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

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

1. ### greenwanderer108Guest

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

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

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

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?

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www.Electronics-Related.com

2. ### Tom BiasiGuest

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 FieldsGuest

---
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_tomGuest

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.