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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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