possible to use Rds(drain to source) as a shunt to measure current across MOSFET?

Discussion in 'General Electronics Chat' started by ak52, Dec 7, 2015.

  1. ak52

    Thread Starter Member

    Oct 15, 2014
    145
    4
    Hello Everyone,

    Would it be a good idea to use the internal Rds of a MOSFET as a high side shunt. or would it be better to get a dedicated shunt resistor separately to measure current passing though the MOSFET?

    Thanks ,
    AK
     
  2. WBahn

    Moderator

    Mar 31, 2012
    17,777
    4,804
    You need a resistance that is a known and stable value. It is unlikely that a channel resistance of a FET is going to qualify.
     
  3. Bordodynov

    Active Member

    May 20, 2015
    643
    188
    Some DC/DC stabilizers on-resistance of the transistor is used to limit the current.
    In this case, voltage control is performed in the open transistor.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    The Rds ON of a MOSFET varies significantly from unit-to-unit and also has a positive coefficient with the MOSFET junction temperature.
    So Rds would not be good for the purpose of measuring the MOSFET ON current unless your accuracy requirements are very low.
     
  5. WBahn

    Moderator

    Mar 31, 2012
    17,777
    4,804
    But performing current limiting is very different from performing current measurement.
     
  6. ak52

    Thread Starter Member

    Oct 15, 2014
    145
    4
    Thanks guys for the replies,looks like it would be better to use a separate resistor.
     
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