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Microelectronics in consumer products > 10. Driving loads > Flashcards

10. Driving loads Flashcards

1
Q

What is a bipolar junction transistor/ what do they do/ how do they work?

A

Often just called transistors. The simplest picture is that a small current into the base causes a much larger current to flow into the collector.

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2
Q

Can you draw a bipolar junction transistor diagram?

A

YES/NO.. draw it

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3
Q

What is the simplest equation to describe the operation of a bipolar junction transistor?

A

Ic = betaI_B, where beta = 50 for a medium power transistor. (Collector current is 50 bigger than the base current)

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4
Q

Draw the simplest circuit for driving a moderate load (up to 500mA) with a bipolar junction transistor?

A

DRAW DIAGRAM

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5
Q

what happens to the transistor when the output from the mC goes low (0V)?

A

The transistor is switched off and very little current flows through the load

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6
Q

what happens to the transistor when the output from the mC goes high?

A

The transistor turns on.. the collector is at about +0.2V and the base is at about +0.7V.

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7
Q

What is used for heaver loads?

A

Darlington pair

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8
Q

What is a darlington pair?

A

2 transistors connected when the base current becomes too large to be supplied by the mC (current > 500mA)

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9
Q

Draw the circuit for a darlington pair?

A

DRAW diagram

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10
Q

What is the max current for each pin of the mc and how do we drive loads that need highers currents than this or need a greater voltage than the mc?

A

Max current from each pin is 15mA. There are a wide range of solutions including:

  1. Single bipolar junction transistor (BJT)
  2. Darlington Pair
  3. Metal-oxide silicon field effect transistor (MOSFET)
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11
Q

What is the aim for all of these solutions to driving loads at higher currents/voltages in terms of power?

A

Either to switch the load fully on or fully off, not to vary its power continuously.

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12
Q

What polarities do BJTs come in?

A

come in 2 polarities: npn and pnp

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13
Q

What does MOSFET stand for?

A

Metal Oxide Silicon Field Effect Transistor

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14
Q

What are MOSFETs used for?

A

Basic devices used to construct digital integrated circuits such as mcs, and bigger ones can be used to switch loads in the same way as bipolar transistors.

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15
Q

What polarities do MOSFETs come in?

A

Again 2 polarities n-channel (npn equivalent) and p-channel (pnp equivalent)

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16
Q

Can you draw a MOSFET diagram for switching a load (nchannel) ?

A

YES OR NO

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17
Q

When does an n channel mosfet turn on?

A

When its gate voltage is more positive then a value called the Threshold Voltage.

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18
Q

Why are MOSFETs very attractive devices to use?

A

Because they are controlled by the Voltage on the Gate rather than the current: the gate is basically one plate of a capacitor. Thus no current flows in steady state, only when the voltage changes.

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19
Q

What happens with inductive loads when they are turned off?

A

They produce a back-emf.

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20
Q

What is a back-emf?

A

A voltage of opposite sign to the supply voltage (V=-L dI/dt)

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21
Q

Why is a back emf bad?

A

Because without proper protection the Back-emf may destroy the transistor and probably the mc as well.

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22
Q

How can we protect against back emf?

A

Connect a diode to short out any reverse voltage across the load.

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23
Q

Can you draw a diagram showing how a protection diode is used in a single BJT with a load circuit?

A

YES OR NO?

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24
Q

Why does the diode have no effect in normal operation?

A

Because it is reverse biased

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25
Q

Why is a second diode sometimes added and where is it added?

A

For extra protection, draw where on prev diagram

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26
Q

What are digital transistors?

A

Transistors where the protection diodes and sometimes the base resistors are built into the drivers.

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27
Q

What is a H-Bridge/ how does it work?

A

A h-bridge allows a DC motor to go in both directions. The circuit provides only one direction of current through the load. The direction of rotation can be changed by reversing the direction of current.

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28
Q

Can you draw h-bridge circuits: with simple switches showing the 2 different directions for the motor and then with MOSFETs?

A

yes or no

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29
Q

How are h-bridges available?

A

As an IC or can be built from discrete components.

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30
Q

What is the obvious solution for controlling the voltage or power in a load continuously?

A

using a DAC.

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31
Q

What is the equation for getting output voltage in a DAC?

A

output voltage = (digital input value/ max value)* reference voltage

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32
Q

What issues can arise with DACs?

A

Similar issues that arise with ADCs. E.g. little point in having a very precise DAC (lots of bits) unless the reference voltage is accurate and stable.

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33
Q

Do most mcs contain DAC?

A

No, very few do.. expect in devices intended for specific applications, such as audio.

34
Q

If you really need a DAC how can you do it?

A

Find a separate one and control it from the mc with I2C or a similar interface.

35
Q

Why are transistors much more efficient when acting as on/off switches than in between?

A

Because they dissipate a lot of power when neither completely on or off

36
Q

What is PWM used for?

A

To control an output continuously by only switching it on and off.

37
Q

How is PWM done?

A

The duty cycle of a square wave is varied to change the average power delivered to a load.

38
Q

Why must the frequency of the square wave be high?

A

So that it is not noticeable

39
Q

What is needed for PWM of sensitive loads?

A

A low pass filter

40
Q

How is smoothing done in inductive loads?

A

They provide smoothing themselves

41
Q

Can you draw diagrams for PWM for a square wave with period of 256 units and pulse lengths: 32, 128 and 224? Showing the average power produced.

A

Yes or no

42
Q

How can PWM be done in software?

A

Using a loop with delays to switch the load on and off.

43
Q

For 50% power what would the delay times in the program be?

A

They would be the same for the on and off delay times

44
Q

Can you draw the flowchart for creating PWM in software?

A

Yes or no?

45
Q

Can you draw the corresponding graphs of variation of counter relating to the output?

