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Op amp Battery Charger Circuit with Auto Cut Off

Op amp Battery Charger Circuit with Auto Cut Off

The post discusses a two opamp IC 741 and LM358 based auto cut off battery charger circuits which are not only accurate with its features but also allows a hassle free and quick setting up of its high/low cut-off threshold limits.

The idea was requested by Mr. Mamdouh.

Circuit Objectives and Requirements

  1. As soon as I connect the external power automatically it will disconnect the battery and supply the system, in the meanwhile charging the battery.
  2. Overcharging protection ( which included in the above design).
  3. Battery low and full charging indications (which included in the above design).
  4. Also i don't know what is the formula to help how to determine the voltage required across my battery to charge it with( battery will be extracted of old laptops.total will be 22V with 6 apms at no load)
  5. Furthermore, I don't know the formula to indicate how long my battery will last, and how to calculate the time if i want a battery to last me two hours.
  6. Also, the cpu fan will supplied by the system too. It would be great too to add the option of a dimmer, my original plan was to vary between 26-30 v not need much more than that.

Circuit Diagram

The Design

In all of my previous battery charger controller circuits I have used a single opamp for executing the full charge auto cut-off, and have employed a hysteresis resistor for enabling the low level charging switch ON for the connected battery.

However calculating this hysteresis resistor correctly for achieving the precise low level restoration becomes slightly difficult and requires some trial and error effort which can be time consuming.

In the above proposed opamp low high battery charger controller circuit two opamp comparator are incorporated instead of one which simplifies the set up procedures and relieves the user from the long procedures.

Referring to the figure we can see two opamps configured as comparators for sensing the battery voltage and for the required cut-off operations.

Assuming the battery is s 12V battery, the lower A2 opamp's 10K preset is set such that its output pin#7 becomes high logic when the battery voltage just crosses the 11V mark (lower discharge threshold), while the upper A1 opamp's preset is adjusted such that its output goes high when the battery voltage touches the higher cut off threshold, say at 14.3V.

Therefore at 11V, the A1 output gets positive but due to the presence of the 1N4148 diode this positive stays ineffective and blocked from moving further to the base of the transistor.

The battery continues to charge, until it reaches 14.3V when the upper opamp activates the relay, and stops the charging supply to the battery.

The situation is instantly latched due to the inclusion of the feedback resistors across pin#1 and pin#3 of A1. The relay becomes locked in this position with the supply completely cut off for the battery.

The battery now begins slowly discharging via the connected load until it reaches its lower discharge threshold level at 11V when the A2 output is forced to go negative or zero. Now the diode at its output becomes forward biased and quickly breaks the latch by grounding the latching feedback signal between the indicated pins of A1.

With this action the relay is instantly deactivated and restored to its initial N/C position and the charging current yet again begins flowing towards the battery.

This opamp low high battery charger circuit can be used as a DC UPS circuit also for ensuring a continuous supply for the load regardless of the mains presence or absence and for getting an uninterrupted supply through out its usage.

The input charging supply could be acquired from a regulated power supply such as an LM338 constant current variable constant voltage circuit externally.

Answers for other additional questions in the request are as given under:

Formula for calculating full charge cut off limit is:

Battery voltage rating + 20%, for example 20% of 12V is 2.4, so 12 + 2.4 = 14.4V is the full charge cut off voltage for a 12V battery

To know the battery back up time the following formula can be used, which gives you the approximate battery back up time.

Backup = 0.7 (Ah / Load Current)

Another alternative design for making an automatic over/under charge cut-off battery charger circuit using two op amps, can be seen below:

How it Works

Assuimg there's no battery connected, the relay contact is at N/C position. Thefeotre when power is switched ON, the op amp circuit is unable to get powered and stays inactive.

Now, suppose a discharged battery is connected across the indicated point, the op amp circuit gets powreed through the battery. Since the battery is at a discharged level, it create s a low potential at (-) input of the upper op amp, which may be less than the (+) pin.

Due to this, the upper op amp output goes high. The transistor and the relay activates, and the relay contact moves from N/C to N/O. This now connects the battery with the input power supply, and it begins charging.

Once the battery is fully charged, the potential at (-) pin of the upper op amp becomes higher than its (+) input, caysung the output pin of the upper op amp to go low. This instantly switches OFF the transistor and the relay.

