Sometimes you may be unable to find a particular inductor the market. This is actually a problem faced by most of the electronic hobbyists and the problem becomes more serious if your project is RF related. The inductors required for RF circuits (antenna, tuner, amplifier etc) are almost impossible to find in the market and the only solution is nothing other than home-brewing them.

With a little practice and patience you can construct almost all air cored inductors at home. The inductance of an air cored inductor can be represented using the simplified formula shown below and to calculate the inductance of an air-core inductor, the same equation may be used.

L  = [d² n²] / [18d + 40l]

• Where’ L ‘  is the inductance in Micro Henries [µH]
• ‘d’ is the diameter of the coil from one wire centre to another wire centre. It should be specifies in inches.
• ‘l’ is the length of the coil specified in inches.
• ‘n’ is the number of turns.

Notes :

• The length of the coil used in the inductor should be equal to or 0.4 times the diameter of the coil.
• As shown in the equation, inductance of the air-core inductor varies as the square of the number of turns.  Thus the value ‘l’ is multiplied four times if the value of ‘n’ is doubled.  The value of  ‘l’ is multiplied by two if the value of ‘n’ is increased up to 40%.

Winding the coil.

• The coil must be first wounded on a plastic former of the adequate diameter (equal to the required core diameter).
• The winding must be tight and adjacent turns must be as close as possible.
• After the winding is complete, slowly withdraw the core without disturbing the coil.
• Now apply a thin layer of epoxy over the coil surface for mechanical support.
• Remove the insulation from the coil ends.

Example

Suppose you want to make an inductor which produces an inductance of 10 μH. The diameter of the coil is 1 inch and the coil length is given by 1.25 inches. You will have to find the number of turns of the coil.

Thus substituting the values in the above equation t

L = 10 inches

d = 1inch

l = 1.25 inches

n = √{L [18d * 40l]} / d = 26

Thus, the number of turns of the coil will be 26.

Number of turns/inch = 20.8

Description.
A lot of electronic circuits in the domain of audio amplifiers are already been published here. This circuit is a little different because it is a four channel amplifier. Each channel of this amplifier can deliver an output of 15Watts into a 4 ohm speaker. The amplifier can be operated from a single 12V DC supply and this makes it possible to use this amplifier in car audio applications too.

The circuit is based on the 15W BTL X 2 channel audio power amplifier IC TA8215 from Toshiba. Even though chip is specifically designed for car audio applications it can be also used for home audio applications. Two TA8215 ICs are used here in order to obtain a 4 channel amplifier system. The circuit is designed almost exactly as per the application diagram in the ICs datasheet. Pins 7 and 19 are the Vcc pins of the ICs internal integrated power amplifier stages and these pins are connected to the positive supply. Pin 9 is the Vcc pin for ICs internal preamplifier and it is also connected to the positive supply. Pins 13 and 14 are the internal power amplifiers ground pins and they are tied together and connected to the ground. The internal preamplifier’s ground pin (pin5) is connected to the common ground through a 10 Ohm resistor which makes the input ground separated from the common ground by a resistance of 10 ohms and this improves the noise rejection. The 100uF capacitor works as a power supply de-coupler. The resistor networks connected to the output lines of each amplifier improves the high frequency stability. The variable resistors (R3, R4, R12 and R13) works as the volume controller for the corresponding channels.

Circuit diagram.

Notes.

• Assembling the circuit on a good quality PCB is a must for obtaining optimum sound quality.
• Use 12V DC for powering the circuit.
• The ICs must be fitted with adequately sized heat sinks.
• R3, R4, R12 and R13 serves as volume controllers.
• K1 to K4 can be 4 Ohm, 20W speakers.
• This amplifier circuit can be used in a variety of applications such as car audio systems, home theater systems, personal audio systems, public address systems etc.

I have already explained in detail the working of a rheostat. To know more about the component click on the link below.

TAKE A LOOK : POTENTIOMETER AND RHEOSTAT – WORKING AND COMPARISON

Although the below explained process is not applicable for any electronic circuits, you will clearly get a clear idea on how a rheostat works.

Components

1. The components needed for the connection are
2. Flashlight bulb and socket [1]
3. Dry cell lantern battery/D-cell battery [2]
4. Wire [About 15 to 17 inches and another one 2 inches]
5. Spring [1]
6. Wire Clippers [A pair]
7. A typical spring can be obtained from a widow roll up. You can even get to buy one at a cheap rate.

