Employability Skills: Brush Up Your Electronics

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This ensures that any fluctuations in load current have a limited effect on the divided output voltage.

The use of pull up and pull down resistors is a common feature in electronics. Closing S1 in the left hand diagram pulls down the voltage at the lower end of Ra by shorting it to the zero line. Current flowing in Ra will then depend solely upon the resistor value and the supply voltage. In the diagram to the right, the voltage at the top of Rb is pulled up to the supply voltage by closing S2.

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With equal value resistors will the current flowing be the same in both circuits when the switches are closed? S1 and S2 could be replaced by transistors acting as switches which effectively become short circuited between the collector and emitter terminals when made to conduct heavily.

Resistor Ra is a collector load and Rb an emitter resistor. When a transistor is biased OFF, i. For T1 the collector voltage would be high, no collector current flowing and for T2 the emitter voltage would be zero, with no emitter current flowing. Biasing ON T1 and its collector output voltage is pulled down and for T2 the emitter voltage is pulled up.

Switch contacts when open provide an interruption of the current flow within a circuit and when closed completes the conducting path. Shown is one of the simplest of schematic diagrams that consists of just three components, indicated by appropriate symbols. Clearly shown are the component connections and the effect of what happens when the switch button is pushed. One of the simplest types of switch has to be the push-to-make, i. Within the pages of a components catalogue you can find dozens of different combinations of switch types.

When selecting a switch there are two main considerations, current rating and the maximum working voltage.

Using a switch that is under-rated can be unreliable and dangerous because of arcing of the contacts or physically expose the user to an electric shock because of a voltage breakdown of the insulation. In this diagram the battery can represent any number of cells connected in series which increase the supply voltage potential difference as each cell is added. Battery cells are usually in multiples of 1.

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To calculate the current I flowing in this simple circuit we can use Ohm's Law by applying the formula shown. Try changing the battery supply voltage and note the changing current. In a practical circuit the more current that flows the brighter the lamp would glow. Increasing the voltage and thereby the current, above that permitted by the bulb and the filament acts like a fuse. Switches are available in many different types.

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Here is an example of a changeover switch that redirects the battery connection to either the lamp or the buzzer. This is a break-before-make switch. Others are make-before-break, where power is connected to both parts of the circuit during changeover. Switches are used as relay contacts, rotary selectors, slider contacts etc.

Here we have given the two devices a different working resistance. This demonstrates that the current drawn from the battery changes as the switch is thrown. As the battery has what is called 'internal resistance' this can cause a reduction in the potential difference PD between the battery terminals as the load resistance increases.

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Electronic buzzers have no moving contacts and therefore do not generate RF interference, but produce a clear penetrating sound. Typical uses are in internal burglar alarm sounders. The output frequency is around Hz with impedance of a few hundred ohms, consuming approximately 35mA when a voltage between four to twenty volts is applied. The staircase switch arrangement derives its name from its purpose in domestic house wiring, where it is often necessary to be able to switch a single lamp from any number of different places.

There are two types of switch used, A and D are normal changeover types.

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The others B and C are called intermediate switches of which there can be any number are crossover types. There is no need for a clever logic diagram to explain or carry out this action, its simply the bulb lights or extinguishes for every switch action. Follow the lamp current path for yourself for all the available switch states. A relay is a device, which is an electrically operated switch.

It works by energising an electromagnet that pulls or releases switch contacts that are make shorted out or break open circuit depending upon whether the relay coil is energised. Close S1 and a current is made to flow in the relay coil. This causes a magnetic field to develop which pulls the central switch contact [b] to the left, making the circuit with [a] for the bulb to light. Release S1 and the magnetic field collapses and the contact [b] returns to its rest position, against the right hand contact [c], causing the buzzer to sound. In use a reverse biased diode should be connected across the energising coil for protection as the high back-EMF that is generated when the current is switched off can easily damage any associated circuitry.

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Relays are available in both single-pole and double-pole switch actions. Typical switch contact resistance is measured as 50mOhms milli-Ohms with operating times of around a couple of milliseconds. Maximum switching currents can exceed 10Amps, but for most applications, a much smaller and less expensive device would suffice. A relay has a mechanical switching operation, not electronic, as is the case of a thyristor semiconductor device.

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The electromagnetic coil resistance is between 80 and 1kOhms depending on the type when designed for an operating voltage of around 5Volts to 30Volts. The life of the contacts can exceed , operations but this depends largely on the amount of current being switched and if it is AC or DC as the former tends to maintain cleaner switch contacts due to its uni-polar direction.

Most switches found in component catalogues are fairly standard, where only the method of mounting or presentation differs. However, there will be times when a specialist action is called for. Typical examples are found in electric cookers and washing machines. As the latter can be very complicated we will concentrate on the former to switch the heating elements in an oven or cooker hot plate.

The thermostat will be bi-metal strip, which bends and makes or breaks a pair of switch contacts to complete the circuit depending upon the temperature setting. But it's the switching action, which is of interest here. The two elements Re1 and Re2 are resistances which are designed to make use of their dissipated heat which is normally wasted. They could be considered as underrated wire wound types, which become hot due to excessive current owing to their low value.

In the 'Low Heat' setting they are connected in series, as their combined resistance increases across the L - N terminals the current flow and therefore heat dissipated will be relatively low. Select for more details of the calculations. As both Re1 and Re2 are always the same we can use a simplified method of finding the equivalent series and parallel values.

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