 7812 IC whose ground pin is elevated to about 2.4V so that the output from the IC gets raised to about 14.4V rather than the normal 12V.

This is required because the attached 12V battery needs to be supplied with a slightly higher potential than its rated value.

How the IC 741 is Configured

The 741 IC stage is configured as a comparator.

Its pin#2 is clamped to a fixed reference voltage of 4.7V using a suitably rated zener diode.

Pin#3 is rigged as the sensing input if the IC via an adjustable preset.

The preset is adjusted such that the potential at pin#3 just exceeds the potential at pin#2 when the battery voltage crosses the 13.5V mark.

As long as the above situation is not sensed, the output of the IC at pin#6 sticks to it initial zero voltage level which in turn keeps the BC547 transistor switched OFF. With BC547 being switched OFF, the TIP122 gets a chance to conduct via the 1K resistor and charges the connected battery.

The battery terminals are directly connected with the modem which is being used for some application.

This allows the modem to remain powered via the external AC/DC adapter while the battery gets charged simultaneously.

The battery is allowed to charge freely until it reaches the over charge threshold when the output at pin#6 of the IC goes high, switching ON the connected BC547 transistor.

The above switching cuts off the base bias to the TIP122 transistor and stops the battery from getting further charged. This does not affect the modem as it continues to acquire power from the external power supply.

During mains failure, the supply from the external adapter gets inhibited, and the modem starts receiving back-up supply from the battery.

Since no relays are used the transition is within micro seconds which keeps the supply to the modem interrupted during power failures or even under heavy power fluctuations.

If the mains stays absent for long, and the battery reaches its over discharge threshold, the situation is immediately indicated with the green LED, which can be also replaced with a buzzer. The modem should be switched OFF then, to stop damage to the battery due to over discharge.

The adjustment of the 100K preset determines the low voltage threshold mark or the lower indication. level.

Once the green LED is lit, it will remain lit until the battery is fully charged, similarly once the red LED illuminates, it will stay illuminated until the green LED lights up or when the battery voltage level falls below the set lower threshold.



Using a PNP BJT for the above Charger Circuit

The above circuit can be also configured in the following manner, here the LED indications get reversed, meaning red LED shows low voltage while the green LED indicates high voltage threshold.

The following circuit also incorporates a current limiting facility which can be used for providing a current controlled charging to the connected battery.


FEEDBACK from Mr. Kapil

Hi Swagat,
Thanks for the circuit.. I really appreciated your swift and kind response..
I have couple of questions on the same.
1) What will be the max current it will support, my device requires atleast 5 amps 12 volts, will this be able to handle that.

2)As per the circuit, I can see, you have directly connected the modem to the battery, but if I am not wrong, this means that modem will keep on taking the power from battery, and battery will not get charged?
Please I clear out this confusion.

Also I am using a li-ion battery, which has a voltage of 12.6 volts on full charge and 11 when discharged.

Also my input volt is also 12volts, I cannot use a higher volt rated adapter.. will it be able to charge my battery at fullest.

Kapil Goel

My Reply

Hi Kapil,

Presently the above shown circuit is rated at 3 amps maximum, so I may have to amend the design to suit your requirements, however the input voltage will need to be above 13V otherwise the battery will never get optimally charged.

The direct connection of the battery with the modem will not affect the battery charging as long as the input source power is active....both outputs will be simultaneously taken care of.Regards.

The modified 5 AMP micro UPS circuit design:




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!

179 thoughts on “Automatic Micro UPS Circuit”

  1. Hello
    Can I use this automatic micro ups circuit for charging a backup battery, 14.8V 2200 MAH LI ION battery
    (Charging current 2A
    Discharging current upto 4.4A)
    So when power cut it will automatically shift to battery

    What is the output voltage and current of this (14.8v battery) ups circuit ?

    I want to use this ups circuit output to run a dc motor rated at 12volt 5amp

    What modifications needed for this requirement?

  2. Sorry Swagatam, i’ve got an additional question. I want to use the cirquit to charge a backupbattery. Ive got one with a capacity of 2800 mAh and another one of 6800 mAh. Wich of the cirquits do you suggest? Do I have to change/adjust components for these? Thank you!

  3. Hi Swatagam. Looks nice and I will try this cirquit. A simple question from a rookie in electronics 😉 : In the cirquit “Using a PNP BJT for the above Charger Circuit” I see 2 diodes connected to the modem and also 1 between the tip127/Battery+ and the modem. What types are they? I can’t find this information. THX! Yvan

    • Hi Yvan, those are ordinary rectifier used for dropping and adjusting the output level for the modem and for the UPS cut-off transition threshold…. you can use a 1N5408 diodes for those.

  4. Hello Swag,

    I am unable to get MC78T12 and was using 78012 which is only 1.5 regulator, could that be the reason for my issues i mentioned earlier. Also in my scenario the voltage at pin 3 is not exceeding the voltage at pin 2 in any scenario?

