Programable Logic Controller questions. PLCS
MODULE TITLE: PROGRAMMABLE LOGIC CONTROLLERS
TOPIC TITLE: INTERFACING
LESSON 3: OUTPUT INTERFACE DESIGN
PLC - 3 - 3
© Teesside University 2011
Published by Teesside University Open Learning (Engineering)
School of Science & Engineering
Teesside University
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INTRODUCTION ________________________________________________________________________________________
The PLC output interface circuit uses signals supplied by the CPU to provide
outputs to control devices found in the outside world.
In this lesson we examine the way digital output interfacing is achieved.
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YOUR AIMS ________________________________________________________________________________________
After this lesson you should be able to:
• state the problems of using a digital logic signal to drive an external
output via an interface circuit
• identify additional electronic components from the circuit diagram
symbols
• describe the function and oper ation of the most impor tant
components typically found in output interface circuits
• understand manufacturers' specifications of output interface circuits
• select suitable output interface circuits to match the requirements of
different loads.
________________________________________________________________________________________
STUDY ADVICE ________________________________________________________________________________________
Circuit diagram symbols are to BS 3939 wherever possible.
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CPU OUTPUTS ________________________________________________________________________________________
When the CPU needs to send signals out in order that some external device
may be controlled it does so by using the logic levels at its disposal. The logic
level can have one of two possible values, i.e logic 'l' or logic '0'. By now you
should know that logic '1' can be assumed to be a +5 V d.c. signal and that the
logic '0' can be assumed to be a 0 V signal.
Whatever the logic level is it needs to be sent to an output port. When we
considered the requirements of an input signal we assumed that its port (and
others) would be part of an integrated circuit called an interface adaptor. A
similar assumption may be made about the port for output signals. That is, the
output port may be part of an interface adaptor. There are, however, other
electronic components which may be used in some manufacturers' designs.
For example, a device called an octal latch may be used instead of an interface
adaptor. In this text we shall continue to assume the use of interface adaptors.
Voltage levels are not the only consideration when dealing with output signals.
The maximum values of current which adaptors can supply are specified in
milliamperes – not tens of milliamperes or hundreds of milliamperes but ones
and twos. Of course, we could overload the adaptor and draw off more current
but this would seriously limit the life expectancy of the device.
The job of an output interface is to use this 5 V, 1 mA signal to provide a
means of controlling a load which has a much larger power requirement.
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PLC OUTPUT INTERFACES ________________________________________________________________________________________
The block diagram of FIGURE 1 shows the basic arrangement.
FIG. 1
The PLC manufacturer does not know what type of load a user will connect.
In most cases manufacturers will design a range of output interfaces which will
cope with most load requirements. The trick, then, is for the user to make the
intelligent choice and select the correct unit. In some PLCs the output
interfaces are made in interchangeable modules. The user buys the PLC void
of output circuitry and then purchases and fits the modules required for that
particular application.
In other PLCs the output interface is built into the design and is not
interchangeable. If the user makes the wrong choice then a completely
different PLC would need to be purchased.
PLC
Micro computer
Interface adaptor
Output interface circuit
External power load
Logic level
signal
Logic level
signal
Power control
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We will now look in more detail at three basic versions of output interface.
(a) Relay output.
(b) Transistor output.
(c) Triac output.
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RELAY OUTPUTS ________________________________________________________________________________________
The first type of output interface circuit to be examined is the relay contact
output. FIGURE 2 shows a possible circuit which we will use as an example.
Relays themselves were covered in the previous topic on the development of
programmable controllers. It is the relay coil driving circuit which is of
interest here.
The interface circuit of FIGURE 2 uses the logic signal from the output
adaptor to switch on and off the transistor (TR1). The transistor itself is acting
as an amplifier in that its collector load current is many times larger than the
current which drives it on i.e. the base current. In this circuit some of the load
current is driven through an indicating LED before all of it is passed through
the relay coil. Clearly if the LED carries a current then so does the relay coil.
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FIG. 2
This arrangement is one possible version, but it must not be assumed to be the
general case. Some PLCs allow the output LED to be illuminated whilst the
physical output of the relay is inhibited. Safety, when commissioning a new
system, may require this feature. The circuit diagram of FIGURE 2 does not
include this refinement.
