Low Voltage Switch/ Cut off /Disconnect for 5S lipo using N channel mosfet

Discussion in 'The Projects Forum' started by Xenocatalyst, Aug 28, 2015.

  1. Xenocatalyst

    Thread Starter New Member

    Aug 5, 2014
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    Hi Could anyone please provide a schematic with component specs/values for a Low voltage switch / cut off using a n channel mosfet.

    I have a power supply consisting of a 5S lithium battery pack and its powering a 35 watt LED via a constant current driver.

    I want the circuit to latch when the battery is connected and unlatch/disconnect when the supply voltage drops to 15 volts.

    I have seen some examples using a FQP47P06 and FQP30N06L and understand the concept but I don't know where to begin to set the correct disconnect threshold.

    Could anyone please help with a schemtaic?

    OK here is a little more.....

    here is a basic schematic. (constant current circuit omitted for clarity)

    Mosfet_zpsemfsud3u.jpg


    Source: 5S Lipo @ 18.5v
    Load: Cree Xhp 70 ~ 12v @ 1050 mA, max 12.8V @ 2400mA
    Mosfet :FQP30N06L or similar

    What I Know:
    he Mosfet allows power flow through the drain and source pins when the gate pin receives a voltage between 1v & 2.5V and higher.
    When the gate pin receives a voltage below 1v the flow between drain and source stops.

    What I Need to Know:
    Using minimal components and keeping generated heat to a minimum, I want the mosfet gate pin to disconnect when the battery voltage drops to 15v.
    So I need the line to the gate to drop from above 2.5v to below 1v when the battery voltage falls below 15v.

    Other requirements:
    Mosfet to be active when battery is connected.
    Mosfet to cut power when battery voltage drops to 15v.
    Kept as simple as possible.
    No buttons to turn it on or off.
    No audible or visual indicator. Easy to obtain components.


    I have seen similar circuits but they use p channel mosfets, I hear that its better to have the n channel as its easier to pull it to ground when the circuit fails than it is to try and supply +5v when the circuit fails.

    I like the simplicity of this circuit http://forum.allaboutcircuits.com/threads/request-guidance-nimh-low-voltage-cut-off.91806/ but not sure if it'll work for my application or if it can be tweaked for it and it is a p channel type.

    Any help you can give would be appreciated.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    If you put your country in when you register, we might have a clue as to what parts you can procure...

    I designed the linked circuit, and with some tweaks, could be adapted to your project. It has something very important; hysteresis, otherwise, as the battery discharges, the circuit cuts-off, battery voltage increases a bit, and without hysteresis the circuit would turn back on, causing the light to flash on/off.
     
    Xenocatalyst likes this.
  3. Xenocatalyst

    Thread Starter New Member

    Aug 5, 2014
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    Well I'm in Australia.
    Over the years I've bought parts online from Element 14 (newark, farnell), RS online, Mouser, Digikey and Sparkfun.
    Otherwise there are the shops in town: Dick Smiths & Jaycar.

    Thanks for explaining the hysteresis for me, that is a feature I'd like to keep.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    In order to help, I need the following information (Answer all numbered questions):

    1. What device turns on the light? (described as "I want the circuit to latch when the battery is connected"). Is that a toggle switch, slide switch, clip lead? i.e. it stays connected even after the load is disconnected.

    2. "I want the circuit to latch when the battery is connected" implies that that the battery is initially charged to a higher voltage than 15V. What is the minimum battery voltage required to enable the circuit to latch on? Is it 18.5V? It cannot be 15V.

    3. Exactly what voltage cuts off the light? 15.00V +- X mV? What is X?

    4. Is the power switch (likely a PFET) regulating the current to the Cree, or is there a separate constant-current driver that will be just switched on/off by the low-voltage disconnect circuit? I need to see a part number/data sheet for the constant-current driver.

    5. What current are you driving the Cree at?

    6. Why are you hung-up on an NFET?
     
  5. Xenocatalyst

    Thread Starter New Member

    Aug 5, 2014
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    1. layout will be battery (5s lipo) -> T connector -> this circuit -> T connector -> switch -> CC LED driver -> LED.
    I will have a manual push on/push off style switch in between this circuit and a constant current LED driver on the positive wire.
    When the mosfet cuts the connection to the load I will physically disconnect the battery (or maybe add another switch) to prevent the this circuit from draining the batteries further.

    2. Battery is rated at 18.5V and max charge is around 21V. I want the gate of the mosfet to become active as soon as the the battery is connected regardless of whether the LED driver is connected but only if the battery voltage is over 16V.