    • Hello Sumit, 7812 will only give a lower current at the output, it cannot be related to IC malfunction.

      If pin#3 voltage is not changing with preset adjustment then either the IC is faulty or the preset is faulty.

      check the response at the center lead of the preset by disconnecting it from pin#3, I hope you have connected the preset leads correctly??

      • Hello Swag,

        I have tried with all possible combinations and definitely there is something incorrect. I have more than four op amps and the result is the same, I am now following the third circuit. Also i have gone ahead and created a live simulation on http://www.falstad.com/circuit/circuitjs.html, if you can possibly take sometime and import the simulation from the text file ( https://filetea.me/n3wLxovfIdCTNuQaAArnlDbMQ), you can see where is the disconnect. Please help in checking as i am stuck…

        • Sumit, unless you are able to do a mind simulation of the circuit, an artificial simulation will be of no use, how are you gong troubleshot the fault in the simulation? Artificial simulation is normally used by the experts to fine tune a mind simulated circuit….

          The circuit above has been already tested by me thoroughly, so how can it be wrong, even if it was not tested still the design makes complete sense.

          So the first thing you should do is understand how the opamp is configured to work, and how the transistors are supposed to react to the opamp output.

          Once you are able to simulate the stages in your mind then troubleshooting will be a matter of minutes and could be done simply with a meter.

          If you think something;s wrong please specify it I will be most happy to correct it.

          just changing the opamps will not help, you must pick up a meter and check the response of the circuit at various nodes, and verify whether things are correctly working or not as per the specifications.

          • Hello Swag,
            I agree completely with you, the file I have shared has a simulation of your circuit diagram, you can import it to the first falstad link I have shared. The point I am trying to make is when you check the simulation it would clarify few thing :

            -Both LEDs will glow simultaneously .
            – the circuit can work as expected only when the 10 k pot is at 2.6 and 100k pot at around 26.8k and the input voltage should be around 15 and other combination
            might swap LEDs but would not cut off charge …
            – I am using it as per the simulation and things seem to work as expected.
            – the file I am sharing is an export of the simulation so u just need to import it to see what I am trying to say…
            – also I have full respect for you and please don’t think I am doing this for any wrong intentions, only think I am trying to figure out is the correct description..
            – you have been kind enough to read through and reply so I have no intentions to challenge your skills. All I am trying to do is update the description for a best suit.

            • Sumit,

              I usually don’t depend on simulators because I know it’s not worth it, because you will never able to reach the actual practical outcome from a simulator’s results, so I will advise you to keep it aside and use it as a secondary tool only.

              and moreover this design is too simple and can be diagnosed through visualization

              If you really want to achieve success, you should get the parts and make it practically, and diagnose the circuit with a meter in case of a fault, very quickly you will be able to get it up and running.

              both LEDs can be never On, you can track the path of the LED current in response to the high and low status of the opamp output, you wil find that only one LED can be ON the other cannot.

              for example in the last circuit when the opamp output is high it will allow the green to light but will block the red LED’s path.

              but yes there’s a glitch, since the emitter of the PNP is connected with a higher potential than the battery it might force the red LED to light, to rectify this you can isolate the IC 741 circuit’s positive line from the battery and connect it with the emitter of the PNP….except the 10k presets upper lead which can be linked with the battery positive….try this and let me know.

  5. Hello sir ,
    I need a 11.1v 44oomah li ion battery charger circuit with constant current and constant voltage mode which are modes used in Li ion battery charging.I have a smps of 12v 2amp.I need to give this input.I have to change 12v to 14.5v for battery input HOw could i do this?
    please help me on
    1) battery charger circuit and this battery has a protection circuit against :sc,over voltage,over current,over discharge,over charge and balance battery charging function
    2) How can i create 12v to 14.5v in smps?

    • Thanks a lot for your reply, I have tried to connect just 2 of the micro charging modules with 2 batteries, while the charger is disconnected the combined output is 8.4v, but once I connect the charger the output is 4.2v and one of the boards out put is near zero v, I came to know that Input – is connected to Out –
      and B -, also Out + is connected to B + but not to Input +, so when the charger is connected, B + of one cell is grounded to Input – !
      is there anything I can do to resolve that issue ?!
      the module has the charger IC TP4056, and overcharge & discharge protection ICs.

      • The output will not combine, rather will show the one which may be higher than the other. The 8V is an inflated value which is shown by the meter due to its high impedance nature, the actual voltage can be confirmed after connecting a load, so if it’s showing 4.2V after connecting the load then that’s fine. You can confirm the same by adding a 1K resistor at the output, this will prevent the meter from showing the inflated value.

