low voltage cutoff circuit

Discussion in 'General Electronics Chat' started by Georacer, Jan 11, 2012.

  1. Georacer

    Thread Starter Moderator

    Nov 25, 2009
    5,142
    1,266
    Hi

    I think this circuit is used to cutoff the voltage source to the output if it's the voltage is 6V or lower. Do I have it correct? I googled "6v low voltage cutoff" and found this somewhat similar looking circuit.

    The red LED is for power indication in the circuit and the white LED is for low voltage indication.

    Is that component on the top left a relay?

    What is that IC on the top right? Someone was saying that the circuit uses operational amplifier. But someone said it could be a LM393 dual comparator. What do you think?

    In what kind of applications this circuit can be used? Could you please list some examples of circuits or electronic products which can be damaged by low voltage if they don't use such voltage cutoff circuit.

    Thank you for the help.

    Regards
    PG

    This is a thread started by me on behalf of PG1995 who is unable to post a new thread here due to AAC technical difficulties.
     
    Last edited: Jan 11, 2012
  2. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    Yes, the item top left is a 10A relay with a 6V coil.
    The IC is likely an LM358 op amp or LM393 comparator.
    The item left of the IC is likely a 3.9V zener used as a voltage reference.
    I cannot guess why there are two pots.
    The main application for the low voltage cut-out is to protect lead-acid batteries from over discharge that tends to kill the battery.
    Regarding loads, many circuits will not operate properly at low voltage, but are unlikely to be damaged.
     
  3. PG1995

    Active Member

    Apr 15, 2011
    753
    5
    Thank you very much, GeoRacer.

    Thank you for the reply, Jim.

    I'm also attaching a picture of the underside of the PCB it might help you.

    Regards
    PG
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    This thing is built with 5% resistors, and the voltage reference device may well be a zener diode of perhaps 5% tolerance. Depending on the circuit used, the result could be an uncertainty in threshold voltages of the order of 15% worst-case, or ±0.9V. That dues not seem close enough to me, so trim adjustments would seem to be in order.

    My guess would be that the trim pots adjust the cut-off threshold and the level at which the connection is restored. There should be positive feedback to introduce hysteresis between the two levels, as otherwise oscillation is likely: the battery voltage can rise on disconnection of the load, possibly leading to immediate reconnection...and so on. This would rapidly wear the relay and may damage the load, so should be avoided.

    The pots might adjust the levels separately, or one pot may set sensitivity, and the other may set the hysteresis between them.

    Of course, this may not necessarily be so - but it seems fairly likely.
     
    PG1995 likes this.
  5. PG1995

    Active Member

    Apr 15, 2011
    753
    5
    That IC is UA741CN.
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Based on PCB I drew a diagram of this circuit.
    I hope that I don't do any mistake.
     
    PG1995 likes this.
  7. PG1995

    Active Member

    Apr 15, 2011
    753
    5
    Many, many thanks, Jony.

    I was waiting for your reply! Can I get a Multisim or CW circuit for this circuit, please? If you are busy and don't have time, it's okay. I really appreciate your help.

    With best wishes
    PG
     
  8. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Download file Low_Volt.zip that I added in the previous post
     
  9. PG1995

    Active Member

    Apr 15, 2011
    753
    5
    Oh, sorry. I didn't notice it.

    But the LED D3 in CW doesn't light up at all even when I lower the voltage to, say, 4V. Is there something wrong? Please let me know.

    Thanks a lot.

    Best regards
    PG
     
  10. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    D3 is CW = clockwise? On which the diagram ?
     
  11. PG1995

    Active Member

    Apr 15, 2011
    753
    5
    CW stands for Circuit Wizard. Sorry for the confusion.
     
  12. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    For B1 = 9V set VR1 = 80% and VR2 = 10K.
    And when D3 is "ON" the relay is also activate, and now change battery voltage from 9 to 8V ---> 7V ---> 6V. And you will see that the D3 is off and relay is not active.
    And now load is disconnect from the battery (load is not included in the circuit diagram).
     
    PG1995 likes this.
  13. PG1995

    Active Member

    Apr 15, 2011
    753
    5
    Thank you very much. I'm very much grateful for all this.

    Best regards
    PG
     
  14. Amlansen

    New Member

    Jun 6, 2013
    1
    0
    @Jony130
    Which software were you used to open .cwz file. please give me the link.
     
  15. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    I use "circuit wizard" to draw the circuit diagram.
     
Loading...