Electric Current The Easy Way

Electric Current The Easy Way: a very simple and qualitative approach to understanding what electric current is and how it flows.

Watching a number of YouTube videos, I realized there is some misconception regarding the electric current and how it flows. Some people don’t understand what the current is made of and whether it flows from positive to negative or vice versa.

So here I am, trying to shedding some light to clear the obscurity on this subject.

This post approaches the subject in a very basic qualitative way. No formulas and no calculations are involved.

Here it is!

From the Webster Dictionary

Current:
A flowing or passing; onward motion. Hence: A body of
fluid moving continuously in a certain direction; a
stream; esp., the swiftest part of it; as, a current of
water or of air; that which resembles a stream in motion;
as, a current of electricity.

So, current is the flow of something, some kind of material thing like the molecules of water in a river.

fresh river_400

But, what is the material that makes the electric current?

Electric current is made of electric charges. These charges have the ability of moving in a medium like, for example, an electric wire. Like the water in a river, charges have to move from a higher level to a lower level of potential energy.

charges_in_wire

For a river, the higher level of potential energy is the higher ground, and so the water flows from a higher ground to a lower ground, from a mountain or a hill toward the valley below or the sea, or the ocean.

Similarly, for an electric current, charges have to move from a higher ground of electric potential energy to a lower ground.

The problem with the electric current is that different kind of charges can make it, depending on the medium, and depending on the kind of charges. So, the definition of higher ground may change.

This seems utterly complicated, and it is. Think if we had to consider the kind of medium and/or the kind of charge every time we need to describe what happens with a current.

So, since we don’t like complications, we make some simplification. We always define a higher ground as a positive electric potential energy level, and we always say that the current flows from the positive potential energy level to the negative.

positive_charges

Hum… Positive? Negative?

Well, yes, because charges can only be positive or negative. Think at the electrons and the protons in the atoms. Those are the basic charges and they are negative for the electrons and positive for the protons.

protons_and_electrons

Wait, what we just said? Electrons are charged negatively and protons positively? How do we know that?

We don’t!

Positive and negative are just made up names that we use because it is convenient to do so.

We could have as well said that electrons are positively charged and protons are negatively charged. But, historically, we have defined the polarity of the charges in a certain way and therefore we continue to do so, because we don’t like changes, and we like concepts to be simple.

So, here we are, saying that an electric current is made of charges. That the charges can be positive or negative. That positive charges like to go from their kind of higher ground, a positive electric potential level, to a lower ground, which is a lower positive electric potential level that we call negative, to distinguish from the other one. And, finally, we say that negative charges like to go from their kind of higher ground, the negative electric potential level, to their kind of lower ground, which now we understand we can call positive electric potential level.

Hum, it seems too much, isn’t it?

And yes, it is: too complicated to use it in every day conversations.

Simplification? Sure, let’s do that. Let’s say that whenever an electric current is involved, we will always say that it is made up of positive charges, and that positive charges always go from positive to negative electric potential level. How’s that? Simple enough?

Now doesn’t matter the medium being an electric wire, were the current is made up of electrons moving through it, or the acid solution in a car battery, where the electric current is made of ions, of both positive and negative kinds, creating two different currents flowing simultaneously in opposite directions.

solution

 

Lesson learned: we like to make things simple. We define the electric current as the flow of positive electric charges in a medium, whatever it is, going from the positive potential level to the negative.

circuit

And that’s it. That’s enough for us. With this definition we can address problems involving electric currents always the same way, without worrying what is really happening behind the scenes. This is a very important concept. All about electrical engineering is based on this definition of current. Well… at least part of it.

More on the Theremin: The Heterodyne Mixer

The heterodyne mixer is the stage of the Theremin where the high frequency signals coming from the pitch reference oscillator and the pitch variable oscillator are combined together to obtain the audio signal.

mixer

Here we are again with another post about the Theremin, which can be considered the first electronic musical instrument ever invented, almost 100 years ago, in 1919, by the Russian physicist Leon Theremin.