A

Yes or no?

46
Q

How is PWM done in hardware?

A

Many mcs contain one or more timers to generate these waveforms; they run automatically after loading the period and duty of the pulse.

47
Q

What module should you look for for PWM in hardware?

A

Capture/compare/PWM module (CCP)

48
Q

How does the CCP module work?

A

The exact same way as the software. The module is based around a counter fed by the clock used by the rest of the mc.

49
Q

What happens each time the counter is incremented?

A

It is compared with 2 registers:

  1. when the counter reaches the value in the duty register the load is switched off
  2. when the counter reaches the value in the period register the counter is reset and the load is switched on (unless duty =0).
50
Q

What ratio sets the duty cycle and hence the average power supplied to the load?

A

duty : period

51
Q

What is a downside of PWM?

A

Because PWM is generated by a counter, there is a trade off between speed and accuracy.

  • High accuracy means duty and counter need a large range
  • this means that the period must also be large and so the loop will be slow
52
Q

What is an example of the downside of PWM (suppose the mc executes an instruction every microsec and 256 possible values of the output are needed)?

A
  • Suppose that 256 possible values of the output are needed (8 bits)
  • The period should therefore be 256 so that duty can range from 0 (fully off) to 255 (fully on)
  • If the mc executes an instruction every microsec, one period takes a min of 256 microsecs = 0.25 ms.. which corresponds to a frequency of 4kHz.
  • This is in hardware, in software each iteration of the loop requires about 4 instruction cycles, even slower.
53
Q

How do remote control servos work?

A

They rotate their output through a range of 90 degrees.

54
Q

How are remote control servos controlled?

A

By sending a stream of pulses at 50 Hz

55
Q

Example uses of remote control servos?

A

To control model aircraft and cars.

56
Q

Can you draw a diagram of a remote control servo?

A

Yes or no

57
Q

Can you draw what the pulse waveforms look like for remote control servos? (show period and pulse duration)

A

yes or no

58
Q

What is the position of the servo dependant on? Examples?

A

Depends on the length of the pulse. E.g. 1 ms –> far left (-45 deg.) 1.5ms –> centred (0 deg.) 2.0ms –> far right (+45 deg.)

59
Q

Where are stepper motors used?

A

Where precise, repeatable, open-loop control of position is required

60
Q

Example use of a stepper motor?

A

moving the head of on ink-jet printer

61
Q

How does the rotor move?

A

In a sequence of steps controlled by pulses applied to the windings on the stator of the motor.

62
Q

Why does a stepper response go naturally with a digital system?

A

Because either the number of steps or the angle moved per step is specified, this is a discrete response which is similar to the behaviour of digital systems.

63
Q

What are the two options for the rotor to be?

A
  1. A cylindrical magnet, with alternate N&S ples around its circumference
  2. A shared piece of soft iron (easily magnitised) which is attracted to the coils on the stator when they are active — (reluctance motor)
64
Q

What is a common number of coils and coil sets?

A

Commonly 4 sets of coils with 6+ coils in each step

65
Q

Can you draw a simple stepper motor with 4 sets of coils and only 2 sets of coils in each set? Showing the different steps to rotate clockwise?

A

Yes or no?

66
Q

What are the other modes stepper motors can be operated in?

A
  • only one coil active at each time –> gives reduced current but also reduced torque
  • half step –> e.g. 1+2, 2, 2+3, 3,…. –> rotor moves through half the angle each time. Disadvantage is torque is different when 1 or 2 coils are active
  • The example drawn needs a bipolar supply since the coils must be energised in two directions, some stepper motors need only a unipolar supply
67
Q

Why can stepper motors not be operated too quickly?

A

Because it takes time for the motor and the load to accelerate and decelerate

68
Q

What are stepper motors prone to?

A

Mechanical resonance, these frequencies must be avoided

69
Q

For the stepper motor drawn, can you draw the corresponding waveforms (assuming unipolar rather than bipolar)?

A

yes or no

70
Q

What are the 3 main aspects of driving a stepper motor?

A
  1. Generation of the clock frequency
  2. Generation of waveforms to provide desired movement (translator)
  3. Power electronics to drive the motor (darlington pairs, H-bridges if bipolar supply is needed)
71
Q

What might the interface between a translator and mc require?

A
  • a clock waveform to define frequency
  • stop/go
  • forward/backward
  • half-step/full-step
72
Q

What sort of range of stepper motors can you get?

A

Very small micromotors (6x9mm) or much bigger stepper motors. Can get flat pancake motors. Can get linear stepper motors.

73
Q

What are linear stepper motors?

A

Drive a screw which turns rotational motion into linear motion.

74
Q

It is easy to keep track of the no. of steps the stepper motor has moved but how do you know the absolute position of the object being moved?

A

In a practical system you may need a reset sequence –> the motor is driven to one end of its range, detected by a separate sensor.

75
Q

What do most other motors apart from stepper motors require?

A

An additional sensor for position

76
Q

What is often used as a position sensor for other motors?

A

Optically encoded disks. The mc must monitor this and drive the motor, so that the system ends up in the desired position.

77
Q

Can you draw a diagram showing a system which included both the motor and its control going to the mc?

A

Yes or no

78
Q

Why do you think you cant get really small stepper motors?

A

Because their construction is complicated

79
Q

When do stepper motors draw most current?

A

When they are holding rather than rotating (no back emf)

80
Q

How to run a stepper motor fast? Why?

A

Accelerate it gradually because of the limited torque available. This is controlled by the stepping frequency which therefore must be variable.

81
Q

Can you draw a diagram showing the rate of rotation/stepping rate vs time for a stepper motor? Pointing out special names rates?

A

yes or no

Microelectronics in consumer products (14 decks)

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