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current measurement

Discussion in 'Electronic Basics' started by prad, May 7, 2006.

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  1. prad

    prad Guest

    how can we measure the current flowing in a circuit without using
    ammeter
    can we have to use a voltage to current convertor?
    if so what are the possible solutions to do so?
    regards
    pradheep
     
  2. Tim Williams

    Tim Williams Guest

    Magnetic field?
     
  3. Ralph Mowery

    Ralph Mowery Guest

    What exectally are you trying to do ? YOu can insert a resistor and measuer
    the voltage across it and calculate the current.
     
  4. Chris

    Chris Guest

    Hi, Pradheep. The easiest way to measure current without an ammeter is
    by measuring the voltage across a series resistor. This works well for
    DC and AC (for higher frequencies, you have to get non-inductive
    resistors). Accuracy is solely dependent on the precision of the
    resistor and the accuracy of the voltmeter. This becomes rather
    wasteful of power at higher currents.

    Another way of measuring either AC or DC current is to use a Hall
    effect sensor to measure the magnetic field, and from that infer the
    current. Commercial hall effect current sensors are available which
    can give you better than 2% accuracy from DC to about 10KHz. These
    sensors generally are made to work on larger currents (more than an
    amp).

    If you are measuring AC current, you may also use a current
    transformer. Accuracy can be better than 1%, and the current and
    frequency range is usually specified by the manufacturer. Make sure
    not to leave the secondary of the current transformer unterminated.

    Good luck
    Chris
     
  5. Phil Allison

    Phil Allison Guest

    "Chris"

    ** The Hall Effect sensors I use ( from Lem) operate up to beyond 100 kHz,
    have better than 1% accuracy and linearity and resolve down to 1mA or
    better.

    http://www.farnell.com/datasheets/51396.pdf


    Dunno where your crummy ones are from .....



    .......... Phil
     
  6. redbelly

    redbelly Guest

    If your DVM has a decent ohmmeter, you don't need to worry about the
    resistor's precision.

    Mark
     
  7. Chris

    Chris Guest

    Hi, Mr. Allison. I'll use Allegro for industrial current measurement.
    As I remember, the Lem units are a bit pricey. I'll definitely check
    them out again now, though.

    Thanks for the heads-up and the link, sir.

    Cheers
    Chris
     
  8. Phil Allison

    Phil Allison Guest

    "Chris"

    ** These the ones you mean ?

    http://www.allegromicro.com/datafile/0750-050.pdf


    That 14 mV noise spec is a real horror !

    13% overall accuracy is a worry too.

    Shame those poor folk in "industry" are so hard up they cannot afford
    something decent.

    :-0




    ......... Phil
     
  9. Phil Allison

    Phil Allison Guest

    "Jamie"

    ** I suppose measuring the voltage at both ends of a current shunt and
    doing the math IS how a code scribbler would do it.





    ......... Phil
     
  10. Pooh Bear

    Pooh Bear Guest

    Rather than pose vague hypothetical questions like this, is there a
    practical problem you have that you need to solve? Would you care to
    describe it if so ?

    Graham
     
  11. Pooh Bear

    Pooh Bear Guest

    The ohms measurement on most DVMs is not greatly accurate.

    Graham
     
  12. Jamie

    Jamie Guest

    if your working with AC then a CT (current Transformer) works well.
    if your working with DC then a hall type current sensor also works
    well.
    but maybe that isn't what you want ?
    if you have the options of inserting something in the line then you
    could use a shunt type series resistor and measure the voltage at both
    ends and do the math
     
  13. redbelly

    redbelly Guest

    How inaccurate are we talking about? I would think it's better than
    the 1% of a precision resistor. If most meters are worse than that,
    I'm surprised.

    Mark
     
  14. Phil Allison

    Phil Allison Guest

    "redbelly"

    ** Forget it - the Poopie Bear cretin does not deal in facts.


    ** Absolutely .


    ** You are being TROLLED by the fourth bear.

    The one Snow White told to **** OFF !!





    ........ Phil
     
  15. Chris

    Chris Guest

    Hi, Mark. A Fluke 77 (typical handheld DMM) can be typically expected
    to get around 1% measurement accuracy for resistors over a couple
    hundred ohms.

    However, current shunt resistors are typically in the low ohm or
    milliohm range, and it's practically impossible to measure these
    accurately without a Kelvin measurement.

    If you have a precision wirewound resistor (0.1% 100 ohm) and a stable
    variable DC power supply, you can make a fairly accurate Kelvin
    measurement with that Fluke 77 by setting the voltage to allow a
    precise current to flow through both the precision wirewound and the
    resistor under test. You can then accurately measure the voltage
    across the test resistor, inferring the resistance to the precision of
    the 100 ohm resistor and the accuracy of the meter. If you're using a
    Fluke 77 on the 200mV range and an 0.1% resistor, you might be looking
    at 0.2% tolerance on your measurement.

    Of course, if you have a bench DMM with built-in Kelvin measurements
    (as I guess you do), that makes your job a lot easier, as you said.

    Cheers
    Chris
     
  16. Pooh Bear

    Pooh Bear Guest

    What meter do you have ? My Fluke's worst performance is on the ohms range.

    Graham
     
  17. Chris

    Chris Guest

    Sadly, yes. :-(

    Cheers
    Chris (hired hand for "those poor folk")
     
  18. Phil Allison

    Phil Allison Guest

    "Chris"

    ** That is very conservative.

    The Fluke 77 spec is for 0.5% basic accuracy and 0.1 ohms resolution.

    Means even a 20 ohm resistor can be tested to within 0.5%.

    DMMs use a *ratio* method to test resistors - ie the external one is
    compared with an internal, reference resistor.

    The reading is just as accurate as that reference +/- 2 digits on the
    display.

    One can verify the accuracy of the meter ANYTIME by keeping a few 0.1%
    resistors on hand to check the range in use.




    ......... Phil
     
  19. redbelly

    redbelly Guest

    I have an Extech model 380282. (Doesn't have the name recognition that
    Fluke has, but it's a pretty good meter and cost 300 $US new.)

    Mark
     
  20. redbelly

    redbelly Guest

    That's a good idea, I'll plan to get a few next time I order parts.

    Thanks Phil,

    Mark
     
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