At that time the Theremin was made out of thermionic valves and used a lot of space and electric power.

theremin_in_concert

Today, thanks to the evolution of electronics in the last century, we can make one that can occupy much less space while also consuming much less power. In fact, this is one of several articles that I have already published on the design and construction of such musical instruments, using solid state components.

Please consult this site archives for the previous articles on the subject and THIS link for schematics and diagrams, which I keep updating as I go in designing and building the pieces of the instrument

A corresponding series on the Theremin is also available on YouTube at THIS link. There, I describe every detail of my project, explaining how the various parts of the device work and how I built everything so far in a very inexpensive way.

In this article I will explore the Mixer stage of the Theremin, describing how it works and how it is used within the Theremin itself.

The mixer is the Theremin stage that combines together the signals from the pitch variable oscillator and the pitch reference oscillator, to create an audio signal that is essentially the sound that the Theremin produces.

The combination of the two input signals is done with a process called heterodyne. It basically consists in multiplying the two input signals by exploiting the non-linear characteristic of transistor Q1, which is carefully polarized outside its linear zone. The result of the multiplication is a new complex signal containing frequencies that are the sum and the difference of the frequencies of the original input signals. Since the frequencies of those input signals are close to each other, their difference falls in the audible range, which is what produces the peculiar sound of the instrument.

Looking at the schematic, you can see that the two input signals are mixed together at the base of transistor Q1, which they reach passing through capacitors C4 and C8, used to decouple the mixer from the direct current superimposed to the input signals.

Transistor Q1 is polarized in the non-linear zone of its characteristics. Because of the non-linearity of the transistor, the two signals end up being multiplied with each other, producing a new, more complex, signal that contains both the sum and the difference of the frequencies of the input signals. This heterodyne process, therefore, applies the following equation to the two input signals:

sin(2πf1t) * sin(2πf2t) = 1/2 cos(2πf1t – 2πf2t) – 1/2 cos(2πf1t + 2πf2t)

where the factors on the left side represent the two sinusoidal input signals, and the resulting complex signal is on the right side of the equation. The above formula is actually a simplification, because it does not take into account the phase shift between the two input signals, which should appear as a phase factor in the parameters of each of the sine waves on the left side of the equation. However, if we did the full calculations, we would see that we would still obtain the same output waves, but each would have an extra amplitude factor that depends on the initial amplitude of the input signals and on their phase shifts.

Anyway, the complex signal obtained at the collector of transistor Q1 is supplied to a Low Pass filter, made up of the components R4, R7, R8, R9, C2, C3, C5, C6 and C7. The filter produces an attenuation of the high frequency element of the complex signal, effectively leaving only the one at low frequency  cos(2πf1t – 2πf2t), which is the audio signal.

That output signal is then passed to the next stage of the Theremin, the VCA, where it acquires the dynamics of the music sound. We will talk about the VCA in a future post.

If you are interested in more information on the Theremin Mixer and how I built it, please watch this companion VIDEO on YouTube.

And, as always,

Happy experiments !!!

Behind The Scenes Of The Theremin Design

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

20180915_153351

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.

20181213_105525

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.

20181213_105610

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.

20180915_153256

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!

An LED Bar Graph VU Meter

VUmeter

Bar graphs VU meters can be easily made with a simple integrated circuit. There are several of them with different characteristics, but they all present the same basic functionality: one or more LED in a row are used to visualize, more or less precisely, the voltage amplitude presented to the input. That voltage can be either a direct current or an alternate current and, in particular, it could be the output of an audio amplifier.

But, what is inside these integrated circuits? How do they make possible this kind of behavior?

Here is the design of a simple gadget that shows how a bar graph VU meter works. Building and using this device is certainly a fun way to learn the principles used to design the bar graph integrated circuits.

bar_graph_vu_meter

The basic element of the circuit is the one made with the components Q1, R1, R10, D1, and D10.

