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Multimeter AC and DC Misuse
Name: Jan
Status: Student
Grade: 9-12
Location: NV
Country: United States
Date: March 2007
Question:
I was using my multimeter to measure the voltage
from an electric socket and found out that on DCV setting the
voltage was .5 and then drops to -.5. On ACV setting the voltage
was 118 volts but never changes to negative. However, I read that
ACV was always changing and DCV should always stay constant. How
is this possible?
Replies:
The multimeter circuitry does not let you use the DC setting to
measure AC voltages. In fact, in some cases it can damage the
meter. What the AC setting is measuring is the root mean square
voltage of the AC signal. It has an electronic "inverter" in the
circuitry that "flips" the negative voltage into a positive signal,
so that what the meter "sees" is a series of "half sine waves".
Vince Calder
Jan -
The reading you get for AC on the DC scale is unreliable. Likely it
is really zero, but the slight variance in the measurement gets you
less than a volt + or -. Wall voltage is set by your power company
and is usually about 118. You run equipment that is listed for 115
or 120 volts on it and they work fine.
Because AC is continuously changing, at any instant it will be
different. The voltage changes so fast that your meter can not show
it. It cycles 60 time per second - we call it 60 Hertz. There are
two ways of measuring AC - peak to peak and RMS (stands for root
mean square). What you are reading on your meter is RMS. That is
kind of like an average.
There is a lot more to the story. What comes to your house is
likely 236 volts peak to peak. Your power box splits it into two
"half-wave," 118 volt sides. Half wave means that you really have a
pulsating DC instead of real AC. However, it acts like AC in your
equipment. If you look outside your house you will see three wire
coming in. Between two of them there is a potential difference of
236 volts. The third wire is a ground wire and the potential
between it and either of the other two is 118 volts.
That is likely more than you needed to know, but let me add just a
bit more. Many countries in the world use DC instead of AC. Thomas
Edison thought the US should do the same. The biggest advantage of
AC is that it can be stepped up or down with a transformer... what
started your inquiry out. Transformers only work on AC.
Larry Krengel
Hi Jan,
Measuring an AC voltage using a high quality, accurate
voltmeter, that set to measure DC Volts, will by definition
result in a reading of 0 Volts. The reason you read a small
residual DC voltage when measuring an AC voltage, was because
of small inaccuracies in your meter. Any true AC voltage has
absolutely no DC component.
The 115 to 120 Volt household AC voltage actually swings
sinusoidally (or smoothly) from zero volts to a peak of about
165 Volts, then back to zero and continues to a peak of -165
Volts and back to zero again. It repeats this 60 times a
second here in North America. It is a good thing that your
meter does not record these very rapid changes, because they
are much too fast for you to follow, and the result would be
just a blur!
An AC Voltmeter is designed to read the so-called "RMS Value"
of the AC voltage. RMS means "Root Mean Square", but you can
think of this as the "average" value of the AC voltage, when
all the peaks and valleys of the rapidly changing AC voltage,
are smoothed out. Think of a car that is constantly
accelerating from 0 to 100 MPH, then back to zero, and
repeating this constantly. In spite of these large speed
fluctuations, the car will have an average speed much lower
than its "peak" speed of 100 MPH.
It is a similar situation with measuring AC voltage. It is of
little interest to know what the peak voltage is, when it is
reached only for a split second before the voltage starts to
fall again, each of the 60 times a second this occurs. It is
much more useful to know what the average voltage is when the
voltage peaks and valleys are averaged out. Just as the
average speed of the car in the above example is needed to
determine how long it will take to travel a certain distance,
the RMS (or "average") value of the rapidly changing AC
voltage is the only voltage value useful to determine how
much electrical power can be delivered. This is why an AC
voltmeter is designed to show this steady RMS (or "average")
voltage value.
The RMS value of the AC voltage in a normal wall outlet is in
fact about the 118 Volts you measured. This is the average of
the rapid changes between 0 V and 165 Volts. The "negative"
you referred to is simply that the electrons momentarily
reverse their direction. The power delivered is the same no
matter which directions the electrons go. Therefore the words
"positive" and "negative" have no meaning when talking about
AC voltage.
Regards,
Bob Wilson.
Jan - AC voltage is changing so fast it repeats itself exactly, every
1/60th of a second.
