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Meter To Measure Current / Calculator Suggestions

Discussion in 'General Electronics' started by Jim Douglas, Feb 7, 2005.

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  1. Jim Douglas

    Jim Douglas Guest

    I have been looking at a new meter to measure current. I wanted another
    method to measure instead of moving cables back and forth between
    voltage/amps.

    I was thinking that as a hobby builder I would want to measure between
    1uA-5A? I am not having alot of luck with finding a meter that will go that
    low. The ones that do go that low don't go anywhere near what I would want
    on the high side. What range do others use, I am just playing around now
    learning transistors, filters, active antenna's and such with the goal of
    building a shortwave transmitter in the future.

    Also I want to get another calculator that I could program to perform
    functions. I have been checking out quite a few on Ebay and don't want to
    spend the HPXX-C model prices, any recommendations there?

    Thanks for taking the time reading this!




    Jim Douglas
    www.genesis-software.com
    Carrollton, TX USA 75006
    Latitude 32.9616
    Longitude 96.8916
     
  2. I find that I very rarely need to measure current (and I've been
    working in electronics for over 40 years).

    However, most DVMs these days will measure currents from 200 uA to 10A
    full scale, which should cover almost any requirement.


    --
    Peter Bennett, VE7CEI
    peterbb4 (at) interchange.ubc.ca
    new newsgroup users info : http://vancouver-webpages.com/nnq
    GPS and NMEA info: http://vancouver-webpages.com/peter
    Vancouver Power Squadron: http://vancouver.powersquadron.ca
     
  3. Hi,

    So what you do is to use more than one meter to cover the range
    you need. 50uA (or lower) analogue meters are easily found in
    surplus and at reasonable prices. Alternatively make one using a
    fixed resistor, an op-amp (or FET) voltage amplifier and an el-cheapo
    DVM. Generally, low current circuits are high resistance, so the
    extra series resistance won't matter.


    Cheers - Joe
     
  4. Ban

    Ban Guest

    Here is a circuit that looks a bit like an Instrumentation Amplifier with
    the property of no voltage drop. It is of course good only for small
    currents <10mA. With a FET-input opamp really small currents can be measured
    with minimum impact on the DUT.
    ___
    +-|___|-+----+
    | R |
    I | |\ |
    o----->----+-|+\ | ___ ___
    | >- +----)-+-|___|-+-|___|-+
    | +-|-/ | | | R1 | R1 |
    | | |/ .-. | | | ===
    | | | | | | | GND
    | | | |R | | |
    | | '-' | | | |\
    Ue=0 | | | | +-|+\ Uout
    | +-------+ | | | >--+----o
    | | | | | +-|-/ |
    | | .-. | | | |/ | Uout=2*R*I
    | | | | | | | |
    | | | |R | | | |
    | | |\ '-' | | | |
    | +-|-\ | | | ___ | ___ |
    V | >- +----+-)-|___|-+-|___|-+
    o-----<----+-|+/ | R1 R1
    I | |/ |
    | ___ |
    +-|___|-+------+
    R
    (created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
    view\fix font
     
  5. beast

    beast Guest

    I don't know about measuring low amps, but I though of a way to
    measure high AC voltages: By having a rated induction coil pushed
    against the measured voltage, the induced voltage would be
    proportional to the voltage present...

    Isn't that so?
     
  6. mike

    mike Guest

    HP3478A will do 3 amps. Has 1uA resolution on 300mA scale.
    My experience has been that whenever I really, really needed ACCURATE
    low current
    measurements, there were a bunch of other issues. The meter resolution
    was the least of my worries.

    HP calculators are nice if you have dedicated calculations and
    absolutely need portability. But a laptop is cheaper. Desktop cheaper
    still. There are some mighty nice math packages available.
    mike

    --
    Return address is VALID.
    Wanted, PCMCIA SCSI Card for HP m820 CDRW.
    FS 500MHz Tek DSOscilloscope TDS540 Make Offer
    http://nm7u.tripod.com/homepage/te.html
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    Bunch of stuff For Sale and Wanted at the link below.
    http://www.geocities.com/SiliconValley/Monitor/4710/
     
  7. DaveM

    DaveM Guest

    How accurate is ACCURATE? What kind of resolution is acceptable? High
    accuracy usually means high resoultion also.
    The HP 428B DC Current Meter is an older vacuum tube technology instrument,
    but it has a clamp-on probe that will let you measure current without having
    to open the current path to measure. Full-scale ranges from 1ma to 10A.
    +/- 3% full scale on all ranges. Find them on Ebay from $25 - $150.
    --
    Dave M
    MasonDG44 at comcast dot net (Just subsitute the appropriate characters in
    the address)

    Never take a laxative and a sleeping pill at the same time!!
     
  8. I have a hung chang analogue meter that measures (dc) current from 5 mA to
    10 A, which I think is sufficient.In all my life I needed to measure current
    only once, when I needed to find out if a battery charger provided the right
    charging current.I trashed my digital multimeter, after finding out it was
    incapable of measuring leaking resistance (hundreds of kohm) correctly.5 mA
    is quite a small current, and 10 quite a large.There are meters with a clamp
    (a small current transformer) that you put around the live conductor in AC
    and measures current indirectly.There are few meters that can measure ac
    current directly, thus it passes through them.
     
  9. SteveB

    SteveB Guest

    Just get a cheap meter that measures the low end. All you then need to
    expand the top range is a shunt resistor, you should be able to wind your
    own as the value will be very low for fsd readings of 5A or more.
     
  10. ctyguy

    ctyguy Guest

    E÷R has always done it for me, but I did use a current range checking for
    car battery leakage with ignition off once.

    FWIW forget using the current ranges, you'll only end up wrecking the DAMN
    DMM
     
  11. SteveB

    SteveB Guest

    A reasonable DMM will have an internal replaceable fuse for the current
    range to protect the meter.
     
  12. Guest

    Get a DMM with current measuring capability up
    to 10 or 20 amps. (It seems that those are the
    two most common sizes for the high side of the
    current measuring capability.) On the low side
    you don't need to measure down to 1 uA. Standard
    for these meters seems to be 200 uA. Almost any
    decent DMM you buy has current measuring capability.
    Even the $2.99 (on sale) Harbor Freight meter
    that Roy mentioned measures 200 uA to 10 Amps. I
    think it may be on sale now. By the way, that is
    a great buy - it even comes with a 9V battery
    included. It's great for non-critical measurements,
    and for "kick-around" use where if it gets damaged
    or lost it doen't matter. And the thing is
    surprisingly accurate.

    If you do enough messing around as a hobbyist, you
    will certainly run into occasions where you want to
    measure the current.

    Ed
     
  13. Jim Douglas

    Jim Douglas Guest

    OK, you guy's with the Harbor Freight got to me and I just completed the
    order for (5) of the $2.99 meters. I was going to order more and put em on
    Ebay. I will have some to give to other's when they ask some questions about
    electronics. Thanks!
     
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