In this post I have explained a simple yet effective low battery cut-off with indicator circuit which can be integrated with cell phone/tablet charger battery packs in order to monitor and avoid battery draining below a set threshold. The idea was requested by Mr. David.
Technical Specifications
Hi Swagatam.
I know it was asked before, but still, if you can help me out, I'd really appreciate it. I'd like a circuit/ic for purely low voltage cut off. I have a 8-pack of AA (NiMH LSD) batteries, which should never discharge below 7.2V. I'm using these with a (car) 12V to 5V USB charger, which I'd like to use on the go.
The setup of simply using a transistor and a variable resistor in conjunction with a TIP122 transistor for handling larger currents, resulted in draining the battery further than the absolute minimum of 7.2V. I'd like this to use this with no relays (as they use too much power). I'm looking at 0.5-1.5A on the primary side (batteries) and 1-2.5A on the secondary 5V side.
For charging both the phone and the tablet. I just hate it, when a circuit like this is used in a 1$ devices. I don't want to buy of the shelf products.
Thank you really much! -David
The Design
The circuit functioning of the proposed cell phone/tablet low battery indicator with cut off can be understood as follows:
Assuming the connected battery pack to be fully charged, the potential at pin#2 is allowed to be at a higher level than at pin#3 by setting the 10k preset appropriately.
The above condition ensures a zero or logic low at the output pin#6 of the IC.
The above low output enables the connected transistor TIP127 to conduct and charge the cell phone or the tablet at its collector.
As the battery pack drains below the mentioned 7.2V mark, pin#2 voltage becomes lower than pin#3 which instantly makes the output of the IC high, switching off the transistor and the load.
The situation is indicated by the red LED which just lights up due to the low battery conditions.
At the above threshold, the output might oscillate for some time due to battery voltage trying to restore at the previous mark as soon as the load cuts off.
Although not necessary, the above outcome can be avoided by adding a resistor network: one across pin#3 and zener cathode, and another across pin#6 and pin#3, the values can be anywhere between 10K and 100K.
Circuit Diagram
How to Set up the Circuit:
It's simple, apply the desired low voltage threshold to the circuit and adjust the preset until the LED just illuminates brightly.
As mentioned earlier, the possible oscillation at the thresholds can be prevented by adding some hysteresis to the above design, it may be done by the following two methods, the second option appears to be more logical and clean.
While setting the preset, make sure the feedback link stays (pin6 to pin3) disconnected, you can connect it back once the preset adjustment is complete.
Note: Please add a 3V zener diode in series with pin#6 of the opamp, (anode to pin#6), in order to counter the off-set voltage problem.
Comments
The above circuit works. I made a mistake with the wrong interpretation of the transistors collector and emitter. So far I can suggest the above diagram to be updated to use 33k ohm resistor instead of the 4k7. This produces 0V when it is cut off instead of 2.45V when 4k7 resistor is used. I also used a 47k variable resistor because I had it before. Thank you Swagatam, I know now, it's a very simple circuit , which I will probably use more than once in my life.
interesting updates David, i am sure the folks are also enjoying it.
Hi, once again I made a prediction about circuits being simple. I was very, very wrong. I wrote that post, when I saw that my phone is being charged, when infact it was discharging. Battery was at 10.xxV, leads before the charger were 7.xxV and the phone received 2.xxV. It was a complete mystery, until I read your post. I am having a little fun with the 3x1W LED chips that arrived today. On the other note I cant get 4.3R resistors to use with LM317.
I will try your idea, so far you were completely right and I was wrong. Will post the update once done.
Thank you David!
However, the use of 33k might hamper the current handling capacity of the transistor, and it won't charge your battery correctly.
Instead you can try connecting a 10k resistor or some other calculated value across the base and ground of the transistor, that will take care of the residual leakage at the opamp output.
More effective idea would be to connect a 3V or any zener diode below 6V in series with the transistor base 4k7 resistor.
Anode will point towards transistor base and cathode towards opamp pin#6….this will result complete switch OFF of the transistor and the load.
A 1$ 5V USB charger? That is why I'll be using it for, though I can't get the circuit to work, the load does not cut off. The threshold (and low battery) LED works fine. I tried it even with an inverter and a PNP transistor.
hello sir,
i like your blog very much. i need a simple circuit for pumping water from public water suply pipes to overtank. the circuit must trigger ON when water supply comes in the pipes (since its daily time is not fixed ) and trigger OFF when the overtank is full or the supply turns dry.
thanks !
It will be difficult for me due to many pending requests, it would be great if you could make the diagram as per the my explanation and show it to me, I'd correct it if required.
correction: the 0.22uF cap should be connected across the common joint and positive of supply, not ground as wrongly suggested above.
Procure the IC 4093 and assemble its pinouts in the following way:
join pins 1,2,5,6,8,9,13,12….this common joint will become one terminal which will go inside water.
also connect a 022uF cap across the above joint and ground close to the IC.
Next join pins 3,4,10,11, join 10k resistor with this common point to the base of a transistor BC547
join the collector with a relay coil, i hope you how to join it, it's very simpe.
emitter will go to ground.
the ground will become another sensor point which must be held close to the sensor point explained above and inside water.
finally join the relay N/O and pole contacts with the motor and AC
power the circuit by applying +12V DC to pin14 of IC and the relay coil.
negative of the supply to pin7 of the IC , transistor emitter.
Thank you very, very much, Swagatam. I will pick up the LM741 and the zener diode asap. I'll post the results when it's done. Your quick response is really appreciated.
-David
Thanks David, it's my pleasure.
I would connect it differently if you had not made the second diagram. Thank you once more! I am just getting the components now (order was misplaced).
I really appreciate your help.
Hi David, I have updated the article with the required diagram, please do it according to it.
I ordered the components. I just want to make sure I connect the resistor for avoiding the oscillation correctly. Pin #3 of the LM741 and the circuit's minus (-) ( upload.wikimedia.org/wikipedia/commons/8/83/Diode_pinout_en_fr.svg )
Did you accidentaly forgot to turn the zener around (in the schematics), or did you meant anode. I am confused. It's my first time working with zener diodes. I just thought they were faster than normal diodes.
Thank you, I hope the components will be ready tomorrow, so I can start and test a prototype then make a product I will carry with me most of the time on the go.
It's my pleasure, David!