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techniques for sensing fairly small currents (<1A)

Discussion in 'Electronic Design' started by Michael, Apr 27, 2007.

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  1. Michael

    Michael Guest

    Hi - I am working on a power board that needs to monitor currents on a
    number of supply lines. Currents should all be fairly small - all less
    than an amp. Largest supply voltage will be 11.1V.

    What methods exist for sensing currents this small? The only one which
    I am familiar with is where a small shunt resistor (preferably a high
    side shunt) is used and then the voltage across that resistor is
    amplified, often with a current sense amplifier.

    Is that it?

    Thanks,

    -Michael
     
  2. Phil Allison

    Phil Allison Guest

    "Michael"

    ** No.

    Try looking for " Hall effect sensors " and current transformers.

    The former operate with AC and DC currents while the latter are AC only.

    Both are non contact ( aka galvanically isolated) sensing methods.



    ....... Phil
     
  3. Michael

    Michael Guest

    Hi Phil - I should mention I'm only interested in DC current sensing.

    I'm aware of hall effect current sensing - but isn't that only viable
    for large currents? A quick google only finds parts designed for
    larger current ranges than what I'm interested in.

    -Michael
     
  4. Phil Allison

    Phil Allison Guest

    "Michael"
    "Phil Allison"

    ** Most linear Hall effect sensors work down to the 1 mA level, just
    ne - Lem, Honeywell etc.

    All YOU need do is run a few turns of insulated wire through the hole to
    increase the sensitivity.



    ....... Phil
     
  5. Keep in mind that many other components, such as FETs, can look a lot
    like resistors, and the addition of a dedicated part may not be
    necessary.

    Why do you prefer high-side to low-side?
     
  6. Michael

    Michael Guest

    Hmm, this is a valid point. The power board will just be stepping down
    a large DC voltage to various smaller DC voltages - so there very well
    might be a good component to grab a current reading from. I'll have to
    look into that further when the design is farther along.
    So that I can use a shared ground.

    -Michael
     
  7. Rich Grise

    Rich Grise Guest

    That seems a little counterintuitive. If you used low-side sensing,
    then all of the sense resistors would have ground in common with the
    supplies. With high-side, you need more sophisticated circuitry
    because the sense resistors will all be floating at their supply
    voltage.

    Hope This Helps!
    Rich
     
  8. IANEE, but not really counter-intuitive IMO. He wants the grounded side of
    his stuff to all be at the same potential, ground. :) If all his stuff
    ran on a kajillion volts, he probably wouldn't want the grounds to all be
    thousands of volts apart from each other. ;-)
     
  9. kell

    kell Guest

    High side current sensing requires more complicated circuitry than
    sensing at ground, but if that's what you want here are a couple of
    ideas. I'm praying the ASCII comes out ok, because I'm using groople
    and they seem to have eliminated the fixed font feature. Thank
    Winfield Hill for the first one and Fred Bloggs for the second.


    + -----+--- Rsense ----+---->
    .. | |
    .. Rc |
    .. +------------, |
    .. | __ | |
    .. c\| / +|--' |
    .. NPN |---< | |
    .. e/| \_-|-----'
    .. |
    .. Rload
    .. |
    .. - -----+-------------------->


    ..
    .. .---------------------. R1
    .. | | 1.24V - (Vled-Vbe)*-----
    .. | | R2+R1
    .. ----+-----[Rs]------+-------> I= ----------------------
    .. | | | Rs* R2/(R1+R2)
    .. | | |
    .. | | |
    .. | [R1] |
    .. | LM185 | |
    .. --- | |
    .. / \ -------------+ ---
    .. --- | \ / led
    .. | | ---
    .. [R4] [R2] |
    .. | | |
    .. | | |
    .. +---------------' |
    .. | |
    .. | |
    .. | |
    .. >| |
    .. |-------------------+
    .. /| |
    .. | |
    .. +---> to comp [R3]
    .. | |
    .. [Rc] |
    .. | |
    .. --- ---
    ..
    ..
     
  10. Michael

    Michael Guest

    Exactly. Even though everything is fairly low voltage, I still have a
    number of supply lines on the same board. If I were to sense ground
    currents I'd have to have a separate ground for each voltage - while
    now I just have one big ground plane.

    -Michael
     
  11. Ken Smith

    Ken Smith Guest

    [... high side current sense ....]
    I prefer doing this:
    Modified ASCII art

    This has the advantage that the minus supply fo the op amp can be
    something other than ground.

    Also you may need to add a capacitor to this or the other version to
    prevent oscillation. Adding an extra resistor in the input path to the
    op-amp also protects it against short circuits in the load.
     
  12. Rich Grise

    Rich Grise Guest

    Oh, well that's different!

    Never mind. ;-)

    Cheers!
    Rich
     
  13. kell

    kell Guest

    And use an op-amp with input range that includes the top rail; a j-fet
    op amp like TL082 would do, or there's the LMC6482, which has a lower
    offset voltage.
     
  14. MooseFET

    MooseFET Guest

     
  15. kell

    kell Guest

     
  16. kell

    kell Guest

     
  17. Fred Bloggs

    Fred Bloggs Guest

    Yep- that's basically it, convert the current to a voltage with
    something called a resistor. The actual circuitry to do this will be a
    function of your intended use. For example are you monitoring for a trip
    or foldback threshold , or are you monitoring for a measurement to be
    displayed, or are you monitoring for some other purpose, and if so, then
    how much range do you need i.e. 10:1, 100:1, 1000:1 etc. As usual we
    will be standing by to give you the best possible answer, as well as
    including a compendium of 10,000 ascii art drawings of typical circuits
    if that's what you desire. We aim to please you in every way possible
    and eagerly await your evaluation of our performance, maybe even
    declaring one or the other of us a "winner," the ultimate purpose of our
    existence...
     
  18. MooseFET

    MooseFET Guest

    The base resistor doesn't do a good job of setting a fixed limit. The
    HFE of a transistor varies a lot from unit to unit.
     
  19. MooseFET

    MooseFET Guest

     
  20. Alex

    Alex Guest

    Hi Michael
    I have used some highend sensor-ic's from Maxim try search for high
    end sensors at maxim.. they can do what you need.
    alex
     
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