Need help with Low voltage protection circuit

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  • flinx
    Senior Member
    • Aug 2010
    • 132
    • USA

    #1

    Need help with Low voltage protection circuit

    I am trying to design a circuit based on this circuit:


    I substituted an IRF5305PBF for the Si4425BDY because I wanted a TO-220 package I think I misunderstood the datasheet as I am not sure what voltage turns on the Mosfet.

    I am using this TL431ACLPR

    I then tried to find a calculator to determine the resistor values for R1 and R2. The resistor values I tried did not work the way I expected.

    The source for the original circuit describes it turning off the mosfet at 18.2v and back on at about 19. No matter what combination of resistors I tried (based on the calculators I found) did that.

    How would I make the circuit work using the mosfet and TL431 I have?
    If not possible with the mosfet what one do I need?
    This device would run off of a 5 cell 18650 battery in series.
    "...off the record, unnamed government sources
    alluded to unsubstantiated innuendos about
    alleged indiscretions and insinuated that they
    are rumored not to be without basis for further
    speculation..."
  • budm
    Badcaps Legend
    • Feb 2010
    • 40746
    • USA

    #2
    Re: Need help with Low voltage protection circuit

    Can you give the link to the original circuit?
    Never stop learning
    Basic LCD TV and Monitor troubleshooting guides.
    http://www.badcaps.net/forum/showthr...956#post305956

    Voltage Regulator (LDO) testing:
    http://www.badcaps.net/forum/showthr...999#post300999

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    http://www.badcaps.net/forum/showthr...er+testing+cfl

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    TV Factory reset codes listing:
    http://www.badcaps.net/forum/showthread.php?t=24809

    Comment

    • flinx
      Senior Member
      • Aug 2010
      • 132
      • USA

      #3
      Re: Need help with Low voltage protection circuit

      Originally posted by budm
      Can you give the link to the original circuit?

      That is the original circuit.
      "...off the record, unnamed government sources
      alluded to unsubstantiated innuendos about
      alleged indiscretions and insinuated that they
      are rumored not to be without basis for further
      speculation..."

      Comment

      • redwire
        Badcaps Legend
        • Dec 2010
        • 3902
        • Canada

        #4
        Re: Need help with Low voltage protection circuit

        I see that circuit all over the web, it's another blogger special.

        The TL431 always draws some current, it needs to power itself. Usually 0.4-1mA minimum, unless you go to low current TLV431.
        So there will always be a voltage drop across R3 4.7k resistor which may be enough to turn on the mosfet a little. You need to add an extra transistor, or changing to N-ch mosfet switching the GND side.
        Note the mosfet max VGS is 20VDC so a protective zener and resistor are needed to protect it.
        This kind of circuit always incorporates a latch, so it does not chatter on and off because disconnecting load always lets battery voltage go back up, LVD turns off and connects load, voltage sags, LVD trips and voltage goes back up... it oscillates which is not good for some loads to have power on/off like that.

        The second circuit with latch taken from http://ka7oei.blogspot.com/2016/08/a...r-12-volt.html
        Attached Files

        Comment

        • stj
          Great Sage 齊天大聖
          • Dec 2009
          • 30965
          • Albion

          #5
          Re: Need help with Low voltage protection circuit

          i cant see how that original schematic could ever work,
          R5 is going to effect your voltage reference, and it will get power from both the load via the load resistance, and the body-diode in the fet

          Comment

          • flinx
            Senior Member
            • Aug 2010
            • 132
            • USA

            #6
            Re: Need help with Low voltage protection circuit

            Lets try a different line of inquiry.

            I need a device like this:
            https://www.ebay.com/itm/16V-Battery...53.m1438.l2649

            But I need it to work with a maximum voltage of 22vdc and then cuts off at 18vdc, hysteresis would be nice but is not absolutely necessary.

            It needs to handle 15 to 20 amps.

            as small as practical.

            It does not need to balance batteries or charge anything.

            It should not cost much more than the above device.

            It would be used in 18v tools and devices that do not have built in battery protection.

            If such a device does not exist then I just need a working circuit (using through hole components) and I will handle the rest.
            Last edited by flinx; 08-15-2020, 04:08 PM.
            "...off the record, unnamed government sources
            alluded to unsubstantiated innuendos about
            alleged indiscretions and insinuated that they
            are rumored not to be without basis for further
            speculation..."

            Comment

            • petehall347
              Badcaps Legend
              • Jan 2015
              • 4426
              • United Kingdom

              #7
              Re: Need help with Low voltage protection circuit

              http://www.diy-electronic-projects.c...ith-Time-Delay
              adjust components to suit current and voltage

              Comment

              • eccerr0r
                Solder Sloth
                • Nov 2012
                • 8689
                • USA

                #8
                Re: Need help with Low voltage protection circuit

                Main problem you have is that the turn on voltage for the MOSFET is dependent on your load. If you want sharp on/off you will need a higher gain than the circuit you have.