    3. Cut off voltage = 15V +- 200mV.

    4. For the moment I am still testing driver boards, I would like to use this but haven't tested it yet (alternatively this). Unfortunately this is the only information I can give you for this. I know its cheap and probably nasty but its if I damage it it's no big loss. Initially i was looking for driver designs to build but realised I was wasting too much time when I could just buy an "off the shelf" solution rather cheaply.
    I also have a Meanwell LDD1200 I may use but this limits the potential of the XHP considerably.

    5. I intend to run 2x XHP70 leds in parallel @ 3 amps (1.5A each).

    6. Somewhere along the line I heard that a n-channel deactivates when the gate voltage drops and that this would continue to protect the batteries from discharging in the event that the circuit malfunctions and the gate loses power.


    I'm open to ideas as electronics isn't my forte.
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Could the PushOn/PushOff switch be between the battery and the rest of the circuitry? That way, there is no leakage possible. That makes the low-voltage disconnect a bit simpler.

    No problem.

    No Problem.

    Those modules are variable-voltage-in/adjustable-voltage-out step-down converters. Those modules are not good at driving a High-power LEDs. You would have to add heat-producing, power-wasting, high-power resistors to convert these constant-voltage supplies to a rather-poor pseudo-constant-current LED driver. Ideally, you should be looking for a switcher that has an adjustable constant-current output expressly designed to drive LEDs.

    A much better way is to get 6V LEDs and run two of them in-series. That way, the single driver needs to have a compliance of >12V, which works well with a 15V input. This makes it so that the two LEDs are driven with identical currents; other wise, each LED would need to have its own C.C. driver. Ideally, you would buy a 2.5A CC LED driver that has a minimum compliance of 12V with a input of 15V to 25Vdc.

    A PFET is a complement of an NFET, so except for polarities, they are equivalent. To turn off a NFET, its gate is connected to the same voltage as its source (usually 0V). To turn off a PFET, its gate is connected to the same voltage as its source (not necessarily 0V) .
     
    Last edited: Sep 13, 2015
  7. Xenocatalyst

    Thread Starter New Member

    Aug 5, 2014
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    Yes, Good Idea.

    My thinking here was that these step down converters have a maximum output rating of 3A and that they would limit the current to the LEDs.
    Are there any CC drivers you could recommend or even one i could build and learn from?


    I decided to go with a parallel connection because running 2 XHP70's in series would draw over 12Amps and I couldn't find a driver that would handle it in a small form factor.
    Why is it that series connection is prefered?

    I'll take your lead here.
     
  8. Xenocatalyst

    Thread Starter New Member

    Aug 5, 2014
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    Also I have already ordered the 12v versions, however they have not yet arrived.
     
  9. MikeML

    AAC Fanatic!

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    They are commercially available. A quick look at DigiKey found these.
    The highest current model that is sold there is this one. It would run your two 12V series-connected XHP70s just fine. Study the data sheet. If you build one, you will have to do the same...
    You have an error in your thinking. The 12V XHP70s should be operated at ~1.5A max! The 6V XHP70s should be operated at 3A max.!(When bolted to a finned-CPU sized aluminum heatsink; dont operate these without a heatsink, you will instantly blow them up).

    Two 12V ones in parallel will consume 2x12x1.5 = 36W. Two 6V ones in series will consume 2x6x3 = 36W. If two 6V LEDs are connected in series, only one 3A driver is needed. If the driver can step-up your battery voltage to about 30V, you can run two 12V ones in series. If your driver is a buck driver (no voltage step-up), then you would have to use two drivers; one for each 12V LED.
     
    Last edited: Sep 17, 2015
  10. Xenocatalyst

    Thread Starter New Member

    Aug 5, 2014
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    I see now, thankyou for explaining that.

    I have something similar, the Meanwell LDD-1200.
    I bought 2 for testing some cree cxa1512's but I haven't been able to get analogue dimming to work with them.

    I already have a few cpu heatsinks ready to use.
     
  11. Xenocatalyst

    Thread Starter New Member

    Aug 5, 2014
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    Thinking about my setup, Using a driver per led is probably better for my application, as I may only need 1 led instead of 2. maybe a modular setup is what I'm after.
     
  12. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Here is the disconnect circuit tweaked for your application. The Load is anything up to about 3A. In the sim, the battery is charging slowly, and the circuit CutsIn. Likely you will have the battery precharged, and then you close a switch between the battery and the input of the Disconnect circuit.

    Pdisch.gif
     
  13. Xenocatalyst

    Thread Starter New Member

    Aug 5, 2014
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    Thank you for your time and effort doing this for me, I'll buy the components and breadboard it.
    If I wanted lower the cut in voltage would reduce the value of R3?

    I do have one more request though,
    I intend to use this battery for other projects as well which may include a high current draw.
    The battery is rated at 4000mah and can sustain a continuous discharge rate of 45C.
    I am not likely to require this amount of current even if I were to use it in a radio controlled car or plane, so what changes would be required to handle a current draw of up to 20C?
     
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