You may wish to consider the design of an output interface circuit which will
allow such control.
R3
C1
D1
R2
R1
Ic
LED
Relay coil
Logic signal
External output
terminals
0 V
+ V
b
c
e
TR1
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CIRCUIT DESIGN AND ANALYSIS
The following section analyses the design of the circuit shown in FIGURE 2.
The purpose of this is to show that the process is straightforward and not at all
difficult.
You need to take note of the approach and the way that assumptions are
brought into the design. Once a value has been assumed then other values
depend upon it.
Current will flow into the interface circuit from the interface adaptor when a
logic 'l' i.e. +5 V has been sent out. The value of current will be limited by the
resistor R1 in series with the base emitter junction of TR1. If the transistor
used for TR1 is silicon then the base emitter voltage dr op will be
approximately 0.6 V. The remaining 5 V – 0.6 V = 4.4 V must be dropped
across the resistor R1 when it carries the base current.
If the base current is assumed to be limited to 1 mA, then by using OHM'S
LAW the value of R1 may be calculated as:
therefore
Unfortunately this 4.4 kΩ is not a standard preferred value and so the nearest suitable value would need to be chosen.
A value of 4.7 kΩ from the E12 range or a value of 4.3 kΩ from the E24 range could be used.
In the above analysis 1 mA has been assumed for the value of base current.
R V
I
R
1
1
4 4 4400
4400 4 4
= = =
= =
. V 1 mA
mV 1 mA
. kΩ Ω
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Explain why this value was chosen.
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If you can't explain then refer back to page 2.
The relay coil will require a larger current than 1 mA. Here again an assumed
value must be used. (The actual relay used must operate with this value.)
Can you suggest a value of relay coil current?
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For this design we shall choose a realistic value of 50 mA. This value of
current must flow through the transistor (TR1) when it is switched on by the
base current of 1 mA. The chosen transistor must, therefore, have a current
gain of fifty.
i.e. collector current = 50 × base current Ic = 50 × 1 mA.
therefore Ic = 50 mA.
The relay used must be a type that will energise when carrying 50 mA through
its coil.
50 mA would be adequate for the purpose of switching on the relay but would
you allow the 50 mA to flow through the LED?
From the work covered in Lesson 2 what would be a reasonable value of current
through the LED?
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Very few LEDs, if any, can safely carry 50 mA.
Typical values were previously stated as between 10 mA and 20 mA.
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We shall assume a value of 10 mA is adequate to illuminate the LED. If this is
the case then 50 mA – 10 mA = 40 mA must be diverted or shunted around
the LED. This is the purpose of the resistor R2.
What ohmic value should R2 have?
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Perhaps you have been able to calculate a suitable value of R2.
If not, don't worry, we shall go through the process.
R2 is connected in parallel with the LED. What we want to happen is that
40 mA will flow through R2 when 10 mA is flowing through the LED. It was
suggested in the previous lesson that an LED carrying 10 mA will have a
voltage drop of about 2.4 volts. R2 is in parallel with the LED and so it will
also have 2.4 V across it.
Using Ohm's law the value of R2 can be calculated.
R V
I2 2 4 0
2400 0
60= = = = . V
4 mA mV
4 mA Ω
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The nearest preferred values are:
62 Ω if the E24 range is used
or 56 Ω if the E12 range is used.
Compare your value for R2 if you did your own calculation.
Were you correct?
FIG. 3
One component not mentioned so far is the diode D1. This diode is connected as
a flywheel diode. The name may not mean anything to you but it has a specific
purpose. It does not carry any of the current which flows through the transistor
because it is connected reverse bias, that is to say, the wrong way round.
D1 carries any current which is generated within the relay coil due to the
electromagnetic energy stored within its magnetic circuit. Normally this
energy holds the relay armature in its energised position but when the
transistor is switched off the stored energy could generate a high voltage which
would damage the transistor. Diode D1 prevents this happening by presenting
a low resistance flow path to the generated voltage.
In practice the value of V shown in FIGURE 2 would need to be about 9 volts.
