Potentiometers and rheostats

Variable resistor, inductor

1. Potentiometers and Rheostats Dr.R.Hepzi Pramila Devamani , Assistant Professor of Physics, V.V.Vanniaperumal College for Women, Virudhunagar

2. Potentiometers These are variable resistors either of carbon or wire – wound type often used for controlling voltage and current in a circuit. They generally have carbon composition resistance element and are connected across a voltage source. They have three terminals, the centre one being connected to the variable arm which is used for varying voltage division in the circuit as shown in the figure.

3. Potentiometers By moving the variable arm B over the fixed resistance R between the points A and C, any part of the input voltage can be tapped off. Since in the figure 5.4, B happens to be at the middle value of R, output voltage is half the input voltage. Most variable resistors used in radios are potentiometers meant for controlling volume ofr tone. When used as a volume control, it picks off a voltage between zero and the full available voltage as shown in figure 5.5. By moving up and down , any desired amount of voltage (signal) can be picked up between maximum signal value at point A and zero signal value at C.

4. Potentiometers The tone control circuit shown if figure 5.6 uses only two terminals on the potentiometer. The resistor allows the capacitor to by-pass to ground either more or less of high frequencies in an audio circuit. When B is at point A, the capacitor becomes a direct by-pass to ground for higher frequencies in the audio signal. Consequently, radio sound becomes ‘ bassy ’. When B is at terminal C, it increases the amount of resistance in series with the capacitor. Hence less amount of high frequencies is by-passed to ground and consequently, there is more trebble in the sound. Potentiometers are commonly used as control devices in amplifiers, TV sets and various types of meters. Typical applications include volume and tone controls, balace controls, linearity and brightness controls in TV receivers.

5. Diagram

6. Rheostats The resistance element of rheostats is made of high resistance wire. It has two terminals and is connected in series with a circuit for adjusting the amount of current flowing through it. Rheostats are commonly used to control relatively high currents such as those found in motor and lamp circuits for controlling its current. Only resistance BC is connected into the circuit. Though similar in construction to potentiometers, they are usually larger in size because they possess much high power rating.

7. Rheostats A given potentiometer can be used as rheostat. One method is just to use two ends only leaving the third end unconnected as shown in figure 5.8 (a). The other method is to wire the third unused terminal to the central terminal as shown in figure. 5.8 (b).

8. Resistor Colour Code Since fixed carbon composition resistors having axial leads are physically small, they are colour -coded to indicate their resistance in ohms. The system is based on the use of colours as numerical values. The dark colours like black and brown correspond to the lowest numbers of zero and one respectively, the light colours to next higher numerals and lastly white colour to nine. The colours used with code and the numbers they represent are given in the table 5.1.

9. Resistors with colour code

10. Thermistors It is a thermal resistor i.e., a temperature-sensitive sensor. Most thermistors have negative temperature coefficient of resistance though devices having positive temperature coefficient are also available.

11. Construction Thermistors are made by heating under pressure (sintering) semiconductor ceramic materials made from mixture of metallic oxides. Various mixtures for this purpose are those of manganese, nickel, cobalt, copper and iron etc. These are pressed into desired shapes and then looked at high and then baked at high temperature for making different types of thermistors. Electrical connections are made either by including wires during shaping process or by silvering the surfaces after sintering.

12. Construction As shown in figure 15.33, thermistors are made in the shape of beads (with diameters ranging from 0.15mm to 2.5 mm), probes (beads sealed in glass rods, discs or washers) upto 25 mm in diameter and rods from 6mm to 50 mm long. Beads may be glass coated or enclosed in evacuated envelopes for protection against corrosion. Washer shaped thermistors can be bolted together for series or parallel connection. The usual symbol for the device is shown in figure 15.33

13. Temperature/Resistance Curve A typical curve is shown in figure 15.34., the thermistor has a resistance of about 3000 Ω at -20 C and 40 Ω at 80 C. Since a change of 100 C has resulted in a 75:1 change in resistance which makes a thermistor such a valuable temperature measurement and control device. There are two ways of changing the resistance of the device internally and externally. In other words, device resistance depends on ambient temperature and self heating.

14. Application Thermistors find many applications such as For measurement and control of temperature as in ovens. For temperature compensation i.e., cancelling the effect of temperature on other electronic devices. As flow rate measuring devices. For regulating ac voltage (unlike zener diode which being polarity sensitive, can regulate only dc voltages). For liquid level detection.

15. Inductor It is another basic component commonly used in electronic circuits. It is nothing but a coil wound on a core or former of some suitable material.

16. Air-core Inductor It consists of number of turns of wire wound on a former made of ordinary card board (fig.5.13a). Since there is air inside the coil, an air-core inductor has the least inductance for a given number of turns and core length.

17. Iron-core Inductor It is that inductor in which a coil of wire is wound over a solid or laminated iron core (fig.5.13b). Putting iron inside an inductor has the effect of increasing its inductance as many times as the relative permeability of iron. In order to avoid eddy current loss, iron core is laminated i.e.,it is made up of thin iron laminations pressed together but insulated from each other. Such an inductor is also called choke.

18. Iron-core Inductor The iron core has been found to work more efficiently at low frequencies if it is in the form of a closed core. i.e., If the core not only goes through the centre of the coil but also surrounds it on its two sides as shown in figure 5.14.

19. Ferrite-core inductor A coil of wire is wound on a solid core made of highly ferromagnetic substance called ferrite. Ferrite is a solid material consisting of fine particles of iron powder embedded in an insulating binder. A ferrite core has minimum current loss. The symbols for different types of inductors are shown in figure 5.15.

20. symbol