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Interconnected PWM

Bipolar PWM with Arduino

Our popular power PWM control circuits use a single transistor to pulse all sorts of loads for power control. The pulses control current in one direction only which is fine for most PWM applications. However sometimes it is required to have a bi directional pulse for driving an AC load such as a transformer.

Ideally a dedicated H-bridge circuit would be used, but it is possible to approximate this using a pair of our PWM circuits being controlled with an Arduino.

Using our Arduino library NanoPWMac we can drive two circuits with a pulse that is equal in length, but with only one circuit active at a time.

Below you can see a simplified diagram of what is happening with a normal single transistor PWM circuit. The pulse signal basically switches power on/off to the load by making and breaking the path for current from the PSU through the load.

Our PWM modules can be linked directly in a master/slave arrangement (see datasheet) so that when one is on, the other is off and visa-versa. This can be used to drive a transformer with AC, but it only really works if you want a 50% duty cycle. This is because if you set the master PWM to 10%, the other one will be 90% therefore driving the transformer unequally and not allowing for proper power control.

To dive a coil with AC and adjustable duty, an Arduino can be connected to two PWM circuits as shown below. The coil must be centre tapped so that each PWM will pull current in opposite directions.

For simplicity the power connections are not shown in the diagram. To link the Arduino to the PWM modules, the SIG jumper is removed from both modules, and a connection from the Arduino pins 9 and 10 is made to the SIG pin marked with a stripe on the OCXi.

The example program in the nanoPWMac library will take a reading from potentiometers connected to A4 and A5 so that these pots can be used to vary frequency and duty independently.

With this setup you can now pulse both circuits in opposition with a waveform like shown below.

Litz Wire

Litz Wire

What is Litz Wire?

Litz wire is a type of cable formed by combining multiple strands of thin insulated wire together side by side. It is used to carry high frequency currents as the insulated strands each carry a portion of the current and prevent losses due to the skin effect. Litz wire is commonly found in radio frequency (RF) applications, and high frequency power circuits such as induction heaters and Tesla Coils.

The strands in litz wire are typically twisted together either as a single bunch, or as multiple bunches twisted to form a large cable. By twisting the wires together it helps to control the magnetic fields around the wires and keep the currents flowing evenly.

What is the skin effect?

She skin effect is a term given to the phenomenon of when high frequency currents tend to flow near the surface (or skin) of an electrical conductor. This occurs due to magnetic fields being induced in the conductor by the changing currents. The magnetic fields make it difficult for the currents to flow anywhere but the outer surface.
With the currents being forced to flow in just part of the conducting wire, the effective resistance of the wire is greater. The higher the frequency, the more loss in the wire due to this resistance.
By using this skin effect calculator, we can see that at a frequency of 1MHz, the effective skin depth is just 65μm (0.065mm), while at 1kHz, the effective depth is 2062μm (2mm) in a copper wire. 

How to make litz wire

Using common magnet wire it is possible to make your own litz wire by twisting it together in bunches. It is not practical for large numbers of strands but is doable for short lengths with a small number of individual wires.

The simplest way is to cut equal lengths of the required magnet wire and to clamp or tie all of one end together. The other ends can be put in the chuck of a hand drill which is used to twist them tightly together.

How to solder litz wire

It can be very difficult to solder litz wire as they can contain thousands of individually insulated strands. It is necessary to remove this insulation before soldering can take place. This can be done by first burning the end to be soldered and then cleaning of the burned insulation with wire wool. Doing this without breaking the fine strands or leaving some of them uncleaned can be quite challenging.

The best way is to use a hot solder bath as this can both burn off the insulation and add the solder. The solder bath must be quite hot and it works best if the litz wire is first dipped into flux to help the solder to stick.

Where can I buy litz wire?

We have a range of litz wire sizes for sale and offer soldering services for ready to use litz cable assemblies. If there is a specific size you need that we do not stock, we can custom make it to your requirements.

Lasers and Interference

Diffraction and Interference

Diffraction through a slitDiffraction is the phenomenon where waves can be bent around obstacles. When coherent light passes through a fine slit some of the rays are diffracted. The varying levels of diffraction cause part of the beam to interfere, thus producing an interference pattern.