        I won’t be able to diagnose the IC board connections because I do not remember the pinout functions.

        you can refer to the following article and check if it helps


      • Thanks a lot for your reply, the 8.4v is real, I can run a DC motor which draws nearly 200mAh, but higher than that the combined circuits can’t provide power, I believe that’s because of the presence of protection ICs DW01A and S8205A, each battery with the connected TP4056 module can provide 4.2v at 1000mAh max, I connected a 12v 21w car lamp that draws 2.2 Amps, it can pull 1Amp from the battery.
        Could an optocoupler be used in isolating ground or combining the circuits ?!
        or It’s impossible to do ?!
        another question please if use one of these charger modules with a battery pack of 6 Li-on 2200mAh cells & a step up DC converter xl6009 to get 12v 2A ?!

        • OK I think got it now, I magnified the image and I could see that the input supplies are taken through the PCB tracks and connected in series, but I am not sure whether the batts can be also connected in that way in back up mode?

          without a schematic it will be difficult to figure out the working of the whole configuration?

          Can you tell what is your actual need then may I can suggest something?

          You can step up 6x2200mAH to 12v 2 amp easily but the charger will not be able to charge a 6x2200mAH pack because the IC is rated to produce only 1000mAh

        • Thanks for your reply, I want to make an advanced Automatic mini UPS so (that I don’t need to worry about overcharge or discharge) for my router it’s 12V 1.5A and some how balance charge the Li-ion cells forming the 11.1v battery pack. The UPS might be in standby mode for long times so not to cause damage to cells, I needed an advanced charger.

          this is the schematic of TP4056 Module I have

          • OK, but as you can see the negative supply and the -B are different on the PCB, so your series configuration will work only for the input supply not with the batteries and is not recommended also…better to buy a single 12.6V cell and use a single 12.6V charger.

  6. HI sir,
    In my Li ion battery pack is of 11.1v and 8800mah. There is a Battery management chip:
    1. Battery Charge Cutoff Voltage: 12.6V 2. Battery Discharge Cutoff Voltage: 8.25V 3. Battery Charge & Discharge Mode Integrated 4. BMS Passive Protection Function: Overcharge Protection, Over discharge Protection, Over Current Protection, Temperature Protection, Balanced Function
    5. Battery Inner Resistance: <=33.75mohm 6. Continuous Discharge Current: 3A 7. Maximum Discharge Current: 5A
    I have a smps of 12v 2a can i apply this supply directly to this battery for charging?
    And for automatic switchover between battery and smps supply I have used relay.Is this possible to give smps supply directly to battery?

    • Hi Sham,

      A 12V supply will not charge your 12.6V battery, even if you keep it connected forever….you will have to increase the SMPS output upto 12.6V, or at least 12.5V.

      Next, you must have an automatic cut-off system between the SMPS and the battery so that the SMPS supply is instantly cut off as soon as the 12.6V level is reached.

      If you satisfy the above two conditions then definitely you can connect the mentioned SMPS with the battery.
      Having said that, a 2amp rate will take more ta 10 hours to charge your 8 AH battery…better to go for a 5amp smps.

    • Thanks sir,
      As I said that there is a bms chip with 11.1v 8800mah li ion battery pack, so there is any need to charge this battery with cc and cv mode ?
      If needed then how can i make? please ,sir help me.

  7. hey, that's what i looking for!! thx for sharing this schematic.
    how i modified that since i just need 2amp max out?

    • Hi, you can adjust the 0.4 ohm resistor value to limit the output at 2 amp, using the following formula

      R = 0.6/2 = 0.3 ohms
      wattage = 0.6 x 2 = 1.2 watts

      • Hello Swag,

        Thank you so much for your efforts in helping with such circuit, i have made the first circuit, my loads are two routers (12v 0.5amp and 5v 0.5 amp), I planed to use an additional 7805 at the output to reduce the power to 5v 0.5amp. My battery is 12v 7.2 AH. I have below question if you can please help :-
        – I am unable to set the lower threshold, if I reduce the input power to 11.0 v and use the 100k var nothing happens the green led is always on ? i can only set it by the 10k pot but then i loose the overcharge cutover.
        – Is Green LED (+) contact is connected to the positive terminal before the 1N5408 ?? because if i connect is towards the battery side the circuit does not behave at all ?

        • Hello Sumit, it could be happening due to leakage voltage from the IC output, you can try connecting a 3V zener diode in between the pin#6 of the IC and the rest of the connections, meaning let the connections at pin#6 be after the zener diode and not directly with pin#6, as done in the following article


          the 100K preset can be connected before the zener or after zener, it is not crucial.

          please note that you must first adjust the 10K preset fo the upper limit ensuring that the feedback 100K disconnected udring the adjustment, once this is done then you can connect back the 100K link and proceed with the lower threshold adjustments.
          preferably you can replace the 100K with a 470K for getting a wider range of adjustment.

          green led can be connected with the positive rail of the IC pin

          • Thank you Swag let me try it, can i use a 3.6v zener instead ? also when i am setting the thresholds the loads and battery should all be disconnected is that rght??

            • Hi Sumit, a 3.6V zener will be OK, and yes there shouldn’t be any battery or load connected while setting up the design….

    • ok thx, i have PS 15v 3amp. n i need to limit the charging current about 100/200mA. what should i do to modified the circuit?

