Supply for a MEMS accelerometer

Discussion in 'The Projects Forum' started by dasiki, Apr 14, 2010.

  1. dasiki

    Thread Starter New Member

    Apr 14, 2010
    3
    0
    I'm testing a MEMS accelerometer with a piezoelectric one, the sensor is mounted on a pcb and requires a voltage supply to work. The data sheet for the sensor gives a supply voltage range of 3.3V to 5V. To get this I've decided to use an old phone charger, it produces 3.7V and as far as I'm aware will work. The problem comes from the fact that the spec sheet quotes a max current supply of 5mA and the charger gives out 355mA. My solution is to simply place a 800 ohm(or roughly equivalent value) resistor in series between the live wire of the charger and the point on the the board wher i connect the supply volatage and connect the ground wire of the charger to the appropriate connection on the board.

    I would appreciate advice on wether or not this will work and if it isn't likely to work, alternative soultions would be very helpful.

    Thanks in advance.
     
  2. Jazz Bass Special

    New Member

    Apr 9, 2010
    24
    5
    The fact that the MEMS IC consumes 5ma's means your power supply must have available 5 ma's at minimum.
    I= V\R
    You do not need to regulate the current to the IC in the manner that you are proposing.
    You might want to limit the overall maximum current that "can" be drawn.
     
  3. dasiki

    Thread Starter New Member

    Apr 14, 2010
    3
    0
    Thanks for the reply.

    So does that mean that my supply is, which is 355mA won't damage the circuit as it will only draw a maximum of 5mA from it? I just read the value on the sheet as kind of a "Do not apply more than this" limit.

    You suggest limiting it, I presume this is not to protect the circuit but more a general safety feature akin to a fuse? Also any suggestions on how to limit it would be really helpful.

    The following is a link to the data sheet, with the specifications on page 3.
    http://www.analog.com/static/imported-files/data_sheets/ADXL001.pdf
     
  4. Jazz Bass Special

    New Member

    Apr 9, 2010
    24
    5
    The MEM datasheet states a min/ max current draw of the IC.
    That is what the manufacturer states as the current that there device, under normal conditions, will draw from the supply.
    This is an important part of any datasheet.
    Imagine designing a circuit that needs ten MEMs.
    Now you know the min/ max current requirement of ten sensors.
    Your power supply design team will have to know the total draw of all devices before they can design an appropriate power supply.
    The end result is: your power supply is fine (overkill, but fine).

    As to current limiting the supply: any limiting that you do will lower your available voltage.
    If you are O/K with the current power supply voltage, let it go.
     
  5. dasiki

    Thread Starter New Member

    Apr 14, 2010
    3
    0
    So today I used to a digital multimeter to check which wire from the charger was live and which was neutral. It gave a voltage of 9.5V, this is much greater than the absolute maximum specified for the PCB and may risk damaging it but I assume its so high because of the low impedance in the multimeter? As I stated above the markings on the outlet say its 3.7V, should I connect it anyway?
     
  6. Jazz Bass Special

    New Member

    Apr 9, 2010
    24
    5
    Sometimes you have to believe what you see.
    Without knowing:
    1- What the power supply is or is capable of.
    2- What the circuit as a load is going do to affect the power supply.

    As to #1; It depends on what type of power supply you are connecting to your PCB.
    Some PS need a load.
    I would put a 1K power resistor across the PS. And then measure the voltage.
    If it drops to what you need, your good to go.
    It is good practice to monitor your input voltage when plugging in the load.
    If it hangs, yeah.
    If it nose dives, there is a problem.
     
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