Light from most sources is incoherent. This means that the many waves coming from the source do not line up with each other, or are not in phase. Common sources of light such as the sun or a light bulb emit photons at random intervals, but as there are so many overall we see it as just a constant source.

Lasers provide a source of coherent light due to the way that they work. All the wavefront’s emitted from a laser line up with each other. We can use this source of coherent light to demonstrate interference from diffraction in by a single slit.

If the beam from a laser is shone through a fine slit, such as that between the edges of two razor blades, we can easily see how the waves are diffracted and produce interference. If the light from the slit is projected onto a paper screen we can observe and measure the patterns produced.

interference patternThis phenomenon is related to Huygen’s Principle. This says that every point on a wave front acts as a source of tiny wavelets that move forward with the same speed as the wave. The wave front at a later instant is the surface that is tangent to the wavelets.

Refraction and the Spectrum

refraction in a prismWhen light enters a denser medium is effectively slowed down. This is due to the light being repeatedly absorbed and re-emited by the atoms in the material. This slowing of the wavefront’s causes the beam to be bent at an angle dependent on the material and the wavelength (colour) of the light.

This image shows how white light is split into its component parts using a prism.

Electricity from Light

Homemade Solar Panels

This shows a simple demonstration of how a homemade solar panel can be used to show the photoelectric effect making electricity from light. Making a solar panel this way would be no where near good enough to generate useful free energy for powering a home, but it great for demonstrating scientific principles.

Solar PanelsOne sheet of copper is given a coating of cupric oxide as this semiconductor will convert sunlight into electricity. To do this a copper sheet is heated over a hob for around 30 minuets. When the sheet has turned black it is allowed to cool. As it cools down the black copper oxide will start to crack up and come off. this is because the materials underneath are contracting at a different rate. it is usually necessary to finish removing some of the black copper oxide by hand.

The remaining layer is covered with cupric oxide which is the important material for producing the photoelectric effect.

If the copper sheets are placed in a strong salt solution in the arrangement shown above, a small voltage will appear between the two sheets. The salt water is really just meant as an electrical conductor, but it also has the effect of making the device operate like a battery. this means it still generates small amounts of electricity even in the dark.

Electricity from Heat

Free Energy from the Environment

This is a simple setup that uses the thermoelectric / seebeck effect for converting heat directly into electricity with no moving parts. There are devices available called Peltier Heat Pumps which are used to keep electronic components cool.

Thermoelectric GeneratorThey are commonly used on processors as they can move heat away from a source under electrical power. When connected to a DC power supply a peltier element will heat up on one side whilst becoming cold on the opposite side. A large heatsink is necessary to dissipate the excess heat so that the other side can remain cold.

These peltier elements can also work in reverse. If it is heated on one side whilst being cooled on the other side a voltage will appear across the terminals. As long as there is a temperature difference between the two sides of the peltier device, then there will be a voltage between the electrical terminals.

Peltier heat pumps are not a very efficient method of generating electricity, but at least the supply of energy is free.

In a practical application a large heatsink could be buried in the ground to remove heat. If the peltier element is placed onto this is can be arranged to be heated on one side by the sun. A small black heatsink on the top of the peltier device could be used to collect heat from the sun that has been focused by a large mirror. Another method might be to use a larger heatsink and to place it inside a greenhouse. With this method it would be important to insulate between the hot and cold heatsinks so that the ground would not be heated directly.

Liquid Metal Experiments

Liquid Metal Experiments

Liquid Metal AlloyMost metals are a tough solid material at room temperature, but some metals can be liquids. The most obvious one is the element Mercury, which has a melting temperature of -38.83 °C. When certain metals are mixed together they can form what is known as an alloy. The atoms of the various metals will bond together to form the new metal which may have unique properties. Below you can learn how an alloy can be made at home which will melt in hot water. The alloys melting temperature is less than that of any of the original metals.

Using three common metals you can make a metal alloy which will melt in hot water. The metals are Bismuth, Lead, and Tin. Bismuth can be found in the form of ‘lead free’ fishing weights, and the other metals can be found together in the form of solder. Most solder is a mix of 60% Tin and 40% Lead, but for this experiment you will need the type that is 40% tin and 60% Lead.