      • Hello Swag,

        Unfortunately tired with the suggestions, its the same. adding a zener just did a change that the green led is on simultaneously with the red (upper limit). lower limit is still an issue, i also tried adding a 600k pot for varied range but no luck. I am thinking to use the second circuit with current eliminating resistor as 0.82 ohms, my battery is 12v 7.2ah sla battery , would it be a good idea to use that design? also would you have any recommendation for taking dual outputs 12v and 5 v @.5ooma (my plan is to regulate the 12v to 5v by 7805)? Thanking you for all your valuable inputs and support.

        • Hi Sumit, the green LED and the red LED are connected with opposite modes so they can never turn ON simultaneously.

          Green LED will be ON when pin6 is 0V, when pin#6 is 0V red will be obviously OFF, similarly when pin#6 is high red will turn ON, and green will be shut OFF because green will now have both its ends at positive.

          you must investigate why your circuit is not working using a meter.

          basically when pin#3 is higher than pin#2 voltage level, pin#6 will be high, and conversely when pin#3 voltage is lower than pin#2 then pin#6 will become 0V…so please check whether these things are taking place or not, you can trace the fault within minutes.

          Sometimes if the IC is not good gives rise to unnatural results….

          you can try any of the opamp based designs from this websire all will work… but before that you must make an effort to understated and grasp the functioning of the opamp and troubleshoot the present design.

          • Hello Swag,

            Thank you once again for the explanation i think i have found the issue, can you please confirm if there is a 10k resistor between the 10k pot variable pin and 741 pin 3? If Yes than i think i have missed this particular component in the design.

  8. Hello Swagatam ..
    I'm really excited in your projects..
    I want you to help me and ask you some questions :
    i have 13.8v 3amps smps ,12v 4.5ah battery and modem&router(1.5A load).
    The question is when i tried to test changeover circuit that you posted the battery discharges and iam really checked the current flews from the battery but most be charging when the mains pw is on ,
    I tried connect the smps to battery directly (dioades exist) ok its charge but when loading at 1.5A both smps& battery supplied the load output. .
    So how can i make automatic ups to remains my network on and battery charges correctly when main ON ?

    • Hello Ahmed,

      sorry, I could not understand your question correctly, because I am not sure how you checked the situation and to which circuit diagram you are referring to??

      If it's the first circuit which is causing the problem, you can simply add a diode just at the right side of thee 1K base resistor of the TIP122 this will stop the battery from self discharging when the mains fails….if possible I'll update the diagram soon with the correction

  9. Hi
    another request in addition to the The modified 5 AMP micro UPS circuit design:

    I want to incorporate switching circuit for a 12 volt DC fan/light which gets powered only on when main fails. The fan/light stays off on the mains supply but powered on automatically when mains fails.


  10. Hi swagatam, im triying to setup the last circuit (5amp) for a 12v battery but i have a lot of problems.

    i remove the link of the 100K preset and apply 14.3v in the place of the battery adjust de 10k preset to get 0.01v difference between pin 2 and 3 the green led is on, put again the link of the 100k preset and reduce the voltage to 11v, the green still on, adjust the 100k preset to get the led in the base of the tip127 (red led) on, if i increase the voltage again to 14.3 the green led doesnt turn on, i tried to 16v and nothing happen, the red is on and the grren is off, but when i touch the 100k preset whit a isolated tweezers the green led turns on and the red off, i decrease the voltage again to 11v and changes (red on and green off), increase the voltage and doesnt change until i dont touch the 100k preset with the tweezers, any idea?

    its correct the point where im applying the variable voltage (in the battery place, not battery conected)
    its necesary to connect the powersupply to the regulator?


    • Hi Fenrir,

      connecting the sample 14.3V input at the battery points will do.

      but monitoring the 0.01V difference is not the correct way of adjusting circuit.

      Initially keep the preset slider towards the ground side, and the 100k preset link disconnected

      apply 14.3V, and slowly move the preset until the green LED just lights up.

      without removing power connect the 100k preset link across its original condition, adjust it at the minimum resistance value.

      Now adjust the power supply value to 11V, and slowly adjust the 100k preset to shut off the green LED.

      The above procedure should fix both the thresholds to their intended cut off positions.

    • thanks Swagatam, It seems to work now
      only i made one change, i replaced the 100k preset for a 10k one
      with the 100k preset never achieved turn off the green led, for the 10k happens in the 11.4v mark, i think its enough, thanks again!

    • That's good Fenrir, however the 100k preset also has the capability of reducing to the 10k or lower mark, so it's strange how the 100k preset did not work…

      .anyway if it's working now, it's great. thanks for updating

  11. Hi Swagatam

    i have a 12 volt 7.5 AH with full automatic battery charger , and my 12 volt 4 AMP device has its own power supply ,

    so i need just the automatic changeover part of your circuit , how can i do that ?

    i mean i wanna use my own battery charger and and my own power supply for load , but i need your circuit to monitor them ! changeover when main is fail and so back !

    thank you for your help

    • Hi Hamzepoor,

      for an automatic changeover just terminate the positives of the two power supplies through individual diodes and connect the ends of the two diodes together. This becomes the positive for the load, ……the negatives of the power supplies can be joined in common and connected with the load's negative.

