Monthly Archives: August 2017

What are Flexible Heaters?

Everyone is familiar with heaters. Although the simplest forms of heaters are sunlight and fire, both are not easy to handle or control. Therefore, people prefer using electricity for generating heat, as this is easy to control, requiring only a switch to shut off or turn on.

An electric heater generates heat by driving current through a resistive element. The power this arrangement consumes is the product of the resistance of the element and the square of the current flowing through it. The resistance radiates a part of the power it consumes as heat. This heat reaches other nearby surfaces through conduction, convection, or radiation, and transfers its energy to them, increasing their temperature.

Controlling an electric heater is a convenient way to keep the heated surface at a specific temperature or at least below a temperature that would cause damage. Initially, the resistive element of these heaters used simple nickel-chrome wires, as these could withstand high temperatures without melting. The heaters generally wrapped the wires around a mass and connected the ends to a power source. However, this arrangement, although effective, was not a practical solution for all applications.

Heaters have now evolved into flexible types, ones designed on flexible material, suitable for attachment to both flat and non-flat surfaces. Usually, temperature-sensing devices accompany these heaters, allowing constant monitoring and adjustments depending on the changes in the ambient surroundings. Flexible heater materials are commonly of two types—polyimide and silicon rubber—with flexible polyimide heaters being more popular.

Designers need to choose the conductor or the resistive element very carefully when designing a flexible heater. Flexible circuits commonly use copper as the standard cost-effective material, and this comes as a pre-laminated material. However, as copper is a good conductor, its resistivity is low. Therefore, heaters designed with regular copper require a lot of surface, and is suitable only for very low resistive designs.

Flexible heaters meant for producing high heat within small areas need a different material than regular copper for higher resistance. Designers thus use different types of nickel-copper alloys, such as Constantan or Inconel instead, which allow much higher resistive circuits within smaller areas. However, as nickel-copper alloys pre-laminated on a polyimide substrate are not commonly available, flexible heaters are more expensive.

The resistance of a heater depends on the target temperature of the material it is heating. After calculating the required resistance, the designer creates a pattern of interlocking serpentine traces of the correct width and length that emit consistent heat across the surface.

Some equipment, exposed to conditions of varying temperatures, use flexible heaters to keep their components at a consistent temperature. In cold countries, automobile manufacturers use flexible heaters for warming the steering wheel or the seat in their cars. Flexible heaters often keep biological samples at typical body temperature of a human or animal for better analysis. Flexible heaters keep batteries and electronics of aircraft warm when they have to operate at high altitudes. Flexible heaters keep critical components within ATMs and handheld electronic devices operating accurately in cold climates. This makes flexible heaters an important element in the electronic industry.

How is Solid Insulation Tested?

Engineers test solid insulation using two important test methods, and both use the Direct Voltage. One of the tests involves checking the resistance of the insulation, while the other tests the leakage current through the insulation at high voltages.

Insulation Resistance Testing

The instrument used for conducting this test is a megohmmeter. The instrument can be hand-cranked, motor driven, or electronic. Regardless of its principle of operation, a megohmmeter generates a direct voltage in the range of 100-15,000 V, and when applied to the insulation, indicates the material’s resistance in megohms.

As the resistance of any insulation material is temperature dependent, all readings need correction to the standard temperature for the class of the equipment under test. Usually, engineers refer to a table for the temperature correction factors for various electrical apparatus.

The value of the resistance of an insulation material is inversely proportional to the volume of insulation under test. For instance, a material a hundred feet long would have one-tenth the insulation resistance of another material a thousand feet long, provided all other conditions remained identical.

Engineers use this test typically to obtain an indication of deterioration trends of the insulation they are testing. However, the value of the insulation resistance by itself is no indication of the weakness of the insulation or the total dielectric strength of the material.

However, if the value of the insulation resistance showed a continuous downward trend, it usually points towards a contamination of the insulation, along with deterioration ahead. Therefore, engineers measure the insulation resistance in four common methods— short-time readings, time-resistance readings, polarization index test, and step-voltage test—to check for deterioration in the insulation system.

Short-Time Readings

This reading is only a rough check of the condition of the insulation. It measures the value of the insulation resistance for a short period, through a spot reading. The reading usually lies on the curve of increasing value of the insulation resistance. When comparing this value with the previous values, if a downward trend is indicated, then the insulation is deteriorating.

Time-Resistance Readings

All good insulation materials show a continued increase in the value of their resistance over the period when the voltage is applied. However, contamination with moisture and dirt decreases the resistance value of an insulation system, indicating contamination.

Indicating conclusive results of the state of the insulation, the time-resistance method is independent of the equipment size and its temperature. The state of the insulation system is derived from a ratio of the time-resistance readings.

Polarization-Index Test

This is a specialized application of the dielectric absorption test. Polarization Index is the ratio of the insulation test at 10 minutes to the insulation test at 1 minute. A ratio of less than one indicates the insulation is deteriorating and needs immediate maintenance. Usually, this test is reserved for dry insulation systems such as for cables, dry type transformers, rotating machines, and so on.

Step-Voltage Test

A controlled voltage method is used to apply voltage in steps to the insulation under test. If the insulation is weak, it will show a reduction in resistance that would not be apparent under lower voltage levels.

