Short guide to choose the right multimeter for your needs.
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.
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.
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.
What are polarity inverters and what are used for.
Sometimes we design and build a circuit that needs a dual power supply. But, in certain cases, we really need just a positive voltage to power a circuit and the negative is only used for some special polarization that doesn’t really need the same amount of power used for the positive. Consider, for example, a circuit with a depletion channel MOSFET that requires a negative voltage just for the polarization of its gate.
In such cases, it is economically better to use a different approach than having a full fledged dual power supply. This approach is called “polarity inversion”, resulting in a device that is able to convert the positive voltage of a power supply into a low current negative voltage.
A polarity inverter is, therefore, a circuit that is capable of taking a positive voltage with respect to the ground and generate a negative voltage also with respect to the ground, so that we can have both a positive and a negative voltage available at the same time to power another circuit, without using a dual power supply.
In principle, the inverter is based on the following circuit.
There are two capacitors and two diodes, and a switch that connects the positive of the first capacitor alternatively to the positive voltage source and to the ground.
When the switch is set toward the positive voltage, capacitor C1 starts charging through the first diode, which closes the circuit toward the ground. Given enough time, the voltage at the capacitor increases up to the input voltage minus the voltage drop on the diode.
For example, if the input voltage is 9V, the capacitor will charge to about 8.4V.
This is represented in the following diagram by the first pulse on Vin and the corresponding voltage on C1.
Now, once the capacitor is charged, we move the switch toward ground. Doing so, we open the circuit that connects capacitor C1 to the input voltage and, instead, we connect the same end of the capacitor toward ground.
This way, the voltage at the capacitor C1 is now providing a forward polarization to the second diode, the one on the right, and therefore we have a closed circuit that goes from capacitor C1, to capacitor C2 and through the second diode.
If we choose the two capacitors with the same capacitance, half of the charges on capacitor C1 will transfer to capacitor C2 and, as a result, both capacitors C1 and C2 will end up with half of the original charge and, therefore, with half of the original voltage that was on C1.
This is represented by the second part of the above diagram, where now the input voltage is zero, but capacitors C1 and C2 are at half the original voltage.
On the next cycle, we move the switch back toward the power supply, so capacitor C1 is again charged to the input voltage. In this case, however, the second diode is inversely polarized, so capacitor C2 is isolated and cannot either charge nor discharge, thus it keeps the previous value of voltage.
Moving the switch back to the ground, C1 gives now some more charge to C2 and, therefore, its voltage drops a bit while C2 voltage, instead, increases more.
And you can now see that if I keep switching back and forth, adding more cycles to the diagram, both C1 and C2 keep retaining more and more charges, and their voltage keep increasing so that, after a number of cycles, C2 has reached about the same voltage as the input.
Now, note how capacitor C2 is connected to the ground on its positive side, and the other end is offering its negative voltage to the the output of the circuit that is thus negative with respect to the ground.
If you look at the last of the four diagrams, in fact, you can see how the output voltage becomes more and more negative with respect to the ground, with a tendency to reach the 8.4 V we mentioned before.
So, if we keep moving the switch back and forth quickly, after reach that state we can sustain it, even if we remove a little amount of charge from C2 at each cycle, due to a load that we could put across its leads.
This circuit is called a charge pump, because is able to pump charges into the second capacitor, even if it is not directly connected to the input voltage.
Note that if we start applying a strong load to the output, C2 won’t be able to recharge fast enough and its voltage will start dropping. And that is why we cannot use this polarity inverter for loads comparable to those that we can put directly on the original power supply.
But, how do we move a switch fast enough to obtain this functionality?
The trick is to replace the mechanical switch with a a solid state one, and control it with a square wave oscillator, the so called astable multivibrator.
One way to do that is to use a 555 timer, like in the following schematic.
The circuit on the right half side is exactly the same as the one in the previous schematic. However, on the left half side, the mechanical switch has been replaced with a 555 timer setup as an astable multivibrator, with a duty cycle close to 0.5.
Pin 3 of the 555, which is the output pin, will move alternatively from the voltage of the power supply to the ground, thus working as if it was the switch of the previous schematic.
The oscillation frequency is provided by R1, R2 and C4, which I calculated in this example to provide a frequency of about 30 kHz with a duty cycle very close to 0.5.
If you would like to know more about the 555 timer, I suggest you to watch the video I made about one year ago where I describe what it is and how it works. Here is the link to the video.
In order to be able to support relatively higher currents with the polarity inverter, we need to be able to recharge the capacitors at a faster pace, which translates in a higher current. One way do so is by using the output of the 555 to pilot a couple of transistors with a high value of beta, the coefficient that express the amplification in current of the transistors. With a higher available current, the capacitors will charge faster and, therefore, it will be possible to handle a higher load current.