      Make sure the primary power supply has voltage a shade higher (by 1 V) than the secondary power supply.

  12. Hi, i made this curcuit
    first of all , i wanna thank you for such a useful site

    i have 2 question :

    1- 78H12A is a fixed 12 volt regulator as you said on your site , how we are getting 13.x volt for charging our battery ?

    2-i used PM600 diode instead of 6A4 , it is possible ? and why my IC741 is getting hot even without any load or battery attached ?

    thank you Swagatam

    • Thanks Shayan,

      1) the inclusion of the series diodes with the GND terminal provides the required extra boost at the output of the IC

      2) check the datasheet of the diode, if it's rated above 6amp then you may use it.

      3) increase the value of the base resistor of the TIP127 to around 10K and see if that stops the IC from getting warm..

  13. Thank you Swagatam ,
    finally i decided to use this circuit built-in charger 😉

    i need 12 volt regulated , but i think the voltage will reduce when battery is discharging , is there any circuit to regulate the voltage on 12 volt ?

  14. Thank you very much
    As Sina Alimorady said , i have my own charger , how can i attach it to this circuit ? i mean which part should be removed and where charger should attached ,

    forgot to mention , the last circuit , i mean 5 AMP

    thank you

  15. Hello Swagtam , thanks for your reply
    the built in charger in this circuit is 1 stage or 2 stage charger , i wanna achieve maximum battery lifetime and performance , and if this circuit doesn't care about that how can i replace my own charger with built in charger of this circuit , i mean where it should be attached and what part of this circuit should be removed ?

    thaanks for your reply

  16. Thank you so much,and there is some questions
    I would appreciate for you describing

    1-What is the current of charging process??

    2-What change we should done to this circuit ,the last one,to make it 1/10 amp charging current ?? Of course its different between 12volt 4.5 amp and 12 volt 55 smp battery

    3-for charging a 55 amp battery it could take an all day long or more to complete!! Is last circuit support such heavy process ?? And if not what changes we have to apply ???

    Thank you very much for knowledge which you are sharing

  17. Thank you so much,but I,m confused,
    Is this circuit,I mean the last one cut off power from modem automatically when main power is not present and battery is going low ??? Or just glow led to notify low battery ???

  18. Hi Swagatam Majumdar,
    first of all i wanna thank you for such great information your are sharing with us,
    in your 5 AMP circuit(the last one) , what changes should apply to prevent battery from deep Dishcharge??
    we know Sealed Lead Acid batteries should not discharge more than 50% , so how we could cut off circuit when battery is low to prevent damage to the battery ans increase its lifetime ?

    thank you again

  19. Hi Swagatam,

    your web site is very good and useful , i really interested in this circuit , but i have a problem , i wanna know in what voltage these circuit cut off battery charging to prevent overcharging of 12 volt 4.5 AMP SLA battery , and how to protect my battery from deep discharging ? i wanna cut off circuit when battery reaches its 50% of charge , how can i do that ?

    thank you so much

    • Thanks Unknown,

      a 12V battery is supposed to cut off at 14.3V, and initiate charging at 11V.

      you can use the first circuit, for your need, adjust the 10k preset for setting over-charge cut off and the 100k preset for the initialization of the process.

  20. Hi,
    Thank you for the circuit, really good one,
    I wanted to ask, if I placed two batteries in series 3.7v same mah i.e 3000mah, so It will have 7.4v and 3000mah,
    what changes shall be done to the circuit….
    Thank you in advance

  21. Hello Swagatam.
    Thank you for an interesting design.
    I am puzzled by two issues regarding the 5A version (the last one) of your circuit, complete with current-limited charging:
    1) I do understand that while TIP 127 is conducting, the output of the regulator is fed to the positive terminal of the battery and it is current-limited by the 0.4 Ohm resistor inserted between the battery's negative terminal and the reference ground for the regulator. However, when the load (in this case the modem) receives its power supply from the battery, the 0.4 Ohm resistor is in line with the load, leading a) to energy wasted as heat and b) a variable voltage drop depending on the current consumed by the load. Could you please comment on that?
    2) Regarding the BC 547, its base is directly connected to the negative battery terminal and its emmiter to the reference ground, i.e. its bias is provided by the voltage drop across the 0.4 Ohm resistor. Under which circumstances will the BC547 conduct, and when it does, will it not just negate the effect of the zener diode that biases input #2 of the 741 by "shortcircuiting" the zener? It seems that I fail to understand the operation of this part of the circuit. Would you care to elaborate please?
    Best Regards,

    • Hello Vangelis,

      yes the 0.4 ohm will come in between the supply during both the occasions, however the dissipation won't be too high, unless the output consumes above the specified level as calculated for the resistor.

      still If you want to avoid this, you can change the position of the resistor and bring it on the ground rail just on the right side of the 4k7 resistor of the green LED…and join the battery negative with the modem negative line.

      a voltage across the 0.4 resistor will develop only when the battery begins consuming above I = V/R

      that's I = 0.6/0.4 = 1.5 amps…you can set other values as per the battery specs.

      once this happens the BC547 is forced to conduct which in turn shorts the pin#2 of the IC to ground…this action instantly reverts the output of the opamp to a high logic shutting of the TIP127…this immediately inhibits the current consumption by the battery….the cycle now begins playing a tug of war keeping things under the specified limits.