Meters, on the other hand, are for the human eye to read, much slower than
1/60th sec,
so they must do some kind of averaging over a longer time,
more than 0.1 second and often less than 1 second.
That is still dozens of AC cycles.
The digital multimeters tend to re-measure 2.5 times per second, (0.4
seconds).
You can see the up-dates of the display,
whenever the reading is not exactly the same as the previous one.
Of the 0.4 seconds, you can assume that it was actually
weighing and averaging the AC voltage for at least 0.2 seconds.
Think about envelope functions:
suppose there is a sine wave, and a computer can measure all the
instantaneous voltages and
average them or whatever, and tell you a single number answer for the
"size" of the sine-wave.
So what functions do you tell your computer to use to print out the peak
voltage of that sine wave?
Different voltmeters do it different ways:
rectifying (taking absolute value) and averaging over a window-time,
or maybe squaring and averaging and taking the square root again
("root-mean-square", "RMS").
Notice both absolute value and RMS never go negative.
The size of a wiggle is either zero or positive.
That's how we define it in our own minds, then we make meters to match.
The reading on an AC meter can appear very steady if slower things are not
changing,
i.e., if the wiggles are steady.
If your AC power is steady and there are not ten different big loads cycling
on and off,
making voltage drops in the small resistance of the house-wires,
then the sine waves repeat themselves very exactly, without changing size a
little now and then.
And the reading on a DC meter can be quite unsteady if slow things are
changing.
Meters are there to report on _something_ real,
and would not be doing their job if they could not change display in response
to changing stimuli.
Besides, the real DC voltage of the AC line is supposed to be simply zero.
One of your wires goes back through the house breakers, up to the power line,
through a transformer up on a power-pole, and back down to your other wire.
Copper-metal-wire all the way, including inside the transformer,
no batteries or diodes or anything that makes a DC voltage.
Transformers can induce wiggles in the voltage on a wire, so they can make
AC,
but they cannot make long-term one-sided voltages. Their DC average output
must be zero.
For DC they are pretty much a short circuit.
If there is a real DC voltage then something is wrong or not very good.
And your meter may have a difficult time seeing the exact (0.01v) zero
average
of a thrashing big 340v peak-to-peak 60/second sine-wave.
If it is averaging for 1/3 second at 60waves/second, that's about 20 waves,
could be 20.5,
then the extra 0.5 wave could have a + excursion without a - excursion to
balance it (or v/v),
and the meter would have to report 1/40th of 120v as its DC average.
That would be 3v. You only saw 0.5v.
So maybe your meter includes some low-pass filtering before the rectifier,
or maybe the averaging window happened to be more like 20.2 waves long at
60Hz.
We never quite know these things; the printed specs usually aren't that
detailed.
There are better ways it could be done; perhaps you can imagine them
yourself.
By the way, I do not recommend using DC ranges, especially low ones (i.e.,
0.2vDC), on the AC line.
120VAC is a sine wave that goes to + and - 170v (120v x 1.414),
and the resistors in the input of your meter might not be enough
to protect sensitive IC's inside if the meter is set to 2v or especially
0.2v.
Some meters might get burned out. Analog meters especially,
because they use smaller resistors to get more sensitive for the
low-voltage ranges.
Ohms ranges can be burnt out by the AC line too.
The safer way to do it is
set a high AC voltage range, and if that's near zero
set a high DC range, and if that's near zero too,
then go down a range or two and try both AC and DC again.
This way you never get blind-sided by a large voltage that your meter
could not see.
Jim S. Swenson
Jan,
First, I will tackle the AC question, as it will help explain the DC answer.
AC, or Alternating current, is in a constant state of reversing itself.
Much like waves on the water, there are points where it is higher
than what you would find on still water, and points where it is
lower. The AC volt reading you read is an average of just how great
this variation is from one side to the other. Since the meter is
reading the magnitude of the difference, without regard to which
line is the high or low at any given time, there is no need to
display these as positive or negative voltages.
DC, or Direct current, is what moves in a single direction. As the
direction can be measured as well as the magnitude of the difference
between the lines, a positive or negative value can be attached to
compare one line with the other. To apply the water analogy I used
above, DC would be more like water flowing in a stream or river.
Ryan Belscamper
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