Here is an example circuit that can provide higher currents:
This circuit is basically identical to the previous one but, instead of applying the output voltage of the 555 directly to the charge pump, made of C1, C2, D1, and D2, the 555 controls the two transistors 8050 and 8550, respectively an NPN and a PNP.
With these transistors, we can still connect the positive lead of C2 to the positive of the power supply and to the ground alternatively, and we can force the charges in and out of the two capacitors to move at a faster pace.
The two resistors R3 and R4 are necessary to limit the amount of current through the base of the transistors. Too much current in there would have two unwanted side effects:
First, the transistors could burn because of too much current.
Second, even if the transistors did not burn, they would still go deep into saturation, which would make them spend more time moving between the on and off states and causing the circuit not to work as expected.
In addition to that, since the voltage at the output of the 555 does not change instantaneously between 0 and Vin, there would be a period, during the transition, where both transistors would be on at the same time. As a result, the input voltage would be short circuited for a little while during each cycle, which is a condition definitively to avoid.
To fix the problem, I added those two Zener diodes to the circuit. The Zener diodes create a gap between 4.7V and 5.1V that will prevent the transistors from being both on at the same time, thus fixing the short circuit problem.
Here is how it works.
During the transition from 0 to 9V on pin 3 of the 555, transistor 8550 will be on in the interval between 0 and 4.7V.
During the interval between 4.7V and 5.1V both transistors will be off and, finally, during the transition between 5.1V and 9V, transistor 8050 will be on.
Viceversa, during the transition from 9V to 0, the opposite sequence will happen: first, transistor 8050 will be on, then both transistors will be off, then transistor 8550 will be on, alone.
And that is why the two zener diodes make sure that the two transistors will never be on at the same time, thus protecting them and the power supply.
The final effect will still be the same: the positive lead of C2 will be alternatively connected to the positive and to the ground, making the charge pump to work, and creating the negative output.
To conclude, polarity inverters have their usefulness in certain situations, but are not good enough to replace a full fledged dual power supply.
So, when do we use one or the other?
We will use the polarity inverter in those cases where only a little load is required on that specific pole, whereas the majority of the load would depend on the single power supply.
Whenever we need considerable and comparable amount of power on both the positive and the negative poles, we will need to use a dual power supply.
And finally, if you would like to see the polarity inverter in action, you may want to watch this video, which I posted back in December 2020.
Whenever a current flows through a wire, it creates a magnetic field surrounding it and, if the current is variable, so will be the magnetic field.
Conversely, if we put a wire in a variable magnetic field, an electromotive force will be generated in the wire, in other words: a voltage. This voltage, in turn, produces a current in the wire when it forms a closed circuit, like a loop.
Another way to describe this phenomenon is this: if we have a variable magnetic field in a volume of space, a variable electric field will be automatically generated in that same space. It is that electric field that generates the voltage on the wire and causes current to flow through it. Again, this concept presents a duality. If I generate a variable current in a wire, a variable electric field will be present on the wire and its surroundings. As a result, a variable magnetic field will be generated in the same volume of space.
In fact, whenever there is a variable electric field, there is also a corresponding variable magnetic field and viceversa. Each of the two variable fields cannot exist without the other. They always both exist at the same time. It is for this reason, that we usually refer to them as a single field called Electromagnetic field.
And because the electromagnetic field changes over time, it also affects its surrounding space and the space surrounding that space, and so forth. Basically, the effects of the electromagnetic field is visible over long distances and so we say that electromagnetic fields propagate in space. We call this an electromagnetic wave.
Light, for example, is an electromagnetic wave and propagates in space at what we call the speed of light.
A physicist named James C. Maxwell, back in 1861, put together all the knowledge of the time and elaborated a set of 4 equations that completely describe how electromagnetic fields work and how electromagnetic waves propagate. These equations are known today as the Maxwell’s Equations.
The concept of electromagnetic waves is widely used in electronics to make devices that can receive and transmit information using this physical property. You know very well these devices. They are called radios, and they can be used on their own or inside a wide variety of more complex devices, like TV sets and cellphones.
Without the electromagnetic waves, the global telecommunications system we enjoy today would not exist.
However, there is a negative side of the electromagnetic fields and waves. Electromagnetic waves are continuously transferred from one circuit to another, or from one part of the same circuit to another as long as variable currents is produced. The problem is that, most often than not, we do not want these kind of interactions.
Whenever an unwanted signal is generated in a circuit because of the electromagnetic fields that surrounds it, we call it a noise, and we try our best to remove it.
The generation of these unwanted signals is generally called Electromagnetic Interference or, in short, EMI.
The action we take to avoid and prevent EMI is called Electromagnetic Shielding, or EMS.
