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November 2017

Inductors in DC Circuits

In addition to an electrical field, such as that which appears between the plates of a charged capacitor, there exists a second type of field in electrical engineering, namely a magnetic field. Whereas an electrical field arises in the proximity of static charges, an magnetic field is associated with moving charge carriers, i.e. an electrical current.

Capacitors in DC Circuits

Capacitors are components, in which static electrical charges are stored. In its most basic structure a capacitor consists of two metal plates, which constitute the electrodes of the capacitor. Due to the charge separation an electrical potential difference (voltage) U is formed between the electrodes. The subsequent graphic image shows an example of a plate capacitor with plate surface area A and plate separation d, which carries the charge Q.

Voltage Dependent Resistor VDR

The resistance of certain semiconductors (e.g. silicon carbide) depends notably on the applied voltage. Such materials are termed varistors or VDRs (Voltage Dependent Resistor).

Photoresistor (LDR)

A resistor whose value changes with light intensity is termed a photoresistor. This type of passive, opto-electronic component is frequently abbreviated to LDR (Light Dependent Resistor). A photoresistor's action is based on an internal, photoelectric effect. The energy (photons) of light impinging on a semiconductor releases valence electrons from their lattice bonds. The higher the light intensity, the greater the number of released charge carriers.

Temperature Dependent PTC Resistors

Some metallic resistors, usually made of platinum or nickel, have a positive temperature coefficient, i.e. their resistance increases with the temperature. Their resistance characteristic is predominantly linear. The diagram below illustrates the resistances of platinum and nickel as a function of the temperature.

Temperature Dependent NTC Resistors

NTC (Negative Temperature Coefficient) resistors are semiconductors made of poly-crystalline, mixed-oxide ceramics and used mainly for temperature measurement. Since they are responsive to heat, they, and the positive-temperature coefficient resistors we shall later, are commonly dubbed with the name, thermistors.

Bridge Circuits

A bridge circuit consists of two voltage dividers connected in parallel as illustrated below.

Voltage Dividers !

Voltage Divider With a Potentiometer

In actual practice, use is mainly made of a continuously adjustable voltage divider in the form of a so-called potentiometer. This device has three connections, one of them is a variable sliding contact permitting the potentiometer's total resistance to be divided into two partial resistances R1 and R2.

Loaded Voltage Divider

The voltage divider can be loaded by connecting it to a load (resistor RL in the diagram below). This load resistance conducts a load current IL, while resistor R2 conducts a parallel current IQ. Resistor R1 conducts the sum of these two currents. The parallel current IQ produces heat loss in resistor R2.

Unloaded Voltage Divider

For the purposes of measurement, it is often necessary to tap sub-voltages from a primary voltage. This is done by means of voltage division. Illustrated below is a voltage divider consisting of two series-connected resistors R1and R2.

Combined Series and Parallel Resistor Circuits

You frequently find both series and parallel connections in a single circuit. Circuits of this kind are sometimes referred to as combined or mixed circuits. It is possible, for example, to connect three resistors both in series or parallel connection or also in combined circuits as shown in the following graphic.

Connecting Resistors in Parallel

Whereas in simple circuits and series connections the current only finds one path through the circuit, in parallel circuit configurations (shunted connection) there is a branching of the circuit (see the following figure).

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