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Measuring Current

Discussion in 'Electronic Basics' started by Jim Douglas, Jan 29, 2005.

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  1. Jim Douglas

    Jim Douglas Guest

    I currently have one nice digital VOM. So I measure voltage, then current,
    back and forth having to move the cable each time, which sucks. I want a
    stand-alone ammeter that I can quickly place within a circuit, I think. I
    would rather not purchase another meter just to measure current and am
    thinking that this might be something easy to build but didn't find alot
    searching so far.

    Do you guy's use a separate amp meter for this stuff? I am just a hobbyist
    that likes playing around with things. One goal is to eventually build a SW
    transmitter. Should I try to purchase, build? I would prefer building. I am
    thinking I would like to be able to measure down to the microampere range.

    Thanks for any input!

    --



    Jim Douglas
    www.genesis-software.com
    Carrollton, TX USA 75006
    Latitude 32.9616
    Longitude 96.8916
     
  2. Art

    Art Guest

    Suggest checking into the Amateur News Groups regarding additional
    information regarding home brew radio transmitting and receiving equipment.
    S.A. rec.radio.amateur.homebrew, etc. Most of the equipment I've worked,
    repaired, etc have an actual separate ammeter (milliamp meter).
     
  3. I read in sci.electronics.design that Jim Douglas
    An analogue ammeter with a decent milliammeter movement is costly, and
    you have to hand-craft the shunts. You could buy a bare digital panel
    meter module, which will make an ammeter with 200 mV drop at full scale
    on all ranges. This could cost less and be easier to make.

    If it were me, I'd buy another inexpensive digital multimeter.
     
  4. JeffM

    JeffM Guest

    I would rather not purchase another meter just to measure current
    Woodgate is right. Buy a cheapie.
    Got one of these in Dallas? http://www.harborfreight.com/
    $2.99 until 2/14/05.
     
  5. CJT

    CJT Guest

    I agree with the other posters that it would probably be cheaper to buy
    another multimeter. However, I would add that your desire to measure in
    the microampere range is probably wishful thinking. The equipment and
    techniques involved in _accurately_ dealing in those ranges are, IMHO,
    beyond what those of what you're likely to encounter in a typical
    hobbyist setup.
     
  6. Or learn to avoid current measurements as much as possible.
    It takes time to open a connection and insert the amperemeter, and then
    you have to restore the connection after measuring.

    In most cases you can measure voltages and calculate the currents.

    Make a few homemade shunts, like 1 Ohm and 0.1 Ohm, so you can insert the
    shunt and measure the voltage over it, when there is no other way to find
    out the current. The shunt can be a resistor or a suitable piece of wire.
     
  7. BFoelsch

    BFoelsch Guest

    Well, I do approximately what you want. I have around the shop 3-4
    miscellaneous meters in plain old sloping front panels. The meters are
    almost free at the swap meet, the cabinets are the expense, but you could
    fashion something yourself. Let's see, I have a 50 microamp, 1 milliamp, 500
    milliamp and 5 amp units.

    Leads are brought out through the top through a dual banana jack. For some
    frequently used current ranges, I put some shunt resistors across a
    stackable dual banana plug. By plugging the right shunt plug into the meter,
    I get some ranges I commonly use; 10 mA, 100 mA, 1A, 15A.

    You can laugh, but it sure beats wiring up range switches. Too, using the
    dual banana jack forces you to unhook the meter from the source of power
    before changing the shunt, which protects the meter from an open shunt
    (burn-out) situation.
     
  8. Jim Douglas

    Jim Douglas Guest

    Thanks for all the good input. I will be picking up another VOM for my
    current measurements.
     
  9. John Smith

    John Smith Guest


    Hi, Jim -

    For my current measurements, I bought an HP 428B (toob-type) off Ebay and
    repaired it. The reason I wanted it was so I would not have to break
    connections to measure current. Of course, one must have enough slack in
    the wire to allow inserting the clamp-on probe.

    Just an idea. Good luck.