                After messing with how to design battery protection, I ended up deciding that you'll end up having to go with the custom ICs to control your FET anyway, and yes they will be SMT. If you don't, you'll be burning a significant amount of power in your nice through hole design, so much so that you'll kill your batteries anyway if you don't choose custom battery protection (like that "U1" component in the example) or more expensive ultra low power/noise amplifiers.

                Comment

                • sam_sam_sam
                  Badcaps Legend
                  • Jul 2011
                  • 6030
                  • USA

                  #9
                  Re: Need help with Low voltage protection circuit

                  Originally posted by flinx
                  This device would run off of a 5 cell 18650 battery in series.
                  What are trying to power with this battery pack

                  Depending on what your power requirements are you might not have to reinvent the wheel
                  If you are just wanting to charge the battery pack
                  Attached Files
                  Last edited by sam_sam_sam; 08-16-2020, 12:23 PM.

                  Comment

                  • flinx
                    Senior Member
                    • Aug 2010
                    • 132
                    • USA

                    #10
                    Re: Need help with Low voltage protection circuit

                    Originally posted by sam_sam_sam
                    What are trying to power with this battery pack

                    Depending on what your power requirements are you might not have to reinvent the wheel
                    anything that require 18 to 21 volts in the category of power tools. but it could also provide power through a buck convert to whatever I might want.

                    the annoying thing is that there are many devices that do this but they are either not the correct voltage range or cost over $100
                    "...off the record, unnamed government sources
                    alluded to unsubstantiated innuendos about
                    alleged indiscretions and insinuated that they
                    are rumored not to be without basis for further
                    speculation..."

                    Comment

                    • sam_sam_sam
                      Badcaps Legend
                      • Jul 2011
                      • 6030
                      • USA

                      #11
                      Re: Need help with Low voltage protection circuit

                      Are you looking to charge the battery batteries or something else

                      Comment

                      • sam_sam_sam
                        Badcaps Legend
                        • Jul 2011
                        • 6030
                        • USA

                        #12
                        Re: Need help with Low voltage protection circuit

                        There might be other solution to this problem
                        Last edited by sam_sam_sam; 08-16-2020, 12:27 PM.

                        Comment

                        • flinx
                          Senior Member
                          • Aug 2010
                          • 132
                          • USA

                          #13
                          Re: Need help with Low voltage protection circuit

                          Originally posted by sam_sam_sam
                          Are you looking to charge the battery batteries or something else
                          No the device would just be there to add over discharge protection when used on devices or with batteries that do not have that.
                          "...off the record, unnamed government sources
                          alluded to unsubstantiated innuendos about
                          alleged indiscretions and insinuated that they
                          are rumored not to be without basis for further
                          speculation..."

                          Comment

                          • sam_sam_sam
                            Badcaps Legend
                            • Jul 2011
                            • 6030
                            • USA

                            #14
                            Re: Need help with Low voltage protection circuit

                            Take a look at this device

                            https://www.ebay.com/itm/Battery-Mod....c100276.m3476

                            Comment

                            • flinx
                              Senior Member
                              • Aug 2010
                              • 132
                              • USA

                              #15
                              Re: Need help with Low voltage protection circuit

                              Originally posted by sam_sam_sam
                              I have tried some of those devices, the problems with them are that they are too large to fit inside of tools and devices, they have displays that draw power and cannot be turned off. That one is meant for controlling the charging of a battery not running a device and then cutting off power.

                              I have some voltage alarms that might be hackable to do what I want with the addition of a relay. I was hoping to use a mosfet to save space. My limited knowledge of them led me to believe they were an instant on/off type O' thing, but I have since learned that is not the case.

                              I have spent 2+ weeks searching for anything and everything and using any combination of words regarding battery protection, under voltage, low voltage, cut off, etc. I have found many devices that do what I want but they all are just outside of the voltage, amperage, or price range of what I need.

                              There is one company that makes exactly what I want but I'm not paying $200 for it. I was going to design my own circuit board but I held off till I could test some designs and that's when I found that most if not all of the circuits I could find do not function as the person who posted the information claims, do not work at all, or only sort of work.

                              I like to keep things as simple and as small as I can, and avoid using things like arduinos to do things that such devices would be overkill.

                              as usual I have apparently tried to do another project that has turned in to a quest for unobtainium devices.
                              "...off the record, unnamed government sources
                              alluded to unsubstantiated innuendos about
                              alleged indiscretions and insinuated that they
                              are rumored not to be without basis for further
                              speculation..."

                              Comment

                              • sam_sam_sam
                                Badcaps Legend
                                • Jul 2011
                                • 6030
                                • USA

                                #16
                                Re: Need help with Low voltage protection circuit

                                What BMS balancing protection board are you using
                                Also can you please post what you are looking for that cost $200.00

                                Comment

                                • flinx
                                  Senior Member
                                  • Aug 2010
                                  • 132
                                  • USA

                                  #17
                                  Re: Need help with Low voltage protection circuit

                                  Originally posted by sam_sam_sam
                                  What BMS balancing protection board are you using
                                  Also can you please post what you are looking for that cost $200.00
                                  I am not using any particular board. If the battery pack has one I do not know what it is, I would also be using 18650 packs that would not have a BMS.