The battery is now disconnected from the charging supply.

The 1N4148 diode across the (+) and the output of the upper op amp latches so that even if the battery begins dropping it has no effect on the relay conition.

However, suppose is not removed from the terminals and a load is connected to it so that it begins discharging.

When the battery discharges below the desired lower level, the potential at pin (-) of the lower op amp goes lower than its (+) input pin. This instantly causes the output of the lower op amp to go high, which is carried to the base of the transistor. The breaks the latch, and switches ON the transistor and the relay to initiate the charging process yet again.

PCB Design

opamp high low battery charger PCB design

Adding a Current Control Stage

The above two designs can be upgraded with a current control by adding a MOSFET based current control module, as shown below:

Adding a Reverse Polarity Protector

A reverse polarity protection can be added to the above designs by adding a diode in series with the negative terminal of the battery, as indicated below:


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!

144 thoughts on “Op amp Battery Charger Circuit with Auto Cut Off”

  1. Hello sir. Actually i wanted to refer the first circuit (load is connected to the supply) other than the modified one. The things i wanted to know is:-
    1. What must be the range of input voltage and current for the safe working of the circuit.
    2. What are the specifications of the relay to be used. Is it dependent on the type of battery to be charged. If yes, then what are it’s specifications.

    Please do help sir.

    • Hello Ashish,

      The IC can tolerate a maximum of 30V, so the input supply should not be increased above this value.
      The relay coil voltage can be as per the battery voltage, if it’s 12V then the relay should be also 12V. The relay contact current rating should 2 times more than the charging current rate of the battery. Charging current should be ideally 10 times lower than the Ah value of the battery for a lead acid battery

      • Thank you so much sir for replying. It means a lot.

        What are the changes i need to do to increase the max input voltage(i.e 30 V)

        So as said above, i need to change the relay every time i need to charge a different battery. Or is there anything that can be done so that i can charge various batteries with one particular relay.

        • Hi Ashish, For different input voltages you can feed the opamp supply pin and the relay coil positive side pin with a 12V fixed voltage through a 7812 IC. Then you can use any voltage from 3 to 30V or higher input and charge different batteries accordingly.

  2. i did not set low voltage and high voltage. my english is not good. i am using translate.
    what (pin 3) and 5 voltage shold be? please help me. thanks.

    • You will need a variable power supply for setting up the opamp presets, please see the discussion with Mr. Engr bashir, I have explained how the presets should be adjusted by applying the high and low voltages. Unfortunately there’s no other method which can be used correctly

  3. Sir,
    Can I use the circuit in 300 watt invert er circuit. and can Tr.bc 547 be used in substitute to Tr.2n2222
    Thanking youl sir.

  4. ok sir, which one i suppose to set as my full charge cutoff 16v or 15.5. ???
    i thought that the battery can be damage it i set it at 16v

    • 15.5V is not correct, you can use 16V, but at 16V you won’t require any cut off, cut off will be required only when you set it at the max 16.8V.

      A single 3.7V requires 4.2V to get fully charged, at 4V also it is quite good and does not require any cut-off

      therefore if you set at 16V that corresponds to 4V for each cell, which means cut off may not be necessary at all.

  5. 1). 14.8V sometime 15.5v,
    2).36000mAh lithium battery constructed by me. using 3.7v lithium battery. the cell connection is 16p4s arrangement =14.80 36000mAh
    3). 16.5v input charger

    • if you have connected 4nos of 3.7V Li-Ion cells in series then the correct full charge level of this battery should be 16.8V and not 15.5V.
      14.8V will be the low discharged voltage level for the battery.

  6. another observation Sir,
    (1) that my charger that i made yesterday it cut off at 14.89v instant of 15.5v. but my battery is 14.8v but sometimes it reached 15.5v when it full lithium battery.

    • Engr Bashir, I wanted to know the specification of the battery, because I have never seen a battery with 15.5V full charge requirement, so please specify the printed value of the battery so that i can help you with better suggestions.

      A preset setting can produce some inaccuracy unless you use a multi-turn type of preset, so slight variation may be there. You can try replacing it with fixed resistors…

  7. yes sir i hv done it as you said but the problem that i am experienceing is that when i set the low battery at 12v and the full charge at 15.5v when i plug charge to test. it just cut off at 14.80v instant 15.5v.
    and when there is no any load on it the relay it just start vibrating and when i connect battery on it , it will stop vibrating and cutt off at 14.80

    • Hi Bashir, Yes the relay may vibrate if battery is not connected, but the problem can be easily solved simply by swapping the N/C contact with N/O. Please see the updated first diagram, I have exchanged the N/C, N/O positions and now the charger should work without any problems.

