A Short Guide To Multimeters

Short guide to choose the right multimeter for your needs.

Picture showing a number of multimeters, both digital and analog.

The very first instrument that an hobbyist buys for the electronics lab is the Multimeter. There are several choices in the market and it is very difficult to make the right decision.

Sometimes it is the amount of available money that forces our hand when choosing the instrument, and sometimes the inexperience drive us to make the wrong judgement.

In this article I will try to provide a set of useful information that will help you make an informed decision. But, always remember that the cheapest solution is not always really the cheapest one. When you are uncertain between a couple of models, always choose the one with the best features. That will avoid future regrets, the moment you realize that the missing features where those that you needed the most. Making the wrong choice may force you, later on, to buy a second instrument because you realize the one you got does not satisfy your needs.

Let’s start with the first categorization: analog versus digital.

In this era of digital devices permeating the market in all electronics categories, the choice of buying a digital multimeter seems obvious. But is it? We need to explore a number of properties and features of both kind o devices. Only after you know pros and cons of each, you will be able to make a decision based on what you envision being the usage that you’ll make with the instrument. And, sometimes, you’ll find that having both kind of instruments is better that having only one.

Picture of a digital multimeter measuring volts.

A digital instrument is definitively easy to read, especially if it has a large display, and it is even better if it tells you the measurement unit along with the value, like in the one depicted on the left. On the other end, to be able to show you a number, the digital instrument needs to make an analog to digital conversion, which takes time to complete. After all the quantity under measurement is always an analog one.

The problem with the analog to digital conversion is that it takes time; it is not instantaneous. As a result, if you are measuring a quantity that slowly changes over time, you could end up with a situation where the quantity changes before the instrument has completed the conversion. In such a case, you would not be able to see any useful reading on the display. The numbers would keep change continuously and inconsistently.

Picture of an analog multimeter measuring volts.

An analog instrument, on the other end, is not that easy to read. Depending on the type of measurement you have to make, and how big is the value under measurement, you will have to find on the display the right scale to use, and that can be confusing, sometimes. The good thing about this instrument, however, is the fact that the needle always responds instantaneously to the quantity being measured.

It might take some time for the needle to reach the right position on the scale, because of its inertia. But that same inertia allows the needle to find itself an average of the value if the quantity is slowly changing over time. Not that the measurement will be precise. It will not, but it will give you an idea of what is going on, while with the digital instrument, you would just see a bunch of numbers continuously changing in a pseudo random way.

Is then the analog instrument better than the digital? Well, wait a minute here. There are still several parameters to evaluate.

Let’s talk about precision. Both digital and analog instruments can have a pretty good precision, but with the analog instrument sometimes you have to guess the best reading, because the needle is not exactly on a mark on the scale, but is instead in between two marks. And things get even worst when accounting for the parallax error. When you look at the mark underneath the needle, depending on the position of your eyes compared to the needle itself and the scale, you may find that moving your head a little bit on one side or the other, the measurement changes. And that’s why the best analog instruments have a mirror on the display. If you look at the needle with one eye only and you position yourself in such a way that you don’t see the reflection of the needle on the mirror, then you are perfectly vertical on top of the needle and you can make a better measurement. And of course this takes time, usually more time than taking a measurement with a digital instrument. Digital instruments, on the other way, may have the precision of just a couple of digits or more. The more the better of course. For a decent lab instrument, you may want to go with instruments, either digital or analog, where you can make a read of at least 3 digits, better 4.

And after all the effort to make a good measurement, you could find yourself with a perfectly useless measure. Why? Because of the impedance that the multimeter puts in parallel to the points under test. And this bring us to yet another difference between analog and digital multimeters.

The majority of analog multimeters present a low input impedance, which means that they draw a certain amount of current from the test points. And this extra current could cause an extra drop of voltage across another resistor that is part of the device under test. The result would be that the reading looses accuracy.

A digital instrument, on the other end, usually has a very high input impedance, in the order of several mega-ohms, compared to the tens of kilo-ohms of the analog instruments. Just because of that, measurements taken with a digital multimeter are usually more accurate.

But then, that is not always true. There are analog multimeters that are equipped with an amplification stage with FET or MOSFET. Those analog multimeters have an input impedance comparable to the one of the digital instruments. Always read the specs of the device you are buying and try to figure out if the input impedance is in the order of the kilo- or mega-ohms. The higher the better.

To conclude, to make the right choice when buying an instrument, normally go with a digital one, unless you already know that the majority of readings you will ever make will be done on slowly changing values. Buy always an instrument that gives you the best precision you can afford, with the highest input impedance. And keep in mind that for a multimeter to be useful in an electronic lab, you want to be able to measure magnitudes very small but not necessarily that high. Shoot for an instrument that will make you read micro-amps and milli-volts, for example.

Last, if you can afford it, you could buy both a digital and an analog instrument. Analog instruments are cheap nowadays, and can be easily found also in the second hand market, like on eBay. And once you have a decent analog instrument, you don’t have necessarily shell a lot of money on a digital instrument. There are decent digital multimeters below the one hundred dollars value. Take a look at the Merchandise section of this web site for such an instrument.

