Topic 2.6.4 – Interfacing Outputs
Learning Objectives:
At the end of this topic you will be able to;
select suitable comparators, transistors, and MOSFET’s for connecting to signal sources and driving outputs such as lamps, buzzers, loudspeakers, motors, solenoids and relays;
state the need for diode protection for comparators, transistors, and MOSFET’s.
Interfacing Outputs.
In our previous topics we have discussed the use of op-amps, comparators, transistors, and MOSFET’s in electronic switching circuits but limited the output devices to LED’s or Lamps in order to concentrate the analysis of the circuits on understanding the processes involved rather than introduce a wide variety of output devices, to complicate matters further.
We have now examined all of the interfacing units needed for the AS course, so it is now worth spending a little time looking at the typical range of output devices you might come across in examination questions. For completeness we will include all of the available output units as a reference point.
1. LED’s – probably the most basic of output indicators. Provides light output in a range of colours.
Symbol.Picture:
Properties.
LED’s are notable for their high degree of efficiency, very long service life and their impact resistance. Current limiting resistors are necessary for safe operation at the low operating voltage, typically 2V with a current requirement of 10-20mA. Damaged easily by reverse voltages of more than 5V. The point-form light source enables precise directing of the light. The radiated power decreases slightly as the temperature increases. LED’s can be driven directly by logic gates, op-amps, and comparators.
2. Lamps – a simple indicator, now being phased out in many low power applications by LED’s. Provides light output usually in white with a range of colours being provided by the use of coloured glass or filters. A wide range of output power is available from a few mW to many kW.
Symbol.Pictures:
Properties.
Lamps are used for a number of general purpose applications, such as panel indicators, to provide illumination for a room or street, to lighting up a stage in the theatre, or in a searchlight scanning the skies as part of a lighting display.
Most lamps cannot be driven directly by logic circuits, or op-amps. Some small lamps needing just a few hundred mA could be driven by some comparators, but they are usually found being used with transistor switches, and MOSFET’s. Mains operated lamps would be connected via a relay (see later).
Component catalogues will specify the operating voltage and current for the lamp which will enable you to design a suitable driving circuit.
3. Buzzers / Sirens – a simple audible output device which will run off an simple DC source.
Symbol.Pictures: Buzzer (left) / Siren (right)
Properties.
Even low power buzzers requiring just 20-30mA can be quite loud, but for alarm systems sirens are used which offer much greater volume up to 115dB at a distance of 1m.
Some low power buzzers requiring less than 20mA can be driven directly by logic circuits, or op-amps. Most low power buzzers needing just a few hundred mA could be driven by some comparators.Usually they are found being used with transistor switches, Sirens usually require the use of a transistor switch or MOSFET. Mains operated sirens would be connected via a relay (see later).
Component catalogues will specify the operating voltage and maximum current rating for the buzzer / siren which will enable you to design a suitable driving circuit.
4. Loudspeakers – another means of producing sound but only works with an a.c. or pulsing signal – will not produce a sound with a constant d.c. source.
Symbol.Pictures:
Properties.
Loudspeakers work by using the electrical current to create a magnetic field around a coil which reacts to the magnetic field of a permanent magnet and causes movement of the paper cone, to set up a sound wave in the air. In order to produce a sound this electrical current must be changing otherwise the cone will stop moving, and therefore loudspeakers are not suitable for use in a purely d.c. circuit.
An astable output may cause a sound to be heard as long as the frequency of operation is high enough.
Most loudspeakers cannot be driven directly by logic circuits, or op-amps or comparators. Loudspeakers are usually found in amplifier circuits incorporating transistors and MOSFET’s but these are outside the scope of the AS course and appear in module ET5, so you will not be asked any questions about these circuits in this examination.
Component catalogues will specify the coil resistance and power for the loudspeaker which will enable you to design a suitable driving circuit.
5. Motors – a simple device for creating movement as the output of an electrical circuit, e.g. model car.Motors come in all different shapes and sizes, with operating currents starting at 300mA running up to several amps depending on the size of the motor.
Symbol.Pictures: Small and Large motors
Properties.
Motors are used in a wide range of electronic products where motion is the required result. The power of the motors varies considerably depending on the application but they all require quite large currents to make them work successfully, particularly at startup where the load current can be very high.