      Please check it and let me know.

  8. Sir i did the first circuit above according to your instructions. but the problem is that, the reply is vibrating but my specifications is as following:-
    -Relay 12v 10A
    -my Battery voltage is 15v to 15.5v
    -Input charger is 16.5v
    -And i want my low level voltage to be 12v and cutoff full charge to be 15.5v.
    please what is the problem???

    • Hi Bashir, did you set the presets correctly? for your case the upper preset must be set to switch ON the relay at 15.5V, and the lower preset must be set to switch OFF the relay at 12V.

      Do it in this way:

      1) Initially keep both the presets sliders to ground level.
      2) Now feed a 15.5V from a variable supply to the circuit (from the battery side), and adjust the upper preset such that the relay just switches ON.
      3) Now slowly reduce the supply to 12V, and adjust the lower preset until the relay is just switched OFF….that’s all

      your circuit is all set now.

  9. Helo sir the circuit am seen here is all about circuit that measures input voltage.
    I want or am taking about the variable DC voltage source that I can adjust to a particular DC voltage of my choice to be able to test my circuit…

  10. Please sir ,I want to build a variable voltage DC voltage source to be powering my circuit for testing.
    Pls help me get the circuit with LCD.

  11. Pls sir ,how can i use or connect this circuit to my solar panels to be able to use it as solar charge controller?
    2) Whatbis the load that is beside the battery in the circuit and what is the meaning of the 2 diode(6A4*2nos) connected to the Load?

  12. Pls sir ,how can i use or connect this circuit to my solar panels to be able to use it as solar charge controller?

    • If you are using the first design, simply apply the solar voltage from the right side, that’s all. Make sure to use a 7812 IC for pin#8 of the opamp if the solar panel voltage is above 30V

  13. Hello sir Swagatam,
    Still on the second schematic.
    1. Please what value of bridge rectifier diodes can I use that can handle any charging current? And
    2. Supposing that I want to charge batteries in parallel, say for example, two 200Ah batteries. Do I have to upgrade the transfo to 40A transfo?

  14. Hello sir Swagatam. I can see that you’ve undated the second schematic. I’ll like to ask that suppose i want to use circuit to charge a battery bank of 36V, 48V and above, where will the pin#8 of LM358 be connected to since from the datasheet, the maximum supply voltage is 32V?

  15. Hello sir Swagatam,
    In the second schematic, it appears that the two input symbols are witched i.e the “+” is in the position of the “-” and vice versa. Or is it correct that way? Please confirm sir. Thank you.

    • Hello Godson, the marking are correctly assigned, and should be connected in the shown manner…the 1st and the 2nd concepts work on different principles, therefore the opamps are configured differently.

  16. Good day sir Swagatam. Please I still need more info as regards the second schematic.
    1. What is the value of C1?
    2. What is the value of the base resistor of the lower transistor?
    3. What is the optimal charging voltage for 24V, 36V, 48V, 60V and 76V?
    4. How can I get a constant voltage from a transfo output because after rectification, the voltage always increases? For example, to charge 12V battery, I need about 14.5V from the transfo. How do I get this voltage from say a 12V transfo or a 15V transfo?
    Anticipating your usual prompt response. Thank you sir.

    • Hi Godson,

      C1 can be a 2200uF/80V capacitor
      you can use 10K for the lower transistor base.
      It’s 14.3V for 12V, so you can find the proportionate values through a simple cross multiplication for the other voltages also.
      the bridge and the filter capacitor will lift the 12V to a 14.3V DC coincidentally, same for the other levels, so the bridge and the filter cap will do it for you.

  17. Hello sir Swagatam,
    I asked for an automatic charger circuit and you referred me to this post. I understand that this circuit for monitoring the the charging. I was actually looking for the circuit that has both the power source and the charger. The charger should be able to charge batteries up to 200Ah and higher voltages such as 24V, 48V and higher. Please kindly help with one. Thank you sir.