If you would like to know more on this topic, and see both the analog and digital multimeters in action, I suggest you to watch this Youtube video.

Happy experiments!

Behind The Scenes Of The Theremin Design

How I design my electronic circuits and prepare the videos to show them to you.

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Did you ever ask yourself where I get the schematics of the Theremin circuits and other gadgets that I present on my YouTube videos? The answer is simple: I do some research on books, on specialized magazines and on the Internet. I see solutions created by other people, if any, and then I think about what would better work for my case. Sometimes it ends up to be a modification of something that somebody else did, maybe for a totally different purpose. Sometimes, I just use the general idea to create something different, new, my own design that is more appropriate for my needs.

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Either way, I usually build a number of prototypes of what I need, then I take some measurements in lab, then I start making further modifications to my original design, until I obtain exactly what I am looking for.

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Also, more often than not, I figure that the circuit I am testing is too sensitive to certain parameters of the circuit itself. Maybe is a capacitor which value needs to be adjusted a little bit, or a connection between two or more components that causes issues because of capacitive or inductive coupling with other components. That is when I try to change my design to reduce such sensitivities, so that the circuit can be assembled by anyone with the exact same results as mine. And this is what is called engineerization, or adjusting the design for mass production.

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And sometimes, to do so, it is not enough to test the single circuit. Instead, I need to connect the circuit with other pieces that have to work together with it, and see if further unwanted interactions happen, so that I can eliminate them or, at least, reduce them so that they become negligible.

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Sometimes this process goes fast, sometimes takes a long time. And that’s why my videos are not published at fixed intervals. Unfortunately, since this is done only as a hobby, I don’t always have enough time to dedicate to my project, so days go by until, finally, I am done. Then I finalize my schematics, I build the last prototype and the final product and, in the process, I also record all these activities so I can end up making a video out of them.

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Then the video editing process starts and, once the video is finally ready, I release it on YouTube for you to watch it.

One day I will be able to do this full time. Who knows, maybe when I retire. Or, maybe, if you all give me a hand, this could become my new full time job (donations, donations, donations). We’ll see.

Thank you for reading this article. And, as usual, happy experiments!

DC Electronic Loads

An electronic load to test DC power supply devices up to 100W.

What do you use when you have to test a new power supply that you just built, or one that you bought and want to know if the declared specs are true?

One thing you’ll need is a passive load that you attach to the power supply output to drain a certain amount of current, both to verify that the power supply is capable of providing that amount of current, and to verify the amount of ripple that was not filtered away by the power supply itself.

rheostatA classic method for doing so is to use a rheostat, which is essentially a potentiometer capable of dissipating the amount of power produced by the power supply. The resistance of the rheostat can be changed and therefore different amount of currents can be used to test the power supply. However, rheostats are big, heavy and cumbersome.

An alternative to rheostats is to have a so-called Electronic Load. These are electronic circuits that are capable to emulate the functionality of a rheostat.

I’m proposing here two simple versions of an Electronic Load, functioning in DC, and capable of dissipating up to 100W, all of this in a very condensed space, and very light in weight.

The first version is a very simplistic one.

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It uses a cascade of three transistors, in a configuration called Darlington. This configuration is effectively equivalent to a single transistor with a gain (hfe) that is the product of the gain of all the transistors in the configuration. This allows for a very little control current flowing into the potentiometer used to regulate the base current, and for a high current available between the collector and the emitter of the transistor Q3, in the above schematics.

This circuit does not need its own power supply, since it gets what it needs directly from the power supply under test. The resistor R1 is calculated based on the highest voltage that the circuit will be able to handle, in this case 50V. It is important to note, however, that using this load at lower voltages will prevent the possibility to use the full excursion of the potentiometer, thus limiting the sensitivity of the circuit for the control of the current.

The next circuit eliminates the sensitivity problem by providing it own power supply to control the base current of the Darlington, thus eliminating the dependency from the external power supply voltage.

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In fact, in this case, the base current can be adjusted with the potentiometer RV1, which is polarized through the resistor R1 and the trimpot TR1, which can be adjusted to maximize the useful range of motion of the potentiometer, regardless of the voltage applied at the input terminals. This way the electronic load can have the same sensitivity for any input voltage. A digital Volt/Am-meter, powered by the same internal battery, provides a visualization of the voltage of the system under test and the current being drained from it.

I will soon publish a video on my YouTube channel that shows the Electronic Load I built for myself. Please watch for that video to come out. And, in the mean time, can you think which one of the above schematics I used? Did I use the simple one, because less expensive? O did I sacrifice a few extra bucks to gain more sensitivity on the regulation of the load?

I strongly suggest to subscribe for free to my YouTube channel, and also to click on the bell icon that appears after the subscription is done. This will allow you to automatically receive an e-mail whenever I publish a new video. This way you won’t have to go periodically to my channel to check for new posts.

Happy Experiments!