In Part 9 of this mini-mega-series, we mentioned the fact that we can use special tools called voltmeters to measure voltage, ammeters to measure current, and ohmmeters to measure resistance. We also have multimeters, which can measure voltage, current, and resistance, along with a variety of other things, depending on the meter.
It's possible to get all of these meters with either analog or digital displays. For the purposes of these columns, however, we will assume we are working with a multimeter of the digital variety.
A digital multimeter is one of the most useful items in any electronic engineer’s toolbox. You can spend a lot of money on professional, industrial-grade multimeters, but you can also get a very serviceable device for not much money at all. For example, I just had a quick Google (while no one was looking) and found the AstroAI Digital Multimeter (with 4.5 stars from 9,361 ratings) for only $12.99 from Amazon Prime (please note that I have NO connection with, and receive NO remuneration from, anyone associated with any of the components, devices, and tools I may mention in these columns).
Take a look at the photo below. On the left we see a regular cheap-and-cheerful digital multimeter that I picked up from RadioShack years ago and that I use all the time. Observe that this doesn’t have any ports (connectors) per se -- the black and red probe leads just come straight out of the case. On the right we see an ETEKCITY Auto-Ranging Digital Multimeter (with 4.5 stars from 973 ratings) that I purchased for $18.99 from Amazon Prime just to be able to talk about it in these columns.
Observe that both of these meters are currently set to measure resistance and are displaying different versions of “OL” meaning “Open Line” (some people may say “Open Loop,” while others may say “Open Circuit”). This 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.
With regard to the ETEKCITY multimeter, observe that this has three ports (connectors). The black probe is plugged into the COM (“common”) port. This is the probe that is usually connected to the ground (0V) or negative (-ve) part of a circuit. The red probe is plugged into the port marked “VΩmAµA,” which indicates that this port can be used to measure voltage, resistance, and current (in both milliamps and microamps). Different meters will have different annotations on this port, like “VΩmA” or “mAVΩ,” but these are just different ways of indicating the same thing.
Also observe the port with the 10A annotation. This is a special port that is intended to be used only when measuring large currents. As a beginner, you should NEVER be interested in measuring currents larger than a few hundred milliamps, so do your best to forget that the 10A port even exists.
The reason I call the RadioShack device a “regular multimeter" is that it’s up to the user to turn the rotary switch to select the most appropriate voltage, current, or resistance range before making the measurement. In the case of resistance, for example, we have five options indicating 200, 2K, 20K, 200K, and 2M ohms (we’ll discuss this more in my next column).
By comparison, in the case of the ETEKCITY multimeter, the “Auto-Ranging” part of its moniker comes from the fact that all we have to do is set its rotary switch to the Ω (resistance) annotation, and it will automatically work out the most appropriate range when we use the probes to measure the value of a resistor.
Which is best -- a regular multimeter or an auto-ranging device? Well, to a large extent this is a matter of individual preference. One way to think about this is to compare the regular multimeter to a stick shift car, while an auto-ranging multimeter is more like an automatic car. It’s a bit harder to learn to drive using a stick shift vehicle, but once you have learned it’s easy to migrate to an automatic automobile should the occasion demand. By comparison, if you know only how to drive an automatic vehicle, then you are going to have a very “bad hair day” if you ever need to take control of a stick shift car in an emergency.
In my next column, we will discuss how to use both of these multimeters to measure resistance. Of course, this means you’ll need some resistors to measure. We will be using a lot of these components in future experiments and they always come in handy, so -- in anticipation of our next column -- may I suggest you purchase a resistor assortment kit.
We’ll opt for a 1/4-watt power rating because this is common, and we won’t need anything higher for what we’ll be doing. Also, for our purposes, we can settle for 5% tolerance parts because it will be easy to read their color codes and they will match our previous discussions in Part 11. Taking all of this into account, something like this resistor kit comprising 1,800 pieces spanning 72 values will do nicely (the comments say their leads are a little on the thin and flimsy side, but overall they have a good rating).
Until next time, as always, I welcome your comments, questions, and suggestions.