    • Godson, The TR1 in the last diagram could be replaced with the charger winding output, the IC pinout should be supplied from a stepped down 12V source..and the relay replaced with a 30 amp relay.

      • Thank you very much for the prompt reply sir. I really appreciate it. So what you are saying is that as long as the transfo, the bridge rectifier and the relay contacts are correctly rated, the circuit will automatically charge any battery capacity and voltage. Is that correct sir?
        Will the zener diodes be changed when using the circuit for higher voltages?
        What is the diode symbols between the 1N4148 at the output of the opamp and the base resistors of the transistors? Are they indicator LEDs? If yes what will they indicate?

        • The charger winding must be rated at 20 amps for your 200 AH battery, if we assume this is correctly placed rest will be automatically handled by the charger circuit.

          the zener diode is not critical, you can use a value anything between 3 and 9V.

          the red/green symbols are the LeDs, in the last design, the upper red LeD will indicate the upper battery limit reached, similarly the lower red LeD will illuminate as long as the level is over the lower threshold.

  18. Dear sir! I want to modify this circuit for 6v 4.5AH sealed lead acid battery. I need to start charging at 5.8 volt and cutoff at 7.1 volt. Please help me.

    • Dear Arul, you just have to follow the preset adjustment steps as explained in the article, and set them as per your mentioned specs

  19. Thank you Mr swagatam…. it worked perfectly.

    Sir pls can you shear a link for a converter that I can use to double my panels voltage…..

    Also a step down converter that convert 24v downward…

    Thank you in advance

  20. Nothings happening while adjusting the lower pot…. while adjusting upper pot relay is vibrating…. I thought I have made mistakes in the connection same happen after I made another…. pls help…. It has cost me lot of money

    • if you do it without understanding then it will be impossible for you to succeed.

      the circuit is very easy to set up if it's done after proper understanding.

      so please first learn and then do the adjustments.

      first apply the lower threshold voltage, and then adjust the lower preset until the red LED lights up.
      after that keep increasing the voltage until the higher threshold level is reached, and now adjust the preset such that the relay just clicks.

      before doing all these keep the presets at the ground level, meaning shift slider to ground level , and then slowly adjust upwards for the setting up as explained above.

  21. .Sorry it lm338 regulator to drop solar voltage to 16v …. my solar reads 20v at max sunlight
    Thank you….

    • Am sorry to say am not sure if these will do what I want… my batteries are not gell. They are lipo, that requires care.. these circuit you refereed me to is extremely off what I want sir. Electronic component r expensive In my city… please I belief these above circuit should do wat I want…..pls help me I have made 2 of the above circuit, it has same outcome…

      Pls carefully attend to this sir.. I have no mistake in my wiring…

      After I finished the circuit. I wanted setting it up . I have two power adaptor one 12v and 16v respectively.

      I followed the way u thought me to setup the circuit by first disconnecting 10k via pin 3. And feed in the cutoff voltage without the battery connected, and adjusting the lower 10k pot until the the red light illuminate…

      I did just these nothing happened both the r and g light are on…

      When I fed 16v and adjust the upper pot. The relay started vibrating…..

      Please Mr swagatam these circuit is my last hope… help me please

    • Abdulakeem, just following me will not help…you will have to grasp the functioning of the circuit only then you will be able set it up correctly even without my help.

      if your cell is LIPO then this circuit may not be suitable for it, because Lipo requires a balanced charger….but anyway the cut off setting up procedure is not relevant to the type of battery…because the setting up needs to be done using a variable power supply first.

      you can also refer to the following article for more info regarding opamps in battery chargers.


  22. Hello thank you… please give me a link to the lm358 regulator circuit…

    Also about the above opamp circuit. I finally finished the circuit… I had lot of problem I would like u to attend to..

    First I couldn't find 6A4 diode… I used 6A1 I start up the circuit with 12.6v at input the relay was vibrating until I removed and connected the diode directly.( Question ) was the 6A1 not equal to the 6A4?..

    AGAIN I tink I still have problems setting up the circuit when I feed in 12.6 there are two led, the two leds lights up and nothing happens even wen I adjusted the upper ND lower pot?… (question) if I input the high cut off example 12.6v, and I adjust the upper pot is the red led soppose to light alone or the green?

    If I have a battery higher than 12v can I still use 12v relay or I have to change to a higher one.

    Also sir my upper op amp is gets hot and pot was blown.

