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ICL7665s confusion

Discussion in 'General Electronics Discussion' started by Shadow1976, Sep 16, 2014.

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  1. Shadow1976


    Dec 7, 2013
    Hi Guys,
    I have just put together as a test a icl7665s over/under voltage detector. works well but have a question maybe someone here could help with.
    I need to create a condition where a relay will be energised when the upper trip point (12.5v) is reached but stay on until the lower trip point (10v) is also reached (battery discharging). I have set it up with these voltages (undervoltage = 10v) and (Overvoltage = 12.5) but dont know what to add to both outputs to acheive this condition. Reason being, I want to give the battery time to fully charge. a little confused. So to simplify, relay on at >12.5v and relay off at <10v.
    tannewtan likes this.
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Use a dual-coil bistable relay (also called a latching relay). Use the upper trip output to set the relay, use the lower trip output to reset the relay by connecting each output to one coil.
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Here's a circuit that may do what you want. It uses a P-channel MOSFET instead of a relay and has a pretty low power consumption.


    It uses the built-in hysteresis feature of the ICL7665 and has threshold voltages of 12.5V (rising) and 10.0V (falling).

    The battery can always be charged, through the body diode inside Q1, but discharge current is controlled by Q1 which is controlled by the ICL7665.

    Assume that the battery voltage is 13.0V, above the high threshold. The ICL7665's comparator on SET1 sees a voltage much higher than its internal 1.3V reference voltage and drives its output low. This causes the internal MOSFET to connect V+ to HYST1 so R1 is bypassed and out-of-circuit, and it also causes the ICL7665 to pull OUT1 low. This effectively connects the battery voltage between the gate and source of Q1, turning Q1 fully ON and connecting the battery to the positive terminal at the right side.

    In this state the ICL7665 is using its falling voltage threshold, which can be calculated as:
    VFALLING = 1.3 + ((1.3 / R3) × R2)
    = 1.3 + ((1.3 / 39000) × 261000)
    = 10.0V

    As the battery discharges, the battery voltage will approach this threshold. When it reaches the threshold, the ICL7665's comparator on SET1 sees a voltage lower than its internal 1.3V reference voltage and drives its output high. This causes the internal MOSFET between V+ and HYST1 to turn OFF, causing R1 to come into circuit and causing the SET1 pin voltage to drop even further, reinforcing the change, and it also causes the ICL7665 to release OUT1 so it is pulled up to the positive rail by RG. This removes the gate bias on Q1 and Q1 turns OFF, protecting the battery from over-discharge.

    In this state the ICL7665 is using its rising voltage threshold, which can be calculated as:
    VRISING = 1.3 + ((1.3 / R3) × (R1 + R2))
    = 1.3 + ((1.3 / 39000) × (75000 + 261000))
    = 12.5V

    The battery can be recharged by current flowing into the positive terminal at the right, which will flow through the body diode that's an internal part of Q1. It's shown on the schematic as part of Q1's circuit symbol. But charging will probably not work with a smart charger that measures the battery's terminal voltage, so you will need to either use a dumb charger, or charge the battery directly (not via the connections at the right side of the diagram).

    Once the battery voltage reaches the rising threshold of 12.5V, the ICL7665 will switch back ON, as described at the start of the circuit description.

    CD is a decoupling capacitor and is required for reliable operation of the ICL7665. It should be a ceramic type. Connect it as directly as possible between pin 8 and pin 4 using short leads.

    The threshold voltages are not precise because the internal 1.3V reference in the ICL7665 is not precise. See the data sheet for specifications. Errors will also be introduced by R1~3. Use 1% resistors here.

    I have listed several part numbers for the MOSFET. They are all available from Digi-Key ( They are listed in order of increasing cost and decreasing RDS(on) resistance. This parameter determines how much voltage is lost across the MOSFET, and how much power it dissipates, as a result of the discharge current flowing through the MOSFET.

    Here's the list of suggestions with their RDS(on) values and a recommended maximum continuous discharge current, assuming that the MOSFET has no heatsink.

    FQP27P06 (USD 1.25) 70 mΩ up to 2A
    NDP6020P 50 mΩ up to 2.5A
    IXTP32P05T 39 mΩ up to 3A
    FQPF47P06 26 mΩ up to 3.5A
    SUP53P06-20-E3 20 mΩ up to 4A
    SUP75P03-07-E3 (USD 2.90) 7 mΩ up to 7A
    Harald Kapp likes this.
  4. Shadow1976


    Dec 7, 2013
    Wow thank kris. I'll build that one. Much appreciated
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