Theremin v.2 Power Supply Design

theremin-v2-power-supply

For the new version of the Theremin, I have chosen to use a dual 12V power supply. This will have more flexibility because it will allow me to use more sophisticated units, possibly using op-amps.

The circuit is very basic: it uses a dual 14V transformer (not shown in the schematic) capable of providing 1.5A at its output.

A dual transformer is made up as in the following picture.

center-tapped-transformer

Is has a primary winding that is connected to the AC power supply outlet, and a secondary winding with a center tapped wire that is usually put to ground on the low voltage circuit side.

Voltage between either end wire of the secondary and the center tapped wire is usually the same (with the exception of specifically made transformers), which we call V.

The voltage measured between the two end wires of the winding is instead two times V or 2V.

Sometimes, instead of having a single secondary winding, we have two, carrying the exact same voltage. In this case, we can connect together the two closest wires and consider that as the center tapped wire. Then everything works as the first kind of transformer.

transformer-trans64

The AC current of the transformer is converted in to a DC current through the usage of a bridge rectifier and the capacitors C1 and C2.

The bridge rectifier converts the sine wave coming from the transformer into a fully rectified wave.

Full-wave_rectified_sine

Then, the capacitor that follows (in this case C1 and C2) starts charging over the ascending sides of the wave and discharging, partially, over the descending sides of the wave, basically filling the wave in between crests and making it look like more a straight horizontal line with some disturbance in it that we call ripple (the red line in the following picture).

ripple

In general, depending on the use of the power supply, we define a maximum value of the ripple that the circuit can handle.

In our case, we need to make sure that the voltage at the input of the regulators never goes below 14.5V, according to the data sheet, otherwise the regulator will not function properly.

The peak voltage provided by the transformer is its RMS value multiplied by the square root of 2, or:

peak_voltage

The minimum voltage we can have at the input of the regulator is:

regfulator_input_voltage

This is the max value of ripple that we can sustain.

To calculate the capacitor necessary to obtain this ripple, we use the following formula:

capacitance calculation

where f is the frequency of the alternate current which, in the USA, is 60Hz, and Ix is the maximum current that the power supply needs to provide.

So, we would need a capacitance value, for C1 and C2, of 2358uF.

However, the Theremin circuit will really not draw 1.5A from the power supply, so we can stay a little conservative, and use the closest value below the calculated one, which is 2200uF.

At this point we can safely say that the voltage on the output of the regulators will be exactly 12V (positive or negative, depending on the output side).

To further help the regulator, and preventing the current through it to go too close to the 1.5A threshold, where the regulator would not work anymore because the ripple becomes too high, we add to the output of each regulator another electrolytic capacitor, this one with a value at least equal to the capacitance value that we did not put at the input side. Since at the input side we put a capacitance of 2200uF instead of 2358uF, we will need a capacitor of at least 158uF.

However, to stay totally safe, I decided to use a capacitor at least 5 times higher, so I used the value of 1000uF for C3 and C4.

And finally, I added an extra capacitor (C5 and C6) to shunt toward ground any RF frequency that would travel back from the Theremin oscillators toward the power supply. A 0.1uF value is what is suggested by the data sheet of the regulator, so I used just that.

Why did I use this capacitor if there was already a 1000uF in there?

The reason hides in the way the electrolytic capacitors behave. In short, the electrolytic capacitors do not work well at high frequencies, so we need to add the extra 0.1uF capacitor, which is not an electrolytic one, to work in that range of frequencies. And since the range of frequencies is much higher than the one of the 110Vac outlet, a very small capacitance is enough to do the job.

 

Theremin v.1 Pitch Reference Oscillator Analysis

This is my first post of this kind, which I plan to use to give insights on how an electronic circuit is designed. In the future, I plan to post a similar article for every new circuit I design and present on my YouTube channel.

This way, people who are interested in such details can go to this site to get them, thus sparing all the others that don’t care from watching them on the original video.

Today, to get started with the new series of posts, I will go into the insights of the design of the Pitch Reference Oscillator used in the Theremin version 1.

I decided to use the Pitch Reference Oscillator because all the other Theremin’s oscillators are based on the same principle, although for the variable ones, an antenna is added to the resonant circuit to make its capacitance increase based on the position of the hand on the player with respect to the antenna. In fact, the antenna adds up to 8pF to the resonating tank, based on the position of the hand of the player.

For reference, you can watch any of the YouTube videos on my channel related to oscillators in the Theremin Project playlist.