Basically, it works by putting a shield in between the area where the source of the Electromagnetic field is located, and the circuit that is negatively affected.
There are several sources of EMI, some occur in nature, others are created by humans, as a by-product of the machines and devices we use daily.
Since the production of EMI is a bad thing, the governments all over the world provide guidelines to help reduce the generation and the effects of EMI. They also define rules that specify the maximum amount of EMI that any device can generate.
This helps manufacturers in creating compatible devices that do not influence each other.
If we had to make a list of all possible sources of EMI, that list would be incredibly long.
Here are just some examples.
Some natural occurring EMI:
– solar magnetic storms, that occur from time to time on the surface of the sun and may cause huge electromagnetic waves that can propagate toward Earth and cause a disruption of electric and electronic devices.
– lightning, especially during thunderstorms.
– Earth’s magnetic field flux, which is caused by moving an electric device across the magnetic field generated by the Earth, thus creating unwanted electric fields and currents inside the device.
And here are some human-created sources of EMI:
– TV sets
– cell phones
– brush motors
– fluorescent bulbs
– microwave ovens
– power generators
– internal combustion car engines
– arc welders
– brownouts and blackouts in the power grid
– electrical power lines, causing the well known humming that can be even heard when getting close to a high voltage power line.
– voltage sags and spikes in circuits
… and the list goes on and on. And each item in this list is potentially a noise generator that can affect and even totally disrupt the functionality of an electronic circuit.
That’s why, all the electronics equipment that is sensitive to EMI needs to be protected with shields. We call this protection “ElectroMagnetic Shielding” or “RF Shielding”.
Whenever we have an electronic device that is sensitive to electrical noise, we need to provide it with some sort of electromagnetic shielding, or EMS, to make sure that the electrical noise generated by the EMI is reduced below a value, or threshold, that the device can safely handle. Sometimes, very sophisticated and sensitive instruments have really low thresholds, a condition that is very difficult to achieve but, nonetheless, necessary.
Examples of electronic devices that need EMS are:
– electric wires used to transport audio, video and radio frequency signals: to protect them we usually do things like surrounding them with a metallic shield. Such cables are called coaxial cables.
– RF equipment: radios, cellphones, TV sets, and so forth, need to be shielded from unwanted noise. In fact, EMI can make sounds difficult to understand, or it can distort pictures and videos, for example.
– Audio equipment: again we don’t want to hear noise when we are listening to our preferred song or band.
– medical equipment: EKG machines, diagnostic equipment and the sort affect human life and we don’t want that the wrong noise at the wrong time causes somebody to die.
– Avionics: what if the airplane instruments stop working during landing because some EMI was generated by the unshielded device of a passenger? That’s why they don’t want you to use any sort of electronic devices during take off and landing.
– certain lab tools, like oscilloscopes: we want them to measure the signals of our circuits under test, not the noise produced by electromagnetic fields in the room where the instrument is located.
And again, the list goes on and on.
Whenever we create a new device, we always need to make sure, first, that it will not cause EMI that can affect other devices; and, second, that it will not receive too much EMI from the surroundings.
The ability of creating Electromagnetic shielding depends on two different properties of the materials: the conductivity, which is the capability of allow the movement of electric charges, and the magnetic permeability, which is the capability of a substance to facilitate the flow of a magnetic field through it. In other words, the permeability works for the magnetism like the conductivity works for the electricity.
The conductivity property allows to shield from electric fields and the permeability property allows to shield from magnetic fields.
Unfortunately, there are no materials that have simultaneously a good conduction and a good permeability. So, we choose one material or another depending on what aspect we need to give priority.
Shielding due to conductivity is achieved by the property of what is called the Faraday Cage.
Michael Faraday was a British scientist that invented the device in 1836. The cage was capable of shielding people from electric strikes and from electrostatic discharges. That device is today known with his name, the Faraday’s cage, and its principle is extensively used to provide electric shielding of all sorts.
It is basically a cage made of conductive material and is capable of shielding whatever is inside of it against electrostatic and electric fields.
It works because an external electric field causes modifications in the distribution of the charges in the conductor. Once the charges are redistributed, they produce on their own another electric field that cancels out the original field and prevents it to enter inside the cage. The better is the conductor of which the cage is made of, the better the cage works.
Note, however, that the cage is supposed to be a continuous covering of conductive material, without holes. Only that way it can prevent electric fields from going inside. In practice, we do need openings in the cage, otherwise it would not be possible to put anything inside of it. But, as long as the openings have a size much smaller than the length of the electric waves that we want to shield, the Faraday’s cage will work as if it was a continuous covering.
So, for lower frequencies we can have bigger openings, but for higher frequencies we need to make them smaller and smaller, up to the point that for very high frequencies we really need a continuous covering.