    Sir. Make a layman discription of setting up this circuit

    Help sir….. thank you

    • Abdulakeem, you must first disconnect the feedback 10k resistor connected at pin3 of the upper opamp in the fist circuit.

      after that adjust the lower preset to illuminate it's red LEd at just over the selected lower threshold value…and similarly adjust the upper preset to cause the relay to activate at the selected upper threshold voltage. once this is done you may reconnect the 10k feedback…that's done…. it's so simple.now you can connect the battery for the charging and the cut off

      I have already explained the procedure in the article.

      sorry which LM358 regulator are you looking for??

  23. Pls sir, I have a solar panel that reads 20v at max sunlight. Can I connect it directly to the circuit?. Am a Lil confused because I set the circuit with 12.6v

    • Abdulakeem, you can use an LM338 based regulator circuit to drop the voltage to 15V and then use it with the above opamp based circuit.

  24. Sorry Mr swagatam, am a lil confused on the transistor in the circuit, is it 2n2222?
    Again setting up the circuit my battery won't exceed 12.3v they are lion packs… sir pls how do I set it up? Am sorry am just a hobbyist I might not understand some things u expected me get

    • yes the transistor is 2N2222 or you can try any other similar.
      you must set up the cut off thresholds without connecting any battery initially, please confirm the cut off operations by manually adjusting the input supply voltages….once confirmed then you can test it with an actual battery.
      I am not sure what is the higher cut off threshold for your Li-ion battery…normally for a 3.7V Li-ion cell its 4.2V

    • By the way did you connect the LEDs which are indicated in the diagram?….make sure to connect them so that you can get the relevant indications while adjusting the circuit

  25. Thank you very much for introducing me to this circuit, this is exactly what I am looking for. Sir my questions

    What's the value of the transistor? Is it a normal 2n2222 or it a n channel mosfet…

    I quote ur reply: *Hi mars, remove the 1N4148 anode from the hysteresis resistor and connect it directly to pin#3 of the upper opamp and check the response…* are you saying the 10k resistor is not needed?

    Again can the relay be just the normal 12v type?

    Also the major problem I always have is setting up the circuit… pls sir help detail setting up the circuit

    • Thank you abdul,

      2N2222is a BJT not a mosfet

      the referred 1N4148 can be seen at the output of the lower opamp, this 1N4148 diode's anode could be directly connected to pin#3 of the upper opamp

      the relay can be any 12V, but the contact current must be capable of handling the the charging current

      set the lower preset such that the lower opamp output just becomes high when the voltage exceeds this lower limit….similarly set the upper preset such that the upper opamp output just goes high when the voltage exceeds this upper limit

  26. Sir I have i have connected the variable power supply and I also got the concept u said but there is a problem I m able to set lower op amp to 11v but not able to set upper limit because as I set the voltage 14.4 the op amp switching at 10v only please help me

    • Sameer, I did not quite understand what you meant by

      "because as I set the voltage 14.4 the op amp switching at 10v"

      anyway you can try the following…remove all connections associated with pin#1 and pin#7 except the LED assembly connections.

      repeat the LED assembly connections for the upper opamp output pin also.

      Now repeat the setting up procedure as explained by me earlier.

      If you understand the concept correctly then you will have any difficulty in setting it up

      alternatively you can just confirm the following:

      ensure that at 11V pin#5 voltage is just higher than pin#6

      similarly just ensure that at 14.4V pin#3 voltage is just above pin#2 voltage
      adjust these by rotating the respective presets.
      Once the above is done rest will automatically fall in place.

  27. sir thank u for reply ,i have rebuilt the circuit but one more problem …
    i have connected the 12v battery as it is new its showing me 12.90v but i am not able to set the lower limit because zener diode is of only 4.7 and at pin 2 and 6 i am getting 3.7v so as soon as i turn the pot opamp switches to +vcc .so please help me with this do i have to change the zener to 13 or14v

    • Sameer, please read the article properly for understanding the setting up procedure, the zener diode does not need any modification.

      you will need a variable power supply for the procedures:

      feed 11V first and adjust the lower preset such that the lower opamp output becomes just high.

      next increase the voltage to 14.4V and adjust the upper preset such that the output of the upper opamp just becomes high….that's all

      once this is done the cut-offs will automatically function for the connected battery.

      while setting up the presets do not connect any battery

  28. no sir i have connected in proper way but as i try to increase voltage ref pot is getting burnt
    sir i have designed other circuit how can i send u its picture or image.