Procedure

• Connect the two Dry cell lantern/D-cell batteries tail-to-tail, so that the positive polarity of one battery is connected to the negative polarity of the other.
• Using a wire cutter, cut the wire in equal lengths. One wire should be at least 8 centimetres long.
• Connect the wires onto the open ends of both the batteries.
• The end of one wire must be connected to the bulb socket with the bulb in it.
• Connect the second wire to one end of the long spring.
• Connect the free end of one wire to one terminal of the light socket.
• Connect the other free wire to one end of the spring.
• Take the two inch wire and connect it to the second terminal of bulb socket.
• Connect the other end of the two inch wire onto the other end of the spring.

How to make a Rheostat

What happens?

As soon as the circuit is in closed loop, the bulb begins to glow. Although the glow intensity is less, when you move the wire through the spring onto the other end where the wire is connected, the bulb starts to glow more brightly. When both the wires are nearby the glow will be in its maximum.

The spring is mainly made of steel wire. Steel wires are not very good conductors of electricity. Thus the resistance of the circuit also increases. If the spring length is long enough you will get to see different stages of the glow. Thus you will get to see the working of a rheostat.

I have already explained the working of a variable resistor and its applications in electronic circuits. To know more about it click on the link below.

TAKE A LOOK : VARIABLE RESISTORS – WORKING AND APPLICATIONS

TAKE A LOOK : WORKING OF RESISTORS

Potentiometer

A potentiometer, also called as POT, is a 3-terminal variable resistor and is used to adjust the resistance in a circuit.

Working of Potentiometer

The working of potentiometer is the same as that of a variable resistor. The construction is also the same. It has a resistive element as the track and a sliding contact called the wiper. The wiper is connected with the help of another sliding contact to another terminal. The position of the wiper depends on the type of POT used. For a panel POT, the wiper is kept in the middle.

The resistive element has a terminal on both the ends and can be linear or logarithmic. It is usually made up of carbon or a mixture of ceramic and metal or even graphite.

There is single turn POTs which changes its entire resistance in one rotation. More accurate POTs called multi-turn POTs are also present. They need about 20 to 30 rotations to change the entire resistance. They are much more accurate than the former. Take a look at the symbol of a POT.

POTs are differentiated according to the track used. There are mainly two of them. They are

Logarithmic Potentiometer

This type of a POT is shortly designated with the letter “A” in the component. For example, a POT with 5 kilo ohm resistance will be given as “5k A”. The track of this material will be a resistive material that may be tapered from one end to the other. It may also be a material whose resistivity varies from one end to the other. As like a logarithmic variable resistor this type of POT will have a logarithmic output. Due to this nature, they are always used in audio circuits. They are much more costlier than other types of POTS.

Linear Potentiometer

This type of a POT is shortly designated with the letter “B” in the component. For example, a POT with 5 kilo ohm resistance will be given as “5k B”. In this device, the track element has a constant cross-section causing a proportional resistance change between the wiper and one end of the terminal. The device depends on the electrical feature and not the resistive feature. This POT is used for a proportional change like adjusting the centering of a CRO.

Uses of a Potentiometer

• POTs are used for controlling the signal level of a circuit and not the power of the circuit
• Volume control on TV’s and other audio equipments.
• Used in joysticks as a position transducer.
• TRIAC switching applications.
• Voltage divider circuits.

Rheostat

A rheostat is also a variable resistor and is a 2-terminal device. It is commonly used for handling higher currents and voltages. One terminal will be connected to the end of the track and the other to a moveable wiper. When the wiper moves from one end to the other, the resistance changes from zero to maximum.

A rheostat can be made out of a potentiometer. The same mechanism is used except that the terminal that is not used will be connected to the wiper. This helps in reducing the variation in resistance. It also helps in gaining more mechanical strength when connected to a PCB.
The track is usually made from a resistance wire which is wound on a heat resisting cylinder.They are wound together to form the shape of a toroid coil. The slider moves from one phase of the coil to another, thus varying the resistance. The sliders are made in the shape of metal fingers and they move across the tracks through tapping method.

Rheostat symbol

Difference Between Potentiometer and Rheostat

• A potentiometer is a three terminal variable resistor, but a rheostat is a two terminal variable resistor.
• A potentiometer can be used as a rheostat but a rheostat cannot be used as a potentiometer.
• Potentiometers are often used to vary voltage and rheostats are used to vary current.