Another aspect of the Faraday’s cage is its bad efficiency at the higher frequencies due to what is called the skin effect.
When a constant current flows inside a conductor, all the movable charges inside the conductor participate in creating the current.
But, when we have an alternate current, the center of the conductor is used less and less with the increase of the frequency. So, at the highest frequencies, if we don’t have a very good conductor, since the current will only move in a thin outer layer of the whole conductor, or the skin, the resistivity will be so high that the electrons inside will not move fast enough to counterbalance the electric field and so the shield will not be effective.
That’s why at the highest frequencies shields are usually made with an external layer of copper or even silver, which is the best known conductor.
When we need to create a shield for a magnetic field, a good conductor doesn’t do any good. For magnetic fields, we need to use materials with high permeability to magnetic fields which, conversely, are poor conductors.
The big difference with magnetism is that we cannot separate north and south poles like we do with the positive and negative charges, Therefore, the lines of a magnetic field always go from the north pole to the south and cannot be interrupted.
What we can do is to merely deviate the field lines and force them to go through a high permeable material, away from the device that we want to protect.
This is a totally different approach from the electric fields, where we actually cut the field lines and prevent them to go inside the Faraday’s cage.
As we already said, there are two kind of shielding: those that work better against electric fields, and those that work better against magnetic fields.
Different materials have been adopted over time to achieve these two goals.
To shield from electric fields, we need to use good conductors. The best conductors, however, like silver and copper, are very expensive. So, whenever possible, the tendency is to use slightly less conductive metals that are still very efficient at the particular frequencies where the shield is supposed to work.
And so you can see shields that are made of aluminum, steel or nickel, and only when strictly necessary copper or even silver is used.
Shields for electric fields can then be shaped as a solid box, or as a mesh, or a metallic foil, depending on the mechanical and electrical needs. To protect plastic boxes, there are even certain conductive paints that can be used to coat the plastic and obtain a relatively good shielding.
To shield from magnetic fields, we need to use a material that is permeable to magnetic fields.
Iron and steel are the obvious examples, but there are some alloys that are particularly good for magnetic shielding, like silicon-iron and mu-metal.
Just to give you an idea, the relative permeability of the air is defined as 1.
Iron with 0.2% impurities has a relative permeability of 5000.
Silicon-iron has a permeability of 7000.
And mu-metal has a permeability of 100,000.
Depending on how thick we can make the shield and the amount of magnetic energy that we want to stop, we will need to choose one solution or the other.
For electronic circuits, however, we deal most of the time with electromagnetic waves and not with constant or quasi-constant fields.
Since electromagnetic waves have both an electric and a magnetic component, and since one cannot exist without the other, it is enough to shield from one of the two component to block the whole wave.
That is why, electronic devices are usually only shielded against the electric fields, which are the easiest shields we can obtain. Doing so, we can stop or reduce the electric component of the waves and, therefore, the whole waves, since the magnetic component will go away as well, just because the electric component is not there anymore.
When creating an EMS, or an electromagnetic shielding device, we need to be careful to make it in such a way that it will actually shield our electronic circuit, rather than actually collect even more noise. Unfortunately, it is very easy to obtain the opposite effect.
For example, if the shielding of a device is not connected to ground, the current that flows in the shield will non be able to be discharged anywhere. As a result, all the energy will be reflected in the surrounding space, thus defeating the purpose of having a shield or, even worst, causing some resonating effect that could create even more noise.
Another cause of trouble is when the shielding is done in such a way that the internal currents move around in a loop. If that happen, the loop will actually cause a concentration of the eletromagnetic waves, thus causing more trouble than not having a shielding at all.
The last issue I would like to mention is the one of the gaps in the shielding which, if too large compared to the wave length of the noise, will not be able to prevent the noise itself to go through. We therefore need to be very careful when making holes in the shielding, for example, to allow wires to go through. Or to allow us to see the display of an instrument. If the opening is too large, compared to the wavelength, the noise will no through and the shielding will be ineffective.
EMS is always a hot topic in electronic devices, whether they are low or high frequencies. We need to spend a lot of thoughts when designing the case of a device and, in case of radio frequency, even the design of the PCB deserves a lot of attention to prevent unwanted coupling of the signals from one section of the PCB to another, which is basically noise that is generated and distributed internally to the circuit itself.
It is also important to distinguish cases where the device is supposed to deal with very low signals versus those cases where the signal is higher.
When dealing with low signals, we may end up with noise that has the same magnitude of the signal being treated, and so we have to put a particular attention to the suppression of the noise, which could otherwise overwhelm the wanted signal.
When dealing with higher level signals, the surrounding noise could sometimes be considered negligible and, in such a case, the EMS becomes less of an issue.
It is important, therefore, to address each case individually. There is no common solution that can be adopted for everything.