                                  I looked in to BMS, BEC, and UBEC options none of them could handle the amps needed or were the wrong voltages, and I do not want to use the device to charge the batteries, nor do I want to build battery packs.

                                  this is a listing for the board in question there are 3 options.
                                  all 3 are very pricey and the boards are covered to prevent seeing the secret inards.
                                  "...off the record, unnamed government sources
                                  alluded to unsubstantiated innuendos about
                                  alleged indiscretions and insinuated that they
                                  are rumored not to be without basis for further
                                  speculation..."

                                  Comment

                                  • flinx
                                    Senior Member
                                    • Aug 2010
                                    • 132
                                    • USA

                                    #18
                                    Re: Need help with Low voltage protection circuit

                                    It is kind of bizarre there are devices used for RC racing cars that have low voltage cutoffs, they make them for 2, 3, 4, 6, 7,and 8 cell lipos, yet for some odd reason they skip 5 cell which would work perfectly for what I need.
                                    "...off the record, unnamed government sources
                                    alluded to unsubstantiated innuendos about
                                    alleged indiscretions and insinuated that they
                                    are rumored not to be without basis for further
                                    speculation..."

                                    Comment

                                    • eccerr0r
                                      Solder Sloth
                                      • Nov 2012
                                      • 8689
                                      • USA

                                      #19
                                      Re: Need help with Low voltage protection circuit

                                      So how much time do you expect the low voltage cutoff to occur and when you *actually* want to charge the pack, mind that with this cutoff design you have to bypass it to actually charge it (unless the drop/loss through the body diode is acceptable, which is not in most cases for lithium packs.) Also keep in mind if you charge and put the pack in cold storage for a week, your cutoff eats power and can still kill the battery that way too. I'd say most common off the shelf parts through hole design will end up emptying out a full pack in a month, and after it empties it, will continue to overdischarge the pack, defeating the purpose of the protection.

                                      In any case I still don't get it: most if not all lithium ion packs available out there have overcharge/discharge protection. Even my HFT cheap crap Li-ion pack. This is due for liability purposes, having packs immolate due to overdischarge is not going to be safe.

                                      Or is this a homemade pack? In this case your design is still insufficient to fully protect the pack.

                                      Also if size is a problem, through hole is not small either, especially with the number of components needed to properly do voltage protection.

                                      What *ARE* you trying to do specifically? Pictures would be good.

                                      Okay I have to make it clear of my question: What is your battery pack. What are you powering with the battery pack. How much are you planning to use the device. Are you going to be leaving the device in a drawer for a few weeks when you're not using it.
                                      Last edited by eccerr0r; 08-16-2020, 02:09 PM.

                                      Comment

                                      • flinx
                                        Senior Member
                                        • Aug 2010
                                        • 132
                                        • USA

                                        #20
                                        Re: Need help with Low voltage protection circuit

                                        this device has nothing to do with battery packs in so much as it would just protect them from being over discharged.

                                        The idea is to be able to use battery packs that do not have protection (there are lots of these) or LIPO packs which do not normally have protection.

                                        I cannot provide pictures as I do not have any device at the moment.

                                        I will give you an example:
                                        Black and decker makes a cordless vacuum cleaner called the dust buster (you may have heard of it). It runs on 16.8v (yes I know I need 18 to 21 but this is just an example).
                                        It has a 4 cell 18650 pack inside that dies after about 2 years because the amp draw on it is 11amps continuous when in use.

                                        When it dies the batteries could be replaced with better batteries OR one could install an adapter on the back for a makita or other 14.4v battery if one were to purchase the cheap chinese version they do not have battery protection in them, heck many of them do not have cell balancing.

                                        The issue is that repairing the vacuum with new batteries (of legit quality) is about the cost of just buying a new vacuum.

                                        Were one to be able to use already owned battery packs one could save a lot of money not having to buy a new vacuum cleaner every 2 years.

                                        This is only one example.

                                        I want to be able to take a device that might have bad batteries and refit that device for using other battery packs allowing the continued use of said device for longer.

                                        The problem I keep running in to is that if I did want to do this ONLY for said vacuum there are devices out there and I would not be asking on forums for help.

                                        Because I would like to do this for 18/20v devices I cannot find a reasonably priced solution. This has turned in to a quest. I generally post questions like this on forums when I have exhausted all other avenues (that I can think of) I then try forums for ideas that I may not have thought of OR sometimes I just don't know the name of the device or circuit I need.

                                        Once I get what I am looking for I post all the information, schematics, and what not on a blog or said forum and my youtube channel so others might be able to use the information and thus save themselves much money and rage in the long run.

                                        Please forgive me if my post sounds angry or whatever tone does not translate in text. I do this a lot, I eventually find or make what I want but sometimes it takes me a while to gather the required knowledge and materials.

                                        PS. the devices would not be stored with batteries attached.
                                        Last edited by flinx; 08-16-2020, 02:35 PM.
                                        "...off the record, unnamed government sources
                                        alluded to unsubstantiated innuendos about
                                        alleged indiscretions and insinuated that they
                                        are rumored not to be without basis for further
                                        speculation..."

                                        Comment

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