    • Thank you Swagatam for your kind reply.
      Just to make sure that I understood you correctly. 1) The 0.4 Ohm resistor will be disconnected from the negative battery terminal, 2) The BC547 base will be connected to the circuit ground via the 0.4 Ohm resistor, 3) The negative battery terminal will be directly connected to the circuit ground and to the load (modem) negative supply line. Is this correct? (Please confirm this and I will try to upload the revised schematic).
      Many thanks for your time.

    • Hi Vangelis,

      only the 2) is not correct, rest is OK.

      just cut the line which runs right under the battery, and connect the 0.4 ohm resistor there.

      the base can be as it's in was in the original situation.

      the earlier 0.4 points can be joined with a wire link.

  22. sir, as of 741 op amp datasheet, pin 4 should be connected to gnd. why i have to connect it to positive rail?

  23. Thanks sir. but i checked the datasheet of 741, it showed to connect pin 4 to ground, not positive. why is that?

    • Hello white dragon, your circuit looks perfect to me, just make two changes in it:

      remove the 2k2 resistor connected across the output and ground pins of 7805, and connect pin#4 of the IC with the OUTPUT (+) of the 7805 supply line instead of the existing position shown in the diagram..

      rest everything is fine.

  24. sir, may i use 4v battery here in this circuit?


    whats need to be changed?

    • yes you can use 4V battery, in that case you will need to use a 7805 IC at the input and remove all the diodes that's connected at the relevant positive rails

      also you won't need diodes at the ground pin of the 7805

    • ok sir, but what about the 4v7 zener diode? also, there is a 1watt 0.4ohm resistance from battery ground. what will be the modified value for 4v bat?

    • in place of the 4.7v zener, you can use 3no 1N4007 diodes with cathodes towards ground and anode towards the IC pinout.

      the 0.4 ohm ids the charging curent limit, it may calculated as

      R = (10 x 0.6)/batteryAH for lead acid or SMF batts.

      R = 0.6/batteryAH for Li-ion or Lipo batts

    • ok sir. i actually want to use 4v 4.5Ah sla type battery. do i need to remove all remove all the diodes in that circuit?

  25. Good day Mr Swagatam!
    The second circuit (current limited charging) you did was I'm looking for. But I'm not that sure if the output I need is going to be I expect. I'm a newbie to this So my questions are:
    1. What will be the value of the resistor on the negative side of the battery so the charging current will be 3amps?
    2. Will the modem (1.5amps) function properly if the output current of the circuit is 5 amps max (I read that it will not overheat)?
    3. Can you give me a list of the parts of the second circuit that I will buy on the electronics shop?

    Sorry for my bad English. Thanks!

    • Goodday Arvind,

      for 3amp limit you can use a R = 0.6/3 = 0.2 ohms 3 watt resistor

      if the output voltage is regulated to match the modem voltage then the 5 amp current will not be an issue.

      click the diagram to enlarge, you'll find all the parts correctly assigned, the resistors are all 1/4 watt except the current limiting resistor.

    • okay. The mc78T12 is the vrm right? what about the LM338 that will allow 5amps max? i read it on your comment above. Thanks

    • okay. last question. I did the second circuit on NI multisim but the output voltage is just 3 volts but correct ampere I used the LM338 as replacement for the MC78T12. is the circuit really outputting 13 volts or I did something wrong on the simulation? Thanks!

  26. Hi Johany, if your battery is a lead acid or SMF type then the input should be more than 300mA to 500mA, if it's a li-ion or lipo then it can be upto 2amps.

  27. Hi Johany, no change would be required, you can use the same set up, just make sure that the input current is not higher than 300mA

  28. Hey Johany, PCB designing could be difficult due to my busy schedule, but I may try to update it some other day for sure.

  29. Hi Johany,

    yes it has an automatic cut of feature

    the color of the resistor has no significance, just a mismatch.

    all are 1/4 watt rated unless specified

  30. Hi Johany, do you mean the battery specifications?

    any battery can be used here below 3 AH, so I think yours will also suit for this circuit application

  31. Thanks in advance, my battery is of laptop having rating of 10.8V 2600mAh. The load is a receiver having rating of 12V 200mA. Do I have to boost the voltage or 10.8V is enough to operate the receiver? How to go for it?

