Just to make sure we’re all tap-dancing to the same drum beat (and I know whereof I speak, because my dear old dad used to be a dancer on the variety hall stage prior to WWII -- see The Times They Are a-Changin’), let’s remind ourselves that in Part 12 we introduced two multimeters -- a regular digital multimeter that I picked up from RadioShack years ago, and an Auto-Ranging device I purchased from Amazon able to talk about in these columns.
At the end of Part 12, I recommended that you purchase a resistor kit spanning a range of resistance values with a 5% tolerance. So, let’s start by selecting two resistors from our kit: a 1 kΩ (1,000 ohms) with brown-black-red bands and a 10 kΩ (10,000 ohms) with brown-black-orange bands (we described the colored bands in Part 11).
For our initial experiments, we’ll use our regular multimeter, which -- in my case -- is my cheap-and-cheerful RadioShack device.
The reason I call the RadioShack device a “regular” multimeter is that it’s up to the user to use the rotary switch to select the most appropriate voltage, current, or resistance range before making the measurement. In the case of resistance (the “OHM” area of the rotary switch), which is the topic of this column, we have five options: 200, 2K, 20K, 200K, and 2M ohms.
Why do we need these options? Well, selecting the appropriate setting allows us to make the most accurate measurement. This can be confusing if you are a beginner, so let me give you an analogy. Suppose you have two devices you can use to measure the length of something -- let’s say a small ruler with millimeter and centimeter markings and a piece of rope that’s 10 meters long with splashes of paint to mark every meter.
Now, let’s assume you want to measure the diameter of a penny coin. Not surprisingly, you will obtain the best results if you use the small ruler. By comparison, if you wish to determine the distance between your house and the home of a friend who lives at the far end of a long, straight street, then you’ll find the rope better meets your measuring requirements. Remember that analogies are always suspect, and this one doubly so (for all sorts of reasons), but the underlying concept is sound.
Another way of thinking about this is that, at the core of an ohmmeter (the portion of a multimeter used to measure resistance) is a potential divider formed from two resistors we might call R1 and R2 (potential dividers were introduced in Part 7).
Let’s suppose that the value of R1 is known because it’s inside the multimeter, while the value of R2 is unknown because it’s the one we’re trying to measure. For the purposes of what we are trying to do here -- that is, measuring the value of R2 as accurately as possible -- it will make our task easier if the values of R1 and R2 are relatively close together. By comparison, if the value of R1 is say vastly different to that of R2 (say 100 times larger or 100 times smaller), then this will make the task of measuring R2 with any useful level of precision much harder.
Take another look at the previous image. We’ve selected the 20K option, but the probes aren’t connected to anything yet, so the meter displays O.L. As we noted in our previous column, this means “Open Line” (some people may say “Open Loop,” while others may say “Open Circuit”), and it is used to indicate infinite resistance when the probes aren’t connected to anything or if there is no conducting path between whatever the probes are connected to.
Interestingly enough, the actual “OL” presentation depends on the selected range. Even more interesting (at least to me) is that the first time I actually noticed this was when I started to write this column. The various display formats for my meter are shown below (I have no idea if other meters do things the same way).
OK, now let’s measure the actual value of our 10 kΩ resistor. Remember that we are currently experimenting with 5% tolerance components, which means we expect a value between 9,500 Ω and 10,500 Ω (see also Part 11 for additional discussions on this topic).
The way we do this is to set the selection on the multimeter to the nearest value above the one we are trying to measure, after which we apply the probes to either side of the resistor to be measured.
Observe that resistors are non-polarized components, which means it doesn’t matter which way round we connect them or -- in this case -- to which leads of the resistor we connect our black and red probes.
Since we are attempting to measure our 10 kΩ resistor, we set the multimeter to the 20K option. In my case, the value displayed on my multimeter was 9.72, which we take to mean 9.72 x 1,000 = 9,720 Ω.
If we set the multimeter to any option smaller than the value we are trying to measure, then it will display some version of OL indicating that the value is out of range. By comparison, the multimeter will give valid readings for any of the options that are higher than the value we are trying to measure, but we will lose accuracy the further away we wander from the value we are trying to measure. In order to illustrate this, I measured my 1 kΩ and 10 kΩ resistors using all of the settings on my multimeter; the results are shown below:
As we see, it can take a little effort to wrap your brain around the task of reading and deciphering the results from the regular meter. It helps if you know the value you are expecting and select the appropriate option on the meter, but what if you have a resistor without any markings or one you believe to have failed? In this case, your best option is to start al the lowest setting (200 on my meter) and work your way up until to reach the first non-OL display.
As an alternative, of course, you might decide to opt for an auto-ranging meter, like the one shown below.
In this case, all you have to do is select the Ω (resistance) setting, and off you go. Now, this is the clever part, because the auto-ranging meter adopts a similar strategy to the one discussed above for the unknown resistor. That is, it switches in different internal resistance values, starting with the lowest and increasing the value until it reaches the first value that returns a non-OL result, which it then displays. Even better, it actually presents you with Ω, kΩ, and MΩ annotations, which makes things much easier.
In my case, my auto-ranging multimeter returned values of 0.979 kΩ (i.e., 979 Ω) for the 1 kΩ resistor, and 9.74 kΩ for the 10 KΩ resistor.
So, which sort of multimeter should you choose? You might be saying to yourself that opting for the auto-ranging device is a “no-brainer” -- and I certainly agree that it will make your life easier if you are a beginner -- but things are not quite so clear-cut as you might suppose.
For example, it takes a little time for the auto-ranging multimeter to select the most appropriate range. This probably won’t be an issue in most cases, but it can cause problems if the value of the resistance is changing, which might be expected (in the case of a light-dependent resistor (LDR), for example) or unexpected (in the case of a failing component exhibiting intermittent shorts, for example). The problem here is that such changes may come and go before the auto-ranging multimeter has time to detect and respond to them, thereby leaving you unaware that such a change occurred at all.
At the end of the day, it’s a case of “You pays your money, and you takes your choice,” as the old English proverb says, which -- in this context -- means that whichever type of multimeter you decide to use is up to you.
In our next column, we are going to start playing with real world circuits and experimenting with light-emitting diodes (LEDs). Oooh, Shiny! Until that frabjous day, as always, I welcome your comments, questions, and suggestions.