Variable resistors find its application in most of the electronic circuits used today. We have been getting a lot of comments with questions on how a variable resistor works and so on. Here are the details.

TAKE A LOOK : POTENTIOMETER AND RHEOSTAT – WORKING AND COMPARISON

TAKE A LOOK : WORKING OF RESISTORS

Types of Variable resistors

What is a Variable Resistor?

A variable resistor is a device that is used to change the resistance according to our needs in an electronic circuit. It can be used as a three terminal as well as a two terminal device. Mostly they are used as a three terminal device. Variable resistors are mostly used for device calibration.

Working of Variable Resistor

As shown in the diagram below, a variable resistor consists of a track which provides the resistance path. Two terminals of the device are connected to both the ends of the track. The third terminal is connected to a wiper that decides the motion of the track. The motion of the wiper through the track helps in increasing and decreasing the resistance.

Variable Resistors

The track is usually made of a mixture of ceramic and metal or can be made of carbon as well. As a resistive material is needed, carbon film type variable resistors are mostly used. They find applications in radio receiver circuits, audio amplifier circuits and TV receivers. For applications of small resistances, the resistance track may just be a coil of wire. The track can be in both the rotary as well as straight versions. In a rotary track some of them may include a switch. The switch will have an operating shaft which can be easily moved in the axial direction with one of its ends moving from the body of variable resistor switch.

The rotary track resistor with has two applications. One is to change the resistance. The switch mechanism is used for the electric contact and non-contact by on/off operation of the switch. There are switch mechanism variable resistors with annular cross-section which are used for the control of equipments. Even more components are added onto this type of a variable resistor so as to make them compatible for complicated electronic circuits. A high-voltage variable resistor such as a focus pack is an example. This device is capable of producing a variable focus voltage as well as a screen voltage. It is also connected to a variable resistance circuit and also a fixed resistance circuit [bleeder resistor] to bring a change in the applied voltage. For this both the fixed and variable resistor are connected in series.

A track made in a straight path is called a slider. As the position of a slider cannot be seen or confirmed according to the adjustment of resistance, a stopping mechanism is usually included to prevent the hazards caused due to over rotation.

Variable Resistance Specification

Various parameters like size, type of track and also resistance is used to define a variable resistance. Usually the spindle diameter of a variable resistor is 6mm.
If the variable resistor has a straight track it is defined in the component by the short form LIN representing a linear track. If it is a rotary track it is represented in short as LOG, as for a logarithmic track.
A common representation is given below.
5K6 LIN  – 5.6 kilo ohm with a linear track.
2M LOG – 2 Mega ohm with a logarithmic track.

In a linear track variable resistor, as the wiper is moved along the track the resistance varies constantly. In such resistors, the specification may not be given on the type. In that case, you will have to assume that it is linear.

In a logarithmic track variable resistor, the resistance does not increase/decrease constantly. As the wiper is moved from one end, the resistance changes at a slower rate and when the wiper is brought to the other end, the resistance changes at a fatser rate. This means that when the wiper is at halfway along the track, the resistance is not half the value of the total resistance. This is specifically applied for volume control as the response of the human ear to sound is also logarithmic. That is, a slow change in the beginning and a rapid change towards the end.

Application of Variable Resistors

There are mainly three types of variable resistors. They are

1. Potentiometer
2. Rheostat
3. Presets

To know more about potentiometers and rheostat click on the link below.

TAKE A LOOK : POTENTIOMETER AND RHEOSTAT – WORKING AND COMPARISON

Out of these presets are just smaller versions of a variable resistor. They can be easily placed on a PCB and can be adjustable when needed. The value of resistance is commonly adjusted with the help of a screw-driver. They are mostly used in applications like adjusting the frequency of an alarm tone or to adjust sensitivity of circuits. Since this device is the cheapest among all the three they are more commonly used amongst all of them. There are also highly precise presets which have multi turn options. In this type, the resistance will increase/decrease only slowly and hence the screw has to be rotated many times. Here also the basic slider and track mechanism is used. The track mechanism is always linear. Take a look at the preset symbol.

Preset Symbol

Most of the variable resistors are placed directly on the PCB. Some are mounted by drilling a hole in the case containing the circuit and is connected to the terminals with the help of a wire.

In recent years, the size as well as the weight of such devices have been greatly reduced which makes it more suitable for any field of electronics.