  32. HI!. sory mi english

    two question !

    I only get 1.2v to the battery.
    if I invest the TIP127 I get 14v , but I do reverse the datasheet ! : S

    It also seems that my potentiometers have no effect on the circuit. how could I check this ?.


    • Hi, did you connect the LEDs? what response do you get from the LEDs?

      you'll need a variable power supply initially for setting up the thresholds…

    • Yes, the led is connected.

      Today, the Green LED ON and the Red LED is OFF . Then i discharged de battery, now de Green LED is OFF and the Red LED is ON.
      Is working properly?


    • hello sir,
      i build this ckt 2.bp.blogspot.com/-tGgvB93ARXs/UXYc3mwTn-I/AAAAAAAAD-o/1dovbzxqK8A/s1600/micro+UPS+circuit.png
      but im facing some issues, when i switch on the ckt the green led glows and red led remains off, but when i touch the regulator body with any metal the green led becomes off and red led glows, also im not getting any response by adjusting 100k preset but when i adjust 10k preset red glow at some instant. also im not getting enough voltage at output…

    • hello muhammad, the 10k preset is for high level cutoff and the 100k preset is for low level switch ON.

      while setting the 10k preset the 100k link must be kept disconnected, once the high level cut off is set (green LED ON) the 100k link may be reconnected, then the voltage may be reduced to the lower level and the 100k preset may be adjusted for setting the low level restoration (RED ON, green OFF)

      the above should be done without any battery connected and by using a variable power supply at the input for the required high, low supply alterations.

  33. I did not understand the last part. you mean the red led will turn off and green one turns on while 100k is being adjusted? if so, the problem now is that the green one never turns on. only when I retire the main supply and battery is used.
    by the way. the green led resistor is 4.7K or 470ohms?

  34. That’s the problem… the led do not responds quickly. I have to return the preset more than 3 or 4 turns to get the shift.
    Should I have to use the diode 1N4007 in serie with 7808 output?

    • just forgot to mention that while doing the above set-up procedures the 100k preset link should be kept disconnected….and reconnected back after the setting is complete.

      once the above procedures are over the 100k preset must be used for setting up the lower restoration point, apply 5V to the circuit and adjust the 100k preset to force the relay to just cut off of switch OFF.

  35. Thanks Swagatam.

    Probably you Can help me. I am facing some issues to set the trimpots correctly. I am using a 7808 voltage regulator and a 6v/1a battery to be charged. How can I calculate the trimpots and zener values to adjust the correct voltages on pin3 and 2 from 741?


    Thank you for your attention.

    • Fransisco, while adjusting the preset do not keep the battery connected, keep the outputs open

      Now apply the input voltage and carefully adjust the preset until the red led just lights up….now flick the preset to and fro to confirm the LED responding with quick ON/OFF effect….finally set the preset keeping the LED just ON…

      however before you do the above make sure the 7808 output has one diode in series with the circuit in order to provide around 7.4V to the circuit and the above setting determines this level as the full charge level for the battery.

  36. Hi Swagatam,
    I would like to know what is the variable resistances total value? You said (for left one) preset 10 K ohm, what is the total value of this one? I mean, is the total value is 50 K ohm and we set it to 10 K ohm?

  37. Hi Swagatam,
    I would like to know if acid-lead battery can be used with this circuit, for example 2 pieces of 6v 4.5 ah batteries. My power requirement is 12v, 1ah for D-link 2750u adsl router+modem. Thanks.

  38. Hey Mr. Swagatam,
    If I want to charge a lead acid battery 1,2ah, I need to change the value of 0.4 ohm to 3 ohm resistor for 1/10h battery charge?

    • Hi Ahmad, use 7806 for the input IC and don't use the diodes with its ground pin, connect the ground pin directly with the ground, rest everything remains as is.

      set the preset for a 4.2V cut off

  39. hi Swagatam..
    1) i replaced the lm317 with lm338 and installed a biger heatsink with fan and it worked,, but the question is.. why do i have only 1.63 amp output?? since the laptop charger is 3.4amp.. why don't i have the complete 3.4amp when i charge a 12v battery?? in the other hand when i used lm317.. it gaves 2.0 amp 😐 !! how did that happen ??
    2)can i get an overcharge protection circuit but simpler than the one above… for 12v battery and for 6v battery..

    thanks again

    • i measured in series with empty battery.. it started with 0.5amp and increased to 1.63 and never increased again..
      and another questions.. since its a laptop charger.. does it have an overcharge protection feature already ?? or should i build a circuit because i modified the output to 14v ??

    • That's strange, the current consumption should be high when the battery is first connected and should gradually become less as it charges, at full full charge it should almost become zero amps…I think something may be not correct either with your circuit or the meter you are using.

      The charger will never have any protection feature, but your laptop will certainly have a built-in over charger cut off included.

