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

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View



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

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

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

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

As far as measuring VDC. With the DMM set to 20 DCV, I measured some
fairly new
batteries (one 9v and 6 1.5volt AA size in series = 9volt)but
for both of these
the readings showed less than 9v...aprox 8.3 - 8.7
volts. Then I tested an old
9volt battery that couldn't even power the
DMM. This gave a reading of a little
more than 4 volts. So why do voltages
vary. I understand that batteries are
depleted of amp/hours, etc., but
wasn't aware that it applied to voltage. Can
someone put light to these
observations...

Now, the tricky part... Measuring amps. I haven't yet gotten to the level
of
measuring individual components, resistors, transistors, diodes, etc.
so no need
for the related ambiguous jargon just yet . Anyhow, The scales
confuse me. On my
particular model (YUGO DT-830B)the DCA selection is 200
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 is
the
maximum amperage this selection will take...or??? What if we don't have
a clue
how much amps a certain load draws, let alone how to read the DMM
scales.

For example, I hooked up a 3 volt motor with 2 AA batteries. As I don't
have a
clue whether a 3 volt motor will draw more than the maximum 200 mA
of the one
red terminal, I assume it was better to use the 10 amp max
unfused red terminal.
So the positive terminal from the battery with the
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 to
interpret
them. With the DMM set to 10A, I get average readings between
.09 all the way up
to .25 Then with it set to 200 m, the readings are
between 1.1 to 2.8.

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

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




        
This message was sent using the sci.electronics.basics web interface on
www.Electronics-Related.com

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



Quoted text here. Click to load it


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



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


On Sun, 02 Oct 2005 05:39:15 -0500, "greenwanderer108"

Quoted text here. Click to load it

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

Here are the meter's spec's:

We've slightly trimmed the long signature. Click to see the full one.
Re: Digital Multimeter : General / Basic questions (DC amps/volts)


  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.

greenwanderer108 wrote:
Quoted text here. Click to load it

Site Timeline