Here is the link to the playlist:

https://www.youtube.com/watch?v=moT9iAaZU-I&list=PL3SNC7XyKklYZOs48Qfh16nTO4t8edDPU

I began the design of the oscillator starting from the base concept of positive feedback amplifier, which can be represented as follows:

feedback_amp

We have an amplifier with a positive gain, which should be 1 when there is an oscillation, and we have a feedback impedance Zf that takes part of the output signal and brings it back to the input of the amplifier. Zf is supposed to be chosen to obtain a positive feedback at the oscillation frequency.

This basic configuration needs now to be modified to insert in it an LC resonating tank. This should be on the output of the amplifier, so its oscillations can be fed back to the input and be sustained with no decay.

oscillator_block_diagram

The amplifier can now be replaced with a real circuit made with a transistor. I choose in this case a common base configuration, which gives me the required gain of almost 1, which is enough to sustain the oscillation.

The Zf will need to provide a phase shift of 90° to compensate for the internal capacitance of the transistor, which will add another 90° for a total of 180°, thus providing a positive feedback, considering that the transistor has actually a negative value of amplification (-1).

Therefore, Zf can be replaced with a simple capacitor that will bring the output signal back to the input with the desired 90° phase shift.

And once all the polarization resistors are added, the circuit looks like the following:

oscillator

In this circuit, R2, R3, R4 and R5 are the polarization resistors, which are calculated based on the specs of the transistor itself.

L1, C4, C6, C7 constitute the resonating tank. The Two variable capacitors are added to allow for a fine tune of the frequency.

C5 is the feedback impedance, or Zf.

The resonating frequency is calculated based on the following formula:

resonating_frequency

where L = L1, and C = C4//C6//C7//CBE = C4 + C6 + C7 + CBE

and CBE is the capacitance between emitter and base.

Temporarily excluding CBE from the calculations, the frequency therefore can vary between 258.2 kHz and 312 kHz. The CBE can be obtained from the transistor data sheet, adjusting for the polarization between base and emitter.

Also, we can slightly affect the resonating frequency of the oscillator by changing the polarization point of the transistor. This can be achieved by a potentiometer added on the base of the transistor itself, as per the following schematic.

final schematic

All the other capacitors in the schematic are used to make the polarization point of the transistor independent from the oscillation frequency, by shunting the oscillation signal toward ground. Thus, their value is such that they behave like conductors with respect to the high frequency of the oscillator.

At the end, considering the increase in capacity given by CBE, the frequency ends up to be centered around 400kHz, which was the required frequency for the pitch reference oscillator.

DC Electronic Load V.3

Back in August 2018, I presented a DC electronic load on my YouTube channel (V.2). For that, I used an old 2N3055 transistor in a Darlington configuration with 2 more transistors to be able to get enough gain to use it.

100W_el_load_v2

Although declared useful for 100W, I was never able to make it work at those powers due to the limited dissipation capabilities of the power transistor and the heat sink. The max power dissipation I could have from that device was about 20W.

Today, at the anniversary of that presentation, I have created a new version of of the DC electronic load. This new version is based on a MOSFET that can work alone as a load, adjusting the current only through an appropriate voltage on its gate, avoiding the need of having a Darlington circuit with multiple transistors.

The schematic of this new version of the DC electronic load is based on a single MOSFET capable of driving the necessary current, up to 5A and a voltage divider connected to the battery, providing the appropriate voltage to the gate of the MOSFET.

electronic_load-v.3

In order to make it work correctly, the trim-pot RV1 needs to be tuned to obtain a voltage of 1.5V on pin 3 of the potentiometer that regulates the amount of current flowing through the MOSFET, which provides a better use of the multi-turn potentiometer that regulates the actual value of the current.

A combination of digital voltmeter and am-meter, like in the previous version of the DC load, takes care of providing information about the power supply under test.

The device is powered through a 9V battery and it is connected in such a way that the voltage is measured through the yellow wire of the digital voltmeter, wile the current is measured putting the am-meter in series with the MOSFET, with the thick red wire on the source, and the thick black wire toward the negative connector, through a 5A fuse that is used, mostly, to protect the am-meter itself against currents too high of those it can handle.

I made a new case for this new version of the DC load. The main difference is the location of the heat-sink, which is now located on the back panel rather than the top of the device. The new heat-sink is also attached to the back panel through 4 separators, which allow for a better air flow and cooling of the unit when it is used for long period of times.