Expanding the GPIO on the Raspberry Pi

Although the single board computer the Raspberry Pi or RBPi has nearly 26 GPIO pins in its earlier models and 40 in its latest, there are times when the project demands more of them. In such cases, a GPIO expander is the only solution, and the MCP23S08, a device that Microchip Technology makes provides an easy way to expand the IO pins of a micro-controller using only a 2-wire serial interface.

The MCP23S08 works with the I2C protocol as a slave device, providing 8-bit, general purpose, bi-directional IO expansion for the I2C bus. It supports a 7-bit slave addressing, with the control byte acting as the read/write bit. Of the slave address, the MCP23S08 fixes the four most significant bits to 0100. This leaves the remaining three bits to be defined by the user as the address bits. Therefore, one can connect up to eight MCP23S08 devices on a common I2C bus at any one time.

It is possible to configure individual bits of the 8-bit GPIO port as either input or output. At the same time, it is also possible to enable the internal pull-up resistor on the port pins to interrupt-on-change. A set of configuration and control registers control these operations. Each resistor has its own address and its power-on reset value, as listed on the datasheet of the MCP23S08.

The first register is the IO direction register, and controls the direction of the data IO. On setting a bit in this register, its corresponding pin assumes an input direction, and if the bit is clear, its corresponding pin works like an output.

With the input polarity register, the user can configure the polarity of the corresponding GPIO port bits. When he/she sets a bit on this register, the corresponding GPIO register bit stores the inverted value present on that pin.

Interrupt-on-change control register controls this feature for each pin. When the user sets a bit on this register, the corresponding pin becomes capable of interrupt-on-change. Of course, for enabling this feature, the user must also configure the INTCON and DEFVAL registers as well.

Interrupt control or INTCON register controls the manner in which the associated pin value compares for the interrupt-on-change feature. If the user sets the bit, MCP23S08 compares the corresponding IO pin against the associated bit in the DEFVAL register. If clear, the MCP23008 compares the corresponding IO pin to its previous value.

The default comparison value or DEFVAL register is for configuring the default comparison value. If the user enables a bit in this register, and the associated pin has an opposite value, it will cause an interrupt.

The PI-SPI-DIN series of IO modules has the RBPi reading 2 modules of eight isolated digital inputs for a total of sixteen inputs and controlling four modules of four relay outputs for a total of 16 relays, all using the IO expander chip MCP23S08.

As the RBPi can select only two chip-select lines, each of the PI-SPI-DIN modules has a jumper selection of five chip selects. The two address lines of the MCP23S08 need to be enabled in the setup routine while determining the port pins as inputs or outputs.

What are Memristors?

What are Memristors and where are they used?

Professor Leon Chua developed the memristor theory in 1971. A scientist discovered the behavior of memristors when he was working in a lab at HP, trying to figure out crossbar switches. Memristors are also known as matrix switches, as they behave as a switch when connecting multiple inputs to several outputs. When Professor Chua looked at the assortment of resistors, capacitors, and inductors for the switching, he noticed a vital missing component, calling it the memory resistor or memristor. In 2006, Stanley Williams developed the practical model of a memristor.

One can consider the memristor as the fourth class of electrical component, after the familiar resistor, capacitor, and inductor. However, unlike the others, memristors exhibit their unique properties only at the nanoscale. Theoretically, memristors maintain a relationship between the time integrals of voltage across and current through two terminals of an element, as they are passive circuit elements.

The resistance of a memristor, or memristance, varies according to its function. It allows access to the history of the applied voltage via tiny read charges. The presence of hysteresis defines the material implementation of its memristive effects. This is its fundamental property, which looks more like a non-linear anomaly. Therefore, memristance is a simple charge dependent resistance, with a unit of Ohm. However, it has its own advantages.

The memristor technology offers lower heat generation as it utilizes less energy. Used in data centers, it offers greater resilience and reliability under interruptions of power. While not consuming any power when idle, memristors are compatible with CMOS interfaces. It is possible to store additional information as memristors allow higher densities to be achieved.

Physically, the memristor has two platinum electrodes across a resistive material, and its resistance depends on its polarity, magnitude, and length. The device retains its resistance even when the voltage is turned off, which makes it a non-volatile memory device. The resistive material can be titanium dioxide or silicon dioxide. As voltage is applied across the terminals, the oxygen atoms within the material disperse towards one of the electrodes. This activity stretches or contracts the material depending on the polarity of the applied voltage, thereby changing the resistance of the memristor.

Depending on their build, memristors are of two types—Iconic thin film and molecular memristors, and magnetic and spin based thermistors.

The material property of iconic thin film and molecular memristors rely more on different material properties of the thin film atomic lattices and application of charge makes these display hysteresis. Using these materials scientists make memristors of Titanium dioxide, ionic or polymers, resonant tunneling diodes, and manganite.

In contrast, magnetic and spin based memristors rely more on the property of the degree of electron spin. Therefore, these systems are aware of the polarization of electronic spin. There are two major types of such memristors—the spintronic memristors, and the spin torque transistor memristors.

With the practical demonstration of memristor manufacturing, their potential application has led to a rapid increase in research for using them in analog and digital circuits, such as programmable logic controllers, computers, and sensors. This has also led to development of theoretical models of memristors—Verilog-A, MATLAB, and Spice.