Virtually all motors cannot be driven directly by logic circuits, op-amps or comparators. Small scale motors requiring a current of less than 2 to 3 A can be driven by transistor switches but the large linear region and the time taken to switch from cut off to saturation can be a problem with transistor circuits. MOSFET switches are the primary source of driver circuits for motors. Mains operated motors would be connected via a relay (see later).
Component catalogues will specify the operating voltage and no load current for motor which will enable you to design a suitable driving circuit.
Motors cause large voltages to be generated when they are switched off which may damage the transistor / MOSFET and therefore diode protection must be included to protect the switching ciruit. (see later)
6. Solenoids – these are used primarily as locking mechanisms. The solenoid consists of a large coil of wire which creates a magnetic field when current flows through it. This attracts an iron core in the shape of a bolt into the coil. When the current is switched off a spring usually pushes the bolt back into place, unless the solenoid is mounted vertically in which case gravity causes the bolt to fall back into place.
Symbol.Pictures:
Properties.
Solenoids as well as being used electronic locks are used in cars to engage the starter motor with the engine when the engine is first started. Solenoids require large currents to be able to create a strong enough magnetic field to attract and hold the iron bolt, and they tend to be physically large.
Solenoids cannot be driven directly by logic circuits, op-amps, comparators or transistor switches. They have to be operated by MOSFET’s due to the very high current demand needed. Mains operated solenoids would be connected via a relay (see later).
Component catalogues will specify the operating voltage and power for the solenoid which will enable you to design a suitable driving circuit.
Solenoids cause large voltages to be generated when they are switched off which may damage the MOSFET and therefore diode protection must be included to protect the switching ciruit. (see later)
7. Relays – an electromechanical switch which uses a low voltage coil to operate a pair of switch contacts that are electrically isolated from the low voltage circuit. This allows mains voltage devices like sirens, motors and solenoids to be controlled by a low voltage control circuit.
Symbol.Picture:
Properties.
Relays are used primarily to switch on mains powered appliances from low voltage control circuits.
The contacts in the relay may be switching very large currents >10A and the fact that the contacts are breaking this current may cause electrical arcing to occur which will eventually cause the contacts to wear out and the relay will need to be replaced. It is for this reason that the MOSFET is now replacing relays for the control of large currents.
Most relays cannot be driven directly by logic circuits, or op-amps. Some small relays needing just a few hundred mA could be driven by some comparators, but they are usually found being used with transistor switches, and rarely with MOSFET’s.
Component catalogues will specify the operating voltage and coil resistance for the relay which will enable you to design a suitable driving circuit.
Relays cause large voltages to be generated when they are switched off which may damage the transistor / MOSFET and therefore diode protection must be included to protect the switching circuit. (see later)
Protecting Devices from switch off voltages.
In the previous discussion about output devices we have mentioned that certain output devices, namely motors, solenoids, and relays can damage transistor switches or MOSFET’s when they switch off because they generate a very high reverse voltage.
You may think that protecting the transistor / MOSFET would be very complicated but in reality it is very easy, and uses a component with which you are already familiar, the silicon diode introduced in topic 2.1.
The diode is connected into a circuit as follows:
The circuit has been shown here with a MOSFET, but it could just as easily be a NPNtransistor, there would be no change to position or orientation of the protection diode. The load represents either a motor, solenoid or relay.
The diode ensures that when the load is switched off, any high voltage generated is clamped by the diode to a maximum of 0.7V, which causes no damage to the transistor or MOSFET.
Sometimes in the examination you are asked to show how relay can be used with a sensing subsystem to operate a mains appliance. For example turning on a mains heater in a greenhouse when the temperature drops below a specified temperature. A possible solution is shown below.
Can you remember why the Schmitt inverter is included in the above circuit ?
No examination questions have been set on this topic as the addition of a protection diode, and use of a relay to switch on mains appliances are the only new items not covered in previous questions. These additions would just be one part of a longer question about transistors / MOSFET’s
You have now completed the ET2 syllabus and will shortly be taking your final exam. Remember practice makes perfect, so keep reviewing the solutions to the questions given and good luck in the examination.
Self Evaluation Review
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select suitable comparators, transistors, and MOSFET’s for connecting to signal sources and driving outputs such as lamps, buzzers, loudspeakers, motors, solenoids and relays.
state the need for diode protection for comparators, transistors, and MOSFET’s.
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