    John
    (kes is not required)
     
  10. DaveM

    DaveM Guest


    Yup!!! I bought three of those HP 428B meters on ebay in one auction. One
    of them had a problem, but was easily repaired. Old technology, but they
    sure save the day when you need to jump around in a circuit and measure
    currents quickly. They are capable of current measurement from 1ma full
    scale to 10A full scale. Great equipment survives the ages.
    --
    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!!
     
  11. Ken Smith

    Ken Smith Guest

    Harbor Freight has DMMs on for $3.99. Buy one for each thing you want to
    measure.
     
  12. Ian Stirling

    Ian Stirling Guest

    I've found common multimeters fairly accurate for measuring very small
    currents.
    The typical 1Mohm input resistance at 200mV full-scale drops 200nA.
    (when I checked it, it seemed to be accurate).
    Even my cheap $4(us) meter has a 200uA range.
     
  13. Like these ones I put up (these particular ones are sold)?

    http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=3870013201&ssPageName=STRK:MESO:IT
    http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=3867723467&ssPageName=STRK:MESO:IT

    The clamp opens up and you can put it round the wire. They are *not*
    for small currents-- scores of mA up to hundreds of A, depending.

    If you put multiple wires through the core (like an AC line cord) they
    will read about zero, unless there is a fault to ground, as they read
    the difference between the two currents.

    They are great for working on power stuff- you can poke around in a
    panel and check motor currents etc., but for electronics you really
    want some DVMs.


    Best regards,
    Spehro Pefhany
     
  14. Rich Grise

    Rich Grise Guest

    It's still a PITA to break the circuit to interpose the meter. If it's
    something that I'll want to read the current for calibration or something,
    I just put a 0R1 or 0R01 resistor in the circuit, and measure its voltage
    drop.

    I've also seen that technique used to increase the range of 0-1 or 1-0-1
    milliammeters - a 1K resistor turns it into a 1K ohm/volt voltmeter - and
    you measure the drop across the shunt. Much easier than trying to match a
    shunt to a given meter's internal resistance.

    Cheers!
    Rich
     
  15. Robert Baer

    Robert Baer Guest

    Err...most DVMs have a 10Meg input resistance, not 1Meg.
    However, one can use a resistive shunt (previously suggested) and
    measure the voltage across it (to 200mV full scale).
     
  16. Ian Stirling

    Ian Stirling Guest

    Though there exist probes like this that using some assumptions about the
    geometry of the wire, and several sensors closely spaced arouind the
    cord, can actually measure the current flowing through a balanced conductor.
     
  17. Is it possible to make a hand-held probe to do this?


    Best regards,
    Spehro Pefhany
     
  18. Ian Stirling

    Ian Stirling Guest

    AIUI, the probe that I referred to was 'just' another clamp-on probe,
    for attachment for the meter of your choice.
    Sorry, I don't recall when.
    It was probably on sci.electronics, on a thread I was involved in.
    "ian stirling" multiple current wire hall
    may or may not turn up something on google.
     
  19. Jim Douglas

    Jim Douglas Guest

    I saw some of those on Ebay today, It seems to have a scope type thing that
    simply wraps around a wire??
    Are they pretty accurate, is the HP a tube based product?
     
  20. DaveM

    DaveM Guest


    No, the listings you referred to are not the HP428 DC current meters that
    were mentioned previously. The HP428 meter uses a small clamp-on probe that
    saves the user from having to open the circuit to measure current. The
    HP428 meter has full-scale ranges from 1ma to 10A. Yes, these are vacuum
    tube models, but easy to maintain and use. Well worth what you'll pay on
    Ebay for these meters if you need to measure small currents without breaking
    the circuit to insert a current meter.
    They are not the hallmark of accuracy when compared to the digital
    instruments of today's technology; +/- 3% full scale on every range, but
    will absolutely get the job done.

    The probes are not hall-effect sensors, but are esentially small
    transformers that are excited by a 40 Khz signal. The output of the
    transformer is modulated by the change in core saturation caused by the DC
    current being measured. You can see great pictures of the meter and probe
    at
    http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=25411&item=3866525538&rd=1
    (for the next couple of weeks anyway).

    --
    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!!
     
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