    • Sameer, if you have connected the center lead of the preset with the (+) pin of the opamp and still the preset is burning then your opamp may be faulty or short circuited….replace it with a new one and check again

    • that means you have connected the pot in the wrong….the center lead must go to the opamp, the other outer leads must go to the positive and negative supply lines.

  29. hi swagatam

    just wondering why you always use opamps for the chargers and not a proper comparator like the LM 311 or the LM 393 and what is the purpose of the 1K resister on pin 1 is the 10K not enough for the histeresis

    • Hi Mars,

      It's merely because opamps like 741 and LM324 are more popular and easily available than LM311 or LM339.

      as you increase the resistor value the hysteresis will decrease and vice versa, here we want the hysteresis to be towards maximum or infinity that's why we have selected the least resistance….

  30. how would that work when most batteries are full at 13.8v and can not even reach 14.4v for cut off and completely discharged at 11.9v and going down below 50% which is 12.2v will damage the battery

    • all good batteries will reach the 14.4V mark, if not then the problem could be with the battery, and in that case the upper regulation would become meaningless since the battery itself is unresponsive above 13.8V.

      The lowest discharge level is 10V but ideally it should not be below 11V.

      at 11.9V the battery can be considered healthy with its charge capacity

  31. hi swagatam
    after playing around with these battery chargers for a while i find that these opamp based chargers that checks the voltage and not the current moves from 12.6 to 14.4 very quickly and the opamp switches on and off but it is not really charging the battery as it only checks the volts
    do you see the same and is there a remedy for better charging

    • Mars, opamps are not responsible for charging the battery, these are just sensors that are supposed to sense the battery voltage at the specified levels and initiate the cut-offs. The actual charging is done by the power supply source whose voltage and current must be correctly rated…for lead acid battery this must be set at 14.4V and current at 1/10th of batt AH.

      With the above rate the battery should take 10 to 14 hours to get fully charged….if this not happening then either the power supply could be malfunctioning or the battery could be faulty.

      The opamp based chargers are more than perfect with their jobs….voltage detection is the right way to go for making automatic battery chargers.

  32. If i use two 12V 7Ah battery in parallel, can i use 12 volt 5A smps to charge the battery and do i want make changes in circuit

    • 5 amp can be a little high, it must not be higher than 3 amp…you can add a 24V automotive lamp in series or use a 5 ohm 5 watt resistor in series.

      the smps voltage must be 14.5V…12V will not do…no other changes would be required

  33. also, this circuit will work with 24V battery ?

    and if so, i'm little confused about the calculation of the 10K variable resisters
    let say my output source is 24V what is my pre-set for the variables 10K ?

    also the output to the load need to be constant at 3Amp with 24V
    would i be able to keep it at 3Amp with 24V at the output, and what kind of voltage at the external power do i need?

    • thanks for responding,

      i was referring to the 10K resister that is connected to pin 3/5 on A1/A2
      those resistors suppose to have 3 pins? if so are they variable resistors and im just gonna keep them at 10K for the sake of having 3 pins resistors.
      sorry it might be a stupid question, but im trying to understand as well as learn.

      thank you.

    • thanks for responding,

      i was referring to the 10K resister that is connected to pin 3/5 on A1/A2
      those resistors suppose to have 3 pins? if so are they variable resistors and im just gonna keep them at 10K for the sake of having 3 pins resistors.
      sorry it might be a stupid question, but im trying to understand as well as learn.

      thank you.

    • those are presets or trimpots, they are variable resistors, their values are suggested to be 10K, meaning the resistance is variable from 0 to 10k

  34. hi MR. Swagatam,

    Thank you very much for the post and the circuit is great. I have a question, is it possible to switch the SPDT relay with a digital switch, as relays aren't really reliable?

    thank you.

    • Hi Mamdouh,
      relays are very reliable devices, but if you don't refer a relay you can use a mosfet, as explained in the above comment.

  35. Hi Swagatam,

    Thanks for nice Circuits. I need circuit idea to lighting the EXIT sign board with below requirement. Will you be able to help me ?

    1) When AC main is there light should lit from AC->DC transformer same time it should charge 6V/12V battery and shut charging automatically when battery is full.

