Connect with us

logic level mosfet resistance

Discussion in 'Electronic Design' started by John O'Flaherty, Jul 31, 2006.

Scroll to continue with content
  1. For a logic level mosfet with RDSon specified at 0.09 ohms at about
    10a, with VGS at 5V, will it act as a .09 ohm resistor even for low
    currents, like 10 to 100mA, giving a drop of 1 to 10 mV, or is there a
    voltage burden? The data sheets that have graphs don't show this area
    very well.
     
  2. colin

    colin Guest

    yes, the data sheets usualy show the V/I curve is equal to rdson below some
    current and it does go all the way down to zero.

    Colin =^.^=
     
  3. Rich Grise

    Rich Grise Guest

    I just wonder why you're using a 10A MOSFET at 10 to 100 mA. :)

    Thanks,
    Rich
     
  4. Andrew Holme

    Andrew Holme Guest

    No. Rds is non-linear.

    See http://freespace.virgin.net/ljmayes.mal/comp/vcr.htm

    This link is from an article about using a JFET as a voltage-variable
    resistor in an audio compressor; however, what it says about Rds linearity
    could be applied to MOSFETs.
     
  5. I would expect it to act like a resistor at currents below
    maximum and a fixed VGS. However, if the answer was important to your
    circuit, I would get a voltmeter and a few selected resistors and
    actually test it. Just put a resistor in series with the mosfet to
    limit the current to your desired level and measure the voltage across
    the mosfet.
     
  6. I want the low RDSon because I'd like to measure a single ground
    referenced voltage and be able to calculate the resistance of a load
    and the power delivered to it. With this arrangement...

    +5V Regulated
    +
    |
    |
    .-.
    | | Rload (variable)
    | |
    '-'
    |
    |------------
    | measure V
    .-.
    | |
    | |10 ohm
    '-'
    |
    |
    ||-+
    Duty Cycle ||<-
    -------||-+
    Control |
    |
    ===
    GND
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    the load current is the measured voltage divided by 10 ohms, and the
    load voltage is 5V-the measured voltage. The measurements will be taken
    on the fly.

    If there's a low power unit that still has low RDSon, that would work
    too. Any suggestions?
     
  7. Thanks, Colin.
     
  8. Tim Wescott

    Tim Wescott Guest

    The following will be insensitive to RDSon. Instead of worrying about
    it you can just get any old logic-level FET. If you want to read the
    voltage at the load you'll have to tap that separately, of course.

    +5V Regulated
    +
    |
    |
    .-.
    | | Rload (variable)
    | |
    '-'
    |
    |
    ||-+
    Duty Cycle ||<-
    -------||-+
    Control |
    o------------
    | measure V
    .-.
    | |
    | |10 ohm
    '-'
    |
    |
    ===
    GND
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  9. Joerg

    Joerg Guest

    Hello John,
    Can't you use a mux and also measure the voltage at the drain? Then a
    subtraction would yield the voltage across the 10ohm resistor.
     
  10. I did want to get by with a single measurement to get all the load
    information- I,V, Power. Plus, with the FET ground referenced, I'll be
    better off by about a volt or so on the drive voltage (for the higher
    end of the current range). I found the IRF3711 for $1.11 that has 5.9
    mOhm (typical) RDSon at 4.5V gate drive, so that will only be 0.059% of
    the 10 ohms.
     
  11. Well, if everything goes according to plan, I'll be able to use a
    single measurement, saving the trouble of multiplexing and the time to
    make the second measurement. It should work ok if I can rely on <10
    mOhm resistance even at lower current levels.
     
  12. Rich Grise

    Rich Grise Guest

    If you're saturating the thing, why not put the current sense resistor
    in the source lead and get a single-ended measurement?

    Good Luck!
    Rich
     
  13. colin

    colin Guest

    You can get current sense fets if all you want to do is measure current -
    they have a fourth terminal wich is conected to a current sense resistor
    through wich a small part of the current flows.

    However the on resistance of a normal FET rises with voltage/current,
    therefore it is lowest at zero volts, some SMPS controller ICs actualy use
    the VDS to monitor the current.

    How accurate do you want to be ?

    Colin =^.^=
     
  14. My hope was to get V, I, R and P of the load with a single measurement.
    I should be able to do that within the needed precision with the
    circuit I had, and have 1 volt or so of extra drive to VGS, as long as
    the RDS is in the area of 10mOhms, and if it remains negligible over a
    range of currents from, say, 5mA to 100mA. The RDS could even be higher
    and be calibrated out, unless it's non-linear and/or temperature
    sensitive. I may not even need to worry about those factors if I start
    wiith the lowest RDS possible. I found a fet with <10mOhm for a little
    over $1, so I'm happy with the way I'm going; I just wasn't sure if the
    simple resistance remained valid at low current levels. The consensus
    seems to be that it does.
     
  15. Not just current :). I want load current, voltage, power and
    resistance with a single voltage measurement, since my circuit will be
    lazy as sin.
    I'll be digitizing with 10 bits, so anything better than 0.1% would be
    a waste. With a power fet running a maximum of 100mA, with 10mV RDS and
    at about 500Hz, endogenous heating won't be a problem! I hadn't heard
    about current-sensing FETS; thanks for mentioning them.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-