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DRV421EVM: DRV421EVM

Rajiv Pandit
Prodigy 20 points
Part Number: DRV421EVM
Other Parts Discussed in Thread: DRV421

Hi,

Am developing a residual current sensor 1-20mA AC/DC and having issues with magnetic shielding as well as the minimum sensitivity of the sensor.

1. The Earth's magnetic field causes the sensor to saturate in certain orientations of the core - am in the process of designing suitable shielding, and would appreciate some guidelines on this.

2. The primary conductor is able to generate flux density of 1~2uT at the gap - is this sufficient to generate an accurate output? Am using a 20K shunt resistor.

regards,

Rajiv

  • 0
    TI__Genius 14825 points
    Rajiv,
    Due to the holidays, it will take a few days to get back to you fully on this thread.
    In the meantime, what shielding have you done so far? Have you tried Mu-Metal or something else?
  • 0 in reply to Jason Bridgmon
    Prodigy 20 points

    Jason,

    I've been running FEMM simulations with a concentric Mu-Metal ring of a slightly larger diameter and cross section and two gaps offset +/- 90 degrees from the location of the sensor.  This seems to carry away most of the earth's flux from the sensing assembly and it's gap. Haven't done any real-time tests as yet. Am currently positioning the sensor assembly carefully to avoid being influenced by the earth field and running my tests.

    BTW, I seem to have posted this in the wrong forum - it should have been in the current sensing forum...

    Regards,

    Rajiv 

  • 0 in reply to Rajiv Pandit
    TI__Mastermind 26485 points

    Rajiv,

    Thanks for your patience.  Mu-metal would be the best shield.  There are a couple of other issues you would need to worry about.  First you would want your magnetic gain to be as high as possible.  This will help with some of the offset (see DRV421 System Parameter Calculator).  This shows you how some of the error contributions can change.  The higher the magnetic gain the better.  The other issue is decreasing the number of turns while still being able to maintain the inductance needed to keep the part stable.  (see Designing with the DRV421: Control loop Stability).  The magnetic core can be magnetized creating an offset as well.  The DRV421 has a demagnetization function but in order for it to function it needs to drive the coil with enough current to function properly.  With a Rshunt of 20K the current will be decreased significantly and may not be able demagnetize the core.  You can bypass these by either placing switches during degauss function or adding diodes to bypass the Rshunt.