    2) When AC mains is not there then EXIT sign light should lit using battery. Again load (light) should be disconnected when low battery.

    Your circuit solves this problem. In this circuit relay is used, i think relay will consume battery current so i want MOSFET and Transistor to be used instead relay. I will use MOSFET or transistor which is cheaper and available with me so I am asking MOSFET as well as transistor.
    Hope there is not much changes to be done to circuit.

    3) In relay / MOSFET / Transistor which is less current consuming ?

    Thanks for great, simple and effective circuits.

    • Hi Raghavendra, yes the above circuit will be quite suitable for your application.

      A relay will consume some current but it will be negligible in comparison to the load current, but if you don't want this you can remove the relay and replace the BC547 transistor with a P channel mosfet and connect the battery across its drain and the negative line of the circuit.

      the load may be connected via another mosfet (N channel), whose gate may be connected with the output of the lower opamp, source with the negative line and the load across its drain and the powitive line of the battery

      mosfet and BJT both will consume no idle current, only a relay will consume some current while it's in the triggered mode.

    • Hi Swagatam, would you please clarifying more about replacing the relay and BC547. the BC547 your referring to in the circuit is the 2N222 if im not mistaken?

      Also as P or N mosfet Chanel they both has, Drain, Gate, and Source.

      I'm confused with both on how to connected with the above circuit.

      Thank you.

    • yes that's correct, I am actually referring to the 2N2222.

      the mosfet connections are explained in the previous comment…sketch the diagram according to the instructions and you will be able to get the idea

  36. after testing the circuit i find the zeners to be losing about 1v as it is only getting about 1 ma with the 10k resisters
    when adjusting the 10k resistors for about 10ma it is much more stable and is almost working at the zener voltage

    • the zener is for creating a fix reference level for the opamps, and 10K will not affect this functionality in any manner….

    • No, that shouldn't happen, opamps are extremely straightforward components, and moreover the preset adjustments are wide apart so instability cannot happen…connect LEDs across the opamp outputs to check the responses during their switching

  37. hi swagatam
    when testing your circuit we find setting A2 does not set the bottom limit for switching the relay but only for the led

    • Hi mars, remove the 1N4148 anode from the hysteresis resistor and connect it directly to pin#3 of the upper opamp and check the response…

  38. Hi Swagatam

    Interesting circuit. However, I think the value of the feedback resistor should be 100K instead of 10K. What do you say?
    BTW, have you tested this circuit?

    • Hi Abu Hafss, the low value resistor is purposely selected to hard latch the upper opamp when it detects the full charge threshold….the delatching operation of this opamp is done by the lower opamp output when the battery reaches the lower threshold.

  39. Please clear i can understand "the 4V constant can be achieved using a emitter follower voltage regulator buffer"

  40. Sorry dear i am so excited from your blog dear I have one more problem
    I have required a boost controller of gprs data logger which have 1A load at the time of network searching or data transmission for 10ms than normal on 50ma sleepmode current is 600uma
    In put 2 to 3.6 volt from lithium primary call with 250ma output

    Output constant 4v 1A
    Due to long life period of battery it must to self cunsuption of circuit is lower qucent current
    I founded a circuit of LTC3125 but this ic is not Easley available
    So that suggest any easy circuit according to availability of spare and lower self consumption
    Awaiting your response friend
    If you like send me your whatsapp no
    Gajendra 9414134388

  41. can the input supply be a solar panel? is this a monitor for batteries or a charger with battery monitor?

    • Hello sir Swagatam Majumdar
      I am a big fan of your circuits and find them very helpful. I have been viewing all of your circuits for long long time now.
      How ever Please can you help me on this circuit. I have built it and connected everything correctly! how ever, You did not mention how to set up the cut off limits. I need to know how to set the turn on voltage for charging and the cut off voltage for when it is full. how do you adjust the 10K preset pots to your desired voltage values..??

      kind Regards Darren

    • Thank you Darren, I appreciate your enthusiasm.

      for the above circuit you just have to set the lower preset to get a high at the output of the lower voltage, this must be done for the lower voltage threshold of the battery.

      Similarly the upper preset should be adjusted to get a high at the output of the upper opamp, and this must be done for the higher threshold or the full charge threshold of the battery.

      Once the above adjustments are done , the circuit can e expected to work automatically and produce the proposed cut off functions.

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