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How to interpret Mosfet RDS ON resistance?

Discussion in 'General Electronics Discussion' started by Rleo6965, Mar 21, 2015.

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  1. Rleo6965


    Jan 22, 2012
    I'm sure you can help me here. I've been searching in the internet how to convert the IRF640 Mosfet Rds (ON) 180M Ω into Rds (ON) 0.0XXX Ω. But I can't find the conversion.

    I'm not sure if its equivalent to Rds (ON) 0.18 Ω or 0.00018 Ω.
  2. TenderTendon


    Dec 20, 2014
    To convert milliohm to ohm, divide by 1000, or 3 decimal places left. Therefore, 180mΩ would be 0.18Ω
    Rleo6965 likes this.
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    In addition to that, several things can be said about Rds(on):

    1. It changes with both Vgs and Ids
    2. beyond a certain Ids it increases rapidly (you probably won't be working in this region in a digital circuit)
    3. It (max) represents a guaranteed value, a specific device may be better (it may even be better than the typical value).
    4. For very low values of Rds(on) the resistance of your soldered connections and the traces on your board, interconnecting wires and junctions may all become significant if your aim is to reduce voltage drop.
    It pays to look at the graph of Vds vs Ids for a range of values of Vgs. This graph may look confusing at first, but it conveys a lot of highly useful information about the static ON state of the mosfet.

    As it stands, 180mΩ is not a particularly low on resistance. 180MΩ would be an astoundingly high on resistance -- it pays to get the case of your multiplier right :)
    Rleo6965 and hevans1944 like this.
  4. Rleo6965


    Jan 22, 2012
    Thanks for the info. How come I missed the meaning of small character m was for milli? ;)
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