A eutectic alloy is one where the ratio of the materials is made to give the lowest melting temperature possible. The eutectic alloy of Bismuth, Lead, and Tin would have the following ratios. 52.53% Bismuth, 32.55% Lead, and 14.92% Tin, by weight. The resulting alloy from this mixture can be expressed with the formula, Bi8Pb5Sn4. When mixed in the correct proportions this metal will melt at 95°C

Another liquid metal can be made that will melt at -20°C. This metal is made from gallium, indium, and tin.

As you can see from this image, This type of metal will easily stick to other materials. This is why mercury is still used in many devices.

Hydrogen Cannon

Hydrogen Cannon

Hydrogen Cannon DiagramThis experiment demonstrates the splitting of water into Hydrogen and Oxygen by electrolysis, and the re combining of these elements in an energetic reaction.

You can never recover the full energy from the Hydrogen fuel due to the lack of efficiency in the process used to make the hydrogen gas. During the electrolysis a current is flowing in the water to seperate the Hydrogen from the Oxygen, it is this current that produces wasted energy in the form of heat.

A plastic tube with one closed end has two electrodes inserted in the base so that they can be connected to wires on the outside. The electrodes should be made from carbon, but any conductor such as thick wire will do for this experiment. The space between the electrodes should be as small as possible but without them touching each other.

When the device is complete the tube should be filled with water until it just covers the electrodes. When the switch is pressed, the battery is connected to the electrodes and the flowing current begins to split the water into Hydrogen and Oxygen.

At some distance above the electrodes there are two more wires that are used for creating a spark to detonate the Hydrogen and Oxygen gas inside. The Piezo Sparker comes from a cheap lighter with click type ignition. When this is pressed it creates a high voltage that is used to create a spark. The two wires inside should be separated so that the spark will jump between them, and should be far enough from the water so that they don’t get drenched when the water is bubbling.

The projectile should fit snugly inside the end of the tube so that it doesn’t fall down to the water. It is important that this object is able to move freely enough to to leave the tube without a lot of resistance or else the whole thing could explode. The best type of projectile is a tube like the main one but slightly smaller so that it fits inside. If it is inserted so that the open end is down inside the larger tube the Hydrogen/Oxygen gas can build up behind it. As the tube is forced out when the gas is detonated it is forced to move in a straight line and can be more accurate than other shaped projectiles.

To fire the cannon the switch should be pressed for around 20 seconds depending on the electrodes and battery used. When enough gas has built up inside, the piezo sparker can be pressed detonating the H2 and O2 gas mixture. The gas expands rapidly forcing out the projectile and the product of the energetic reaction is just water again. The energy released in the explosion is almost the same as the energy supplied by the battery but it is released in a fraction of a second.

SHM Resonance

Resonance and Simple Harmonic Motion

SHM DemosntrationThis experiments is a simple way to demonstrate the principles of resonance and simple harmonic motion.

This diagram show four identical springs attached to a support. Each spring has weight with a different mass attached to the end. If a weight is pulled and released, it will set it in motion bouncing up and down until all its energy is dissipated through friction. As each spring holds a different mass, they will each have a different fundamental or resonant frequency. The first spring with the most mass will bounce slowest. We say that this has the lowest resonant frequency out of the four springs. The frequency is simply the number of times the mass bounces up and down each second and is measured in Hertz (Hz).

If the support holding the springs is moved up and down at a set speed, we will see the weights begin to move up and down buy different amounts. The amount of displacement of each mass will be dependant upon the frequency that the support is oscillating at.

SHM AnimationWe will see the maximum displacement when the frequency of the moving support is the same as the resonant frequency of one of the weights. For example; If the resonant frequency of the first mass and spring is 10Hz then it will move the most when the support is moved up and down at 10Hz. This is known as resonance. If the support is moving at 9Hz or 11Hz then there will be significantly less movement of the mass. This is because resonance usually occurs when the frequencies match almost exactly.

The exact same principles are used in radios and televisions in order to tune into a specific station. Instead of oscillating springs, it is the electrical currents that are oscillating. By using different components it is possible to make a circuit that has a resonant frequency that is the same as the radio wave that we wish to detect. At resonance the amplitude of the signal in the circuit will be much higher than any of the other signals which allow us to use just one channel at a time.