Tuesday, March 27, 2012

How to use an Analog Meter

Bruce Coscia

One of the most puzzling things to electronics students is the mystery of the device known as an analog meter. Part of this is due to the ease of using a digital meter, as all of the numbers are just there.

For example, a measurement of 20.22 on the 200V range is just simply read as 20.22 volts. There is no need to remember if the scale is AC or DC, or for that matter which one of the scales to use. But therein lays the problem: What do we do when we are confronted with an analog meter? Do we run in fear, or tell our supervisor “I am sorry but I only work with digital meters”? The answer to those questions is an emphatic, energetic and really loud NO!!!!

Learning how to read an analog meter really is not that tough, and there are some uses that a digital meter could not (or should not) be used for. This particular topic will be covered in some minor but directed detail in the following article. Sometimes it just takes a bit of practice and focus, but the good thing is that it is relatively painless.

We will cover the different scales contained on the meter you can obtain through CIE; the good thing is that while there may be some minor differences in meters and that the one you use might not be exactly the same, the concepts and processes to read one are the same no matter what. As we move forward, we shall take a look at the ohms scale as well as the different arcs for reading DC and AC voltages.

Our journey starts with the most basic of functions, reading the resistors (measured in ohms).

This is an ohm scale and is used exclusively for reading resistance. Note that the zero is on the right hand side of this particular scale, and usually only goes to that reading when the meter leads are shorted together. The scale itself is fixed, and is only read slightly differently dependent upon the range. For example, note that the scale from zero to 1 and again from 1 to 2 is broken into the same increments (four marks = 5 equal spaces).

If we divide the change by the number of spaces, we have a change of 1 over 5 spaces which yields the following value: 1/5=0.2; this means that each line is increasing by 0.2 (0.2, 0.4, 0.6, 0.8, 1.0, 1.2, etc.). After 2 the scale changes again, now becoming 0.5 per line (2.0, 2.5, 3.0, 3.5, etc.) and remains that way until 10. At 10 we again change increments and it becomes 1 per mark (10, 11, 12, 13, etc.). At 20 we change again, but now each mark is worth 2 (22, 24, 26, etc.). At 50 we change to 5 per increment, and after 100 we go to 20 per increment.

The best way to use this is to choose a range that keeps the value between 2 and 20 whenever possible. This provides you with the most accurate measurement, but if the value cannot be scale adjusted above 20 or below 20 then it becomes a “guesstimate” at best but it is still valid. From here we need to move forward to the voltage function of our VOM while examining the difference in how we measure DC versus AC.

This meter scale shows the DC volt/amp meter setup. You will notice that instead of being one scale, there are actually three. One of the more notable differences between this and the ohms is that the zero starts on the left, and the numbers increase as we move to the right. Another difference is that these are linear scales, which means the increments do not change across the scale, unlike the ohm scale.

The top scale is the 10 scale, and each of the increments show an increase of 0.2. The middle scale is the 50, and those increments increase by 1. The last scale (and the bottom one) is the 250 scale with increments of 5. Since these scales are linear but separate, we need to use the range switch to determine which scale to use and how to multiply it.

For this example, we shall say that our meter has the following ranges: 0.1V, 0.5V, 2.5V, 10V, 50V, 250V and 1000V. Each one of these ranges uses a specific scale, and it work like this. All of the ranges that have a 1 in them (0.1, 10 and 1000) use the 10 scale. All of the ranges that have the numbers 2 and 5 in them (2.5 and 250) use the 250 scale, while all of the ranges that have just the 5 in them (0.5 and 50) use the 50 scale. The only difference in the measurements is how much we multiply by. In other words, if you think about it the ranges represent the maximum value that can be read on that range. This means that if you can picture the scale now being the same as the range - i.e. the 0.1V range uses the 10 scale, so now the 10 really represents 0.1V, and all of the increments are worth 0.02 for that range and scale combination. That makes the multiplier for that range 0.01. For the 10V range just read everything as is, and when using the 1000V range we have to multiply by 100. This same concept holds true for the other ranges and scales; the range determines the scale and the multiplier.

Last but not least, we will add all of the scales into one image. This one shows the previous two types of scales we talked about, but it also adds one more for the AC readings.

Notice that the AC scale is separate from the DC scales, and is the bottom of our picture (usually shown in red on the actual meter, while the DC is typically black). We use the same values for the AC readings but must use the lower scale.

For example, if we took this reading it could be 50 on the ohms scale, 2.8 or 14 or 70 on the DC (depending on the range), or it could be 3 or 15 or 75 on the AC (again depending on the range). The meter itself is not very tolerant of reversing connections, which is what makes it different from the digital meter so many of us have become accustomed to. It also does not provide the accuracy of a three decimal place display.

So what makes the analog meter something we should all have in our collection? IT’S THE SPEED!!! The analog meter will respond to even the quickest spikes, which shows we can at least determine if there is activity or not. The time it takes a digital meter to sample and respond means that it usually has missed the event and now we think that the whole system is dead, which may or may not be the truth of the matter.

While this is not the most extensive coverage of the meter scales, it is directly focused on the part that gives most of us the biggest cause for consternation. Reading the meter scales is not so bad once you get the hang of it, but like anything worth knowing it takes practice.

Bruce Coscia is an instructor at the Cleveland Institute of Electronics.

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