Potential dividers

·

Fixed resistors in series

Potential dividers divide up the voltage within a circuit, so that parts of a circuit only receive the voltage they require. Potential dividers usually consist of two or more resistors arranged in series across a power supply.

·

Fixed resistor in series with an LDR

Potential dividers form an important part of sensor circuits. For example, an LDR or thermistor can be used in place of one of the resistors, with the output voltage signal being directed to an op-amp or IC.

·

Variable resistor in series with a thermistor

Instead of using a fixed resistor, a variable resistor allows the output voltage to be adjusted. A low-value fixed resistor should be placed in series with the variable resistor to prevent the full power of the circuit from being routed down the output voltage path if the variable resistor is accidentally moved to a low resistance.

Common uses of potential dividers

Potential dividers are important in both transistor-switching circuits and op-amp comparator circuits. The diagram shows a darkness-sensor circuit with a transistor used as a switch. When the LDR senses a drop in light, the LED is switched on.

· When the LDR has light falling on it, its resistance is low, usually around 400 ohms.

· When the LDR is covered up, the resistance increases (often to many kilo-ohms).

When the resistance of the LDR is small, its share of the voltage supply is small too; this means that the output voltage (Vout) from the voltage divider is small and the transistor is switched off.

In the dark the large resistance of the LDR takes a large share of the voltage supply so Vout is large and the transistor and LED both switch on.

Capacitors

A capacitor is a discrete component that can store an electrical charge. The larger the capacitance the more charge it can store.

The unit of measurement of capacitance is the farad. Often you will see capacitors of much less than a farad. These will be measured in microfarads (one millionth of a farad or 1/1,000,000) or picofarads (one million-millionth of a farad or 1/1,000,000,000,000).

There are two types of capacitor:

· polarised or electrolytic capacitors

· non-polarised or non-electrolytic capacitors

Дата добавления: 2015-10-26