Variable resistors

Variable resistors consist of a resistance track with connections at both ends and a wiper which moves along the track. The track may be made from carbon, cermets (ceramic and metal mixture) or a coil of wire (for low resistances).

Rheostat

In a rheostat two terminals are used: one connected to the end of the track, the other to the moveable wiper. The resistance changes between the two terminals from zero up to the maximum resistance.

Rheostats are often used to vary current, for example to control the brightness of a lamp.

Potentiometer

Potentiometers have all three terminals connected. They are used to vary voltage, for example to control the volume (loudness) in radio or TV.

Presets

Presets are small variable resistors. They are adjusted only when the circuit is built. Presets are adjusted with a small screwdriver.

Text 3

Michael Faraday (1791 –1867)

Michael Faraday was an English chemist and physicist who worked the fields of electromagnetism and electrochemistry. The unit of capacitance the farad was named after him.

Capacitors

Capacitors store electric charge. They are used to smooth varying DC supplies by acting as a reservoir of charge. They are also used in filter circuits because capacitors easily pass AC (changing) signals but they block DC (constant) signals.

Capacitance is a measure of a capacitor's ability to store charge. A large capacitance means that more charge can be stored. Capacitance is measured in farads, symbol F. However, 1F is very large, so prefixes are used to show the smaller values.

Three prefixes (multipliers) are used, µ (micro), n (nano) and p (pico):

· µ means 10^-6 (millionth), so 1000000µF = 1F

· n means 10^-9 (thousand-millionth), so 1000nF = 1µF

· p means 10^-12 (million-millionth), so 1000pF = 1nF

There are many types of capacitor but they can be divided into two groups, polarized and unpolarized.

1)Polarised capacitors (Electrolytic Capacitors, large values, 1µF +)

Electrolytic capacitors are polarized and they must be connected correctly. One of their leads is marked + or -.

2) Unpolarized capacitors (small values, up to 1µF)

Small value capacitors are not polarised and may be connected either way round. They are not damaged by heat when soldering. They have high voltage ratings of at least 50V, usually 250V or so.

Many small value capacitors have their value printed but without a multiplier. For example 0.1 means 0.1µF = 100nF.

Sometimes the multiplier is used in place of the decimal point:
For example: 4n7 means 4.7nF.

Variable capacitors

Vocabulary

to store charge накапливать заряд

to smooth сглаживать

voltage rating максимально допустимое напряжение

track дорожка

wiper ползунок

terminalклемма; ввод / вывод

multiplier множитель, коэффициент

in place of вместо

Exercise 1. Translate into English.

1. Принципиальные схемы показывают соединения компонентов.

2. Резисторы используются для уменьшения тока в цепи.

3. Когда ток проходит через резистор его температура повышается.

4. Постоянные сопротивления имеют постоянную величину.

5. Если величина сопротивления может быть изменена, то это – переменное сопротивление.

6. Конденсаторы легко проводят переменный ток, но они блокируют постоянный ток.

7. Емкость - это мера способности конденсатора накапливать заряд.

Text 4

Relays

A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and connects or disconnects the switch contacts. The coil current can be on or off, so all relays have two switch positions.

Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits; the link is magnetic and mechanical.

The coil of a relay passes a relatively large current, typically 30mA for a 12V relay, but it can be as much as 100mA for relays designed to operate from lower voltages.

Relays can have many sets of switch contacts.
The relay's switch connections are usually labeled COM, NC and NO:

· COM = Common, always connect to this terminal; it is the moving part of the switch.

· NC = Normally Closed, COM is connected to this terminal when the relay coil is off.

· NO = Normally Open, COM is connected to this terminal when the relay coil is on.

Connect to COM and NO if you want the switched circuit to be on when the relay coil is on.

Connect to COM and NC if you want the switched circuit to be on when the relay coil is off.

Diagram showing the operation of electromagnetic relay.

Exercise 2. Translate into English.

1.Ток, текущий через катушку реле создает магнитное поле, которое притягивает рычаг и замыкает или размыкает контакты выключателя.

2. Внутри реле нет электрического соединения между двумя цепями.

3. Все реле имеют два положения переключения.

4. Реле могут иметь много контактных групп переключения.

Text 5

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