  40. hi again..
    please i want to make a 12v charger from a laptop adatpter with 19v/3.5amp
    how to regulate 19v to 14 or 13.5 to make it charge the battery??
    and would it cut off if the battery is full ?? i mean how to make a simple overcharge protection circuit??
    thank you a lot

    • You can use a LM338 variable voltage regulator circuit and use it for getting the required 13.5V from the 19V input.

      after adjusting it to 13.5V you will need to operate the above circuit with this voltage for acquiring the automatic high/low cut-offs.

    • Sorry for the inconvenience
      but what is the difference between LM338 and LM317?
      bucause i tried to get 13.5 volt with LM317 but no success..
      and please what resistor should i use with LM338?
      and what resistor should i use with LM317??
      thank u again

    • thank u again Swagatam
      i used lm317 because i want the max 1.5amp and i got everything ok
      i got 13.6 v but the ic get too hot.. is that normal?? of course i installed a heat sink but it still get hot and the volt is decreasing slowly.. should i install a fan ??
      or should i use lm338 ??
      thaaaaaaaaaaank you a lot

    • you are welcome Ahmed,
      you may increase the heatsink dimensions, or as you said use LM338 which would run much cooler…

  41. hello Swagatam..
    i have 2 questions please:
    1. can i make this circuit without the charger part?? cuz i have an automated charger.. so i dont like it to be charger twice..
    2. can i use an acid lead battery with 70 amp "car battery" instead of the one you recommneded..
    thank you a lot

    • hello Ahmad, do you mean you want to replace the MC78T12 section with you charger output?? yes it can be done.

      any battery can be used here as long as the voltage specification is 12V

  42. Hi,
    my English is not so good. I have four questions to the last schema (5 Amps).
    1. If I am correct, this circuit keeps battery in safe – it prevents battery from overcharging, when battery is fully charged, it cut off from power. And when battery voltage drops down under given threshold it starts charging. Am I right?
    2. I see in your schema Li-Ion battery. Can I use for example Lead Acid battery 12V/1Ah?
    3. Battery voltage should not drop under given threshold for avoiding damaging. This circuit does not cut off battery when it happens, right? RED led will lit instead, right? If I put to RED led some kind of relay or one or two MOSFETs to cut off battery, it could work?
    4. Can you confirm me that RED led will lit only when battery voltage is low under safe limit and GREEN led will lit only when battery is fully charged?

    Thank you!

    • Hi
      1) yes that's right.
      2)All types of rechargeable battery can be used here for the required actions.
      3) yes that's correct
      4) yes that's correct too!

    • Hi Swagatam
      This is a very usefull circuit you shared with us. Very much appreciated, i have some questions in mind , If you may help me out ( i am nill in understanding circuits, but i can build one)

      Considering the The modified 5 AMP micro UPS circuit design:


      1. can you include battery protection feature in the circuit so it protects battery from over discharging and automatically cuts off the battery when its in low voltage.

      2.I want to have multiple outputs for multiple devices, like Laptop (running on this circuit with car charger) an LED light source and 12 Volt Fan {definitely using theses under power outages} so DO I ATTACH THESE DEVICES TO THE SINGLE OUTPUT ALL IN ONE(parallel), can this effect the working of each other?

      3. I want to use the Car battery or 100 Ampere battery, how fast it could charge the battery of high ampere, do it need modification to do so, if yes please modify the circuit.

      4. Please specify power input adapter (15volts) amperes?

      5. Can you share a working circuit picture, so i have help in identifying the components?

      6. Please do share the component details, coz i cant understand the diagram symbols. (kindly)

      Very thanks and regards

    • Thanks Solaris,

      I am sorry, due to lack of time I won't be able to fulfill all the mentioned requests.

      for the components you can simply write down exactly as given in the diagram, all the parts are standard, and the retailer will understand what to be given.

      I think the following circuit would be more suitable for your need since it satisfies most of the features in it.


      however please be cautioned, that these designs require a reasonably experts knowledge regarding electronics, especially of opamps, and BJTs…so if you think you are very new then it could be extremely difficult for you to succeed with these designs.

      for a 100 amp batt you will a 10 amp 14V DC input…and the output transistors will need to be replaced with TIP142 for NPN, and TIP147 for PNP

      I will recommend you to build a basic design first and modify it later on once you succeed with the initial one.

    • Hi Abu-Hafss,

      you can use a 555 boost circuit for it, I have already published a few of those in this blog, and guess you have read them all

  43. Dear, another query. I can use instead of a red LED indicator low loads a relay to make the outage charge the battery to avoid damage?. I contribute it as an idea and at the same time consultation. thank you!

    • Hi,
      Very useful circuit, thanks for sharing it, I want to ask that What is the difference between 0.4 ohm resistance and 100 K Var? 0.4 ohm resistance determines battery charge current, But i dont understand what is main function 100 K variable resistance?

    • Hi thanks, the 100k pot sets the low voltage restoration threshold for the battery…it decides when the charging needs to be restored for the battery

  44. i have only battery and solar unit.how can i get it charged and low battery cutoff…and also how to stop the over charging????

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