This block of components then replicates several times to increase the number of LEDs used in the bar graph. In this particular case, the same circuit is replicated 8 more times, for a total of 9 LEDs in the bar.

You can also see that each block, or stage, receives as the input the output from the previous stage.

Diodes form D10 to D17 are used to provide a different threshold to each stage. In fact, let’s say that the first stage is triggered when the voltage on the anode of D10 reaches about 3V. In order to trigger the second stage we will need 3V on that stage, but that means that the voltage at the first stage has to go up to 3+0.6V, or 3.6V. The extra 0.6V is the forward voltage of diode D10.

Similarly, to reach each further stage, we will need an input voltage 0.6V higher for each stage we want to light up.

In the end, to light up the last stage we will need an input voltage of at least 3 + 8 times 0.6V, or 7.8 volts.

Once the threshold is reached in a stage, the corresponding transistor switches on and starts conducting a current that is only limited by the LED and the series resistor which, in this case, is 330 ohms. With the values in the circuit, the LED current will be about 20 mA.

So, when we apply a voltage to the input terminals, depending how high the voltage is we will see a number of consecutive LEDs lighting up, while the remaining will stay off because their respective stages have not been triggered yet or, in other words, the voltage at those stages hasn’t yet reached the threshold imposed by the diodes.

Note also that resistors R10 to R18 are not all of the same value. This is because by the time the voltage reaches the threshold in the last stage with transistor Q9, the voltage on the previous transistors is higher and higher while we move to the left of the circuit, since we need to add back the 0.6V that the diodes are dropping. Therefore, to avoid damage to the transistors on the left, we need to increase the base resistor when moving from the right to the left of the circuit.

Another thing to notice is the trimpot located at the connectors for the input signal. The circuit as it is, is capable of handling signals up to the value of the power supply, which is 9V.

However, we can adjust the trimpot to handle higher signals, just by moving the trimpot cursor toward the end that is connected to ground, in order to get only a fraction of the actual input signal.

Conversely, if you had a very small input signal, that could not trigger even the first stage of this circuit. In that case, you could still add a very simple amplifier between the source of the signal and the input of this circuit, which would help increase the level of the signal to the required value.

Finally, you see that the bar graph meter is powered with a 9V power supply. I used such value so you can use a 9V battery, if you like to try the circuit.

However, if you wanted to use this circuit as part of a more complex system having a higher value of the power supply, you could just modify resistors from R1 to R9 and use the power supply of that system.

For example, if you planned to use 12V instead of 9, you would use resistors of 470 ohm rather than 330, and everything would work just fine.

Remember, however, that 9V is the minimum voltage you can use to correctly power the bar graph circuit. You can only increase the power supply voltage to a higher value and increase accordingly the resistors R1 to R9. That is because we need a power supply that exceeds the voltage at the base of the leftmost transistor Q1, which can be as high as 7.8 volts, as we said before.

The circuit can be easily assembled on a per-board, like in the case in the frnt picture of this post. There, I used a 3 prongs connector to provide the power supply and the input for the external signal.

Once the circuit is assembled, set the trimpot with the cursor toward the ground side, then power up the device and put a signal to its input. The signal should be the highest possible with the amplifier to which you are attaching the VU meter. Then, adjust the trimpot until all the LEDs are lighted up. Now the circuit is tuned and you can input to it any signal that changes between 0V and the max you used for the tuning.

For further information, and to see the VU meter in action, take a look at this video that I posted on my YouTube channel.

Happy experiments!!!

Another Theremin Post

Announcing the new video on the design, implementation and testing of the power supply module of the Theremin.

You can find the video at this link.

Also, refer to the link on the right column about the Theremin project to find all the files so far published on the construction of this unusual electronic musical instrument (schematics, 3D designs, work art).

And, finally, don’t forget to look also at the other videos on my YouTube channel.

 

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.

100W_el_load_v1

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.

100W_el_load_v2

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!