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Digital Multimeter : General / Basic questions (DC amps/volts)

Discussion in 'Electronic Basics' started by greenwanderer108, Oct 2, 2005.

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  1. In my electronics enlightenment journey, I find myself indulging into th
    fundamentals of understanding the basics of circuits---starting wit
    understanding DMM readings.

    I want to make sure I have the fundamentals first before I blow any fuse
    or circuits from making simple measurements.

    Surfing the various threads regarding the Digital Multimeter, I understan
    that Volts must be measured in paralell with the black lead (com) on th
    negative and the read lead (Volt 200mA max)

    And amps must be measured in series with the actual current runnin
    through using blac (com) and red (either 10 ADC 10 amp max unfused or th
    Volt-ohm 200 mA max fused)

    As far as measuring VDC. With the DMM set to 20 DCV, I measured som
    fairly new batteries (one 9v and 6 1.5volt AA size in series = 9volt)bu
    for both of these the readings showed less than 9v...aprox 8.3 - 8.
    volts. Then I tested an old 9volt battery that couldn't even power th
    DMM. This gave a reading of a little more than 4 volts. So why do voltage
    vary. I understand that batteries are depleted of amp/hours, etc., bu
    wasn't aware that it applied to voltage. Can someone put light to thes
    observations...

    Now, the tricky part... Measuring amps. I haven't yet gotten to the leve
    of measuring individual components, resistors, transistors, diodes, etc
    so no need for the related ambiguous jargon just yet . Anyhow, The scale
    confuse me. On my particular model (YUGO DT-830B)the DCA selection is 20
    microamps, 2000 micro amps, 20 miliamps, 200 miliamps, and 10 amps.

    Does, 10 amps mean that it's the maximum amps it can read, 200 miliamps i
    the maximum amperage this selection will take...or??? What if we don't hav
    a clue how much amps a certain load draws, let alone how to read the DM
    scales.

    For example, I hooked up a 3 volt motor with 2 AA batteries. As I don'
    have a clue whether a 3 volt motor will draw more than the maximum 200 m
    of the one red terminal, I assume it was better to use the 10 amp ma
    unfused red terminal. So the positive terminal from the battery with th
    red lead, and the black lead in series on to the load (motor).

    All kinds of varying readings jumping around. And I'm not sure how t
    interpret them. With the DMM set to 10A, I get average readings betwee
    .09 all the way up to .25 Then with it set to 200 m, the readings ar
    between 1.1 to 2.8.

    So how do I interpret these into amps/miliamps? The assumption is tha
    such a small motor can't possibly be drawing 1 or 2 amps, well, I don'
    really know. That's why I'm posting.

    So on top of interpreting the confusing scales of amps, also I would lik
    to know why I can't get a steady still reading of volts or amps with th
    running motor.

    Also, is it possible to measure amp hours of battery cells with the DMM
    If so, how must one proceed?

    Thanks in advance.





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  2. Tom Biasi

    Tom Biasi Guest


    The batteries increase their internal resistance as they are depleting.
    Think of them as a voltage source with a resister in series.
    The ranges on your DMM show maximum for that setting.
    If you are set to the 200mA range, the meter will measure 200mA maximum on
    that range.
    A reading of 1.1 on that scale means 1.1 mA. If you load that little motor
    even slightly the current will go up.
    You can not measure AH with that meter. The best that you can do is load the
    battery with its rated load and see if the voltage drops below the
    manufacturers spec.
    Good Luck,
    Tom
     
  3. John Fields

    John Fields Guest

    ---
    No. There is no such thing as "Volt 200mA max".

    Here are the meter's spec's:

    DCV: 0-200m ±0.25% -2-20-200-1kV ±0.5%
    ACV: 0-200V-750V ±1.2%
    DCA: 0-200u-2m-20m ±1.0% -200mA ±1.2% -10A ±2.0% Fused
    R: 0-200-2k-20k-200k ±0.8% -2Mohm ±1.0%
    hFE measurement
    Transistor test
    Diode test
    Battery: 9V


    Looking at DCV (DC Voltage) we have 5 ranges:

    0-200mV ±0.25%
    0-2V ±0.5%
    0-20V ±0.5%
    0-200V ±0.5%
    0-1000V ±0.5%

    When you measure voltage you place the leads in parallel with
    whatever you want to measure, but the placement of the leads isn't
    critical since all that will happen if you get them backwards is
    that the meter will indicate a negative voltage. What you _should_
    do is place the black lead (after making sure that it's plugged into
    the black jack on the meter) on what you want your reference to be.
    ---
    ---
    Or any of the other current ranges.
    ---
    ---
    As someone else posted, when batteries become depleted their
    internal resistance rises, so the voltage they can deliver to a load
    decreases because of the votage drop across the internal resistance.
    ---
    ---
    The different current ranges indicate what the maximum current is
    for that range before the meter will indicate an overload. The 10
    amp range is unfused, so you can put a great deal of current through
    that range. Be especially careful on that range, since it's totally
    unprotected and you could damage the meter, or worse, if you let a
    lot of current through there.
    ---
    ---
    That's exactly right. If you're unsure of the voltage or current,
    always start on the highest range and work your way down.
    ---
    ---
    With the meter set on the 10A scale, the readings are displayed in
    amperes, so .09 would be 0.09 amps (90 milliamps) and .25 would be
    0.25 amps (250mA, or one quarter of an amp).

    On the milliamp scales, the readings are displayed in milliamperes,
    so 1.1 would be 1.1 milliamperes, and 2.8 would be 2.8 milliamperes.
    ---
    ---
    The motor is making a huge amount of electrical noise when it's
    running, disrupting the meter readings. Try putting a large-valued
    electrolytic capacitor (10,000µF or more, and watch the polarity)
    across the motor and see what happens.
    ---
     
  4. w_tom

    w_tom Guest

    On a chart with amp-hours across the horizontal axis, and
    with voltage on the vertical axis, then a battery curve tends
    to hold voltage constant until battery approaches end of
    life. Then battery voltage starts dropping quickly. What
    voltage is too low? For a nine volt battery, some appliances
    declare a battery at 8.7 volts or lower to be dead.

    Voltage to amp-hour relationship requires that chart. Also
    that relationship changes when a rechargeable battery wears
    out. This curve between voltage and amp-hours is unique for
    different battery technologies.

    For meter readings - always go to the maximum setting for
    first measurement. With experience, then one has a feel for
    what need be set. Voltage half of meter is very forgiving.
    Current side of meter is not. Always go to maximum setting -
    10 amps - for your first current measurement.

    You will learn that some batteries of same voltage but
    different technologies put out large currents whereas others
    output low currents. First type are designed for high power
    applications - ie electric drill. Second type for long term,
    low power applications - ie smoke detector. There are even
    high current and low current versions of NiCd batteries. So
    which current setting do you use first? Highest because you
    don't know and have respect for your meter.

    BTW, don't take a current measurement of lead acid
    batteries. They are designed for many tens or many hundreds
    of amps - will blow out the current meter fuse and maybe
    more. Smaller batteries have higher internal resistors that
    will keep current below 10 amps. All batteries contain
    internal resistance which is why they have a maximum short
    circuit current - as measured by the amp meter.
     
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