Here is an OpenSCAD view of the box and the corresponding code to create it.

v3_box_view

$fa=0.5;
$fs=0.5;

//main section
rotate([180, 0, 0]) translate([0, 10, -2])
{
// front panel
difference()
{
cube([150, 80, 2]);
translate([27.5, 40, -1]) cube([45.8, 27.7, 4]);
translate([52, 20, -1]) cylinder(d=6.2, h=4);
translate([120, 60 , -1]) cylinder(d=9, h=4);
translate([108, 20 , -1]) cylinder(d=9, h=4);
translate([132, 20 , -1]) cylinder(d=9, h=4);
translate([3,3,0.5]) linear_extrude(height=2) text(“eleneasy.com – DC load – 25W max.”, size = 6);
translate([36,18,0.5]) linear_extrude(height=2) text(“off”, size=5);
translate([61,18,0.5]) linear_extrude(height=2) text(“on”, size=5);
translate([109,44,0.5]) linear_extrude(height=2) text(“current”, size=5);
}
translate([102.25, 15, 0]) cube([2, 10, 2]);
translate([126.25, 15, 0]) cube([2, 10, 2]);
translate([111.75, 15, 0]) cube([2, 10, 2]);
translate([135.75, 15, 0]) cube([2, 10, 2]);
translate([12, 20, -36]) cube([27, 2, 35]);
translate([39, 4, -36]) cube([2, 18, 35]);

// left panel
translate([0, 0, -60]) cube([2, 80, 60]);

// right panel
translate([148, 0, -60]) cube([2, 80, 60]);

// bottom panel
translate([0, 0, -60]) cube([150, 2, 60]);

// top panel
translate([0, 78, -60])
{
cube([150, 2, 60]);
}

// screws supports
translate([2, 2, -58]) difference()
{
cube([10, 10, 58]);
translate([5, 5, -1]) cylinder(d=2, h=16);
}
translate([138, 2, -58]) difference()
{
cube([10, 10, 58]);
translate([5, 5, -1]) cylinder(d=2, h=16);
}
translate([2, 68, -58]) difference()
{
cube([10, 10, 58]);
translate([5, 5, -1]) cylinder(d=2, h=16);
}
translate([138, 68, -58]) difference()
{
cube([10, 10, 58]);
translate([5, 5, -1]) cylinder(d=2, h=16);
}
}

// back cover
translate([0, 10, 0]) difference()
{
cube([146, 76, 2]);
translate([5, 5, -1]) cylinder(d=4, h=4);
translate([5, 71, -1]) cylinder(d=4, h=4);
translate([141, 5, -1]) cylinder(d=4, h=4);
translate([141, 71, -1]) cylinder(d=4, h=4);
translate([73, 38, -1]) cylinder(d=40, h=4);
translate([126, 60, -1]) cylinder(d=12.5, h=4);
translate([130.5,51,0.5]) rotate([0, 0, 180]) linear_extrude(height=2) text(“5A”, size=5);
translate([(146-55)/2, (76-50)/2, -1]) cylinder(d=4, h=4);
translate([146-(146-55)/2, (76-50)/2, -1]) cylinder(d=4, h=4);
translate([146-(146-55)/2, 76-(76-50)/2, -1]) cylinder(d=4, h=4);
translate([(146-55)/2, 76-(76-50)/2, -1]) cylinder(d=4, h=4);
}

Assembling the circuit is pretty straightforward, and it is done partially in the air and partially  on a perforated board.

We just need to make sure we provide the cables with the right thickness for the current we need to support.

In my case, I used stranded cables with an 18 gauge. These cables are necessary between the thick am-meter cables, the MOSFET source and drain, and the external connectors.

Every other connection can be made with 22 gauge cables.

And finally, the heat-sink should have a resistance of 0.82 Centigrade degrees per watt or less, to prevent the MOSFET from becoming too hot. Note that this will not save the MOSFET in case you draw a current too high. The product between the current and the voltage as provided by the measurements display must never exceed 25W, and the current should never exceed 5A, or the MOSFET will burn.

The tuning is done by measuring the voltage between the terminal 3 of the potentiometer and the ground, with the circuit on, but not connected to any external power supply. The trim-pot has to be adjusted such that the measured voltage equals 1.5V, which is just below the minimum voltage necessary to make the MOSFET conduct current. This way, when turning on the apparatus with the potentiometer all the way to the counter-clockwise position, there will be no current. Then, moving the potentiometer in the clockwise direction, current will start flowing.

Testing of the unit is done attaching it to a power supply that provides different test voltages while we adjust the current with the multi-turn potentiometer on the DC load unit. Just make sure not to exceed 25W of power at any given time. Doing so could damage the MOSFET itself.

I plan to use this DC load in all my future projects that require a power supply of 25W or less, to test the power supply itself. Besides checking that the power supply works fine, you could also check that the ripple of the output voltage does not exceeds your requirements. That can be done connecting the power supply output to an oscilloscope while the DC load draws the current.

And finally, here are a couple of picture of the finished device.

20190816_112540.jpg

20190816_112614

Happy experiments!