To relay coil
R2 56 Ω
+ V
10 mA40 mA
2.4 V
50 mA
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On the load side of FIGURE 2 a resistor R3 is connected in series with a
capacitor C1 across the relay contacts. This network is not provided so that it
shares the load current with the contacts. If the relay contacts are going to
switch highly inductive loads then sparking will take place at the contacts.
Sparking and arcing seriously reduce the life of the contacts as well as there
being the possibility of sparks igniting any flammable gases which may be
present. R3 and C1 are provided to reduce the arcing. The values of R3 and C1 depend upon the characteristics of the load.
Relay outputs are often termed volt free because the manufacturer does not
provide a supply to the contacts. The manufacturer has no idea what load will
be connected or whether the supply for the load needs to be a.c. or d.c., low
voltage or extra low voltage, etc.
Relay outputs are very useful because they can be used to switch either a.c. or
d.c. loads. The rating of the relay contacts is, however, different for each type
of supply and so this is one of the manufacturer's specifications which must be
checked by the user against the requirements of the load.
Typical contact ratings may be, for example, 2 A @ 24 V d.c. or 2 A @ 240 V
a.c.
At this point you should refer back to the diagram of FIGURE 2 and write
down the values of, and reasons for, each component in the circuit.
Do this without reference to the lesson if you can.
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________________________________________________________________________________________
TRANSISTOR OUTPUTS ________________________________________________________________________________________
The second type of output interface that we will look at is the transistor output.
Small loads which require a low value d.c. supply are catered for by the use of
transistorised output circuits. Such outputs are ideal for the control of
pneumatic solenoid valves or indicator panels. The user must provide the
supply for the load. The maximum load voltage obviously must not exceed the
maximum voltage rating of the output transistor. In practice a manufacturer
may use a power transistor with a maximum voltage rating of about 35 volts
whilst specifying that the user should not exceed 24 volts. A wise engineer
will follow the manufacturer's recommendations.
Turn your attention now to the circuit diagram of FIGURE 4.
There are no new symbols in this diagram although one component, the
inverter (IC1), has not been explained.
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FIG. 4
CIRCUIT OPERATION
You should be able, by now, to examine the circuit, pick out parts which have
been covered, and make a reasonable decision regarding the function of each
component. Similarities between this circuit and those designed or examined
in previous lessons should be of great help.
What then is the purpose of the inverter (IC1)? This logic gate accepts a logic
input and provides a logic output in the following way.
(a) The gate accepts either of the two possible logic levels but changes
whichever input it receives to the opposite level output.
D1
Logic signal
External load
terminals
+ 5V
TR2
D 2
Power transistor
56 ΩR2
R1 200 Ω
OP 1
10 mA
Inverter IC1
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i.e. If a logic 'l' is applied to the input the output will become a logic '0'
and if a logic '0' is applied then the output will become a logic 'l'. The
input is inverted to provide the output.
(b) A suitably chosen inverter can also act as an amplifier of current
(sometimes called a buffer stage).
In the circuit of FIGURE 4 both of these points are important.
If a logic 'l' signal is sent from the interface adaptor into the inverter then the
output will become a logic '0'. A logic '0' (previously assumed to be 0 volts) at
the inverter output will allow a current to flow through the LED, through the
R1 – OP1 combination, through the resistor R2 and into the output of the
inverter gate. This may seem strange but it happens this way. (This circuit has
been chosen to illustrate this point.)
The current flowing into the inverter input will be less than 1 mA but it
controls the flow of the larger current into the inverter output.
Now assume the value of the LED (D1) current to be 10 mA and attempt to
answer the following three questions.
1. What value of voltage do you assume to be dropped across D1?
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2. What is the purpose of R1? (You have already seen the R1 – OP1 arrangement in
the circuit of FIGURE 5, page 10 of Lesson 2.)
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3. What is the purpose of R2?
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Answers may be found on page 28.
When the adaptor signal is logic 'l' the LED will illuminate and the
phototransistor will switch on. The phototransistor obtains its current through
the external supply but it is unlikely that a phototransistor would be able to
carry the full load current. For this reason the phototransistor of FIGURE 4 is
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being used to switch on and off another transistor (TR2). The second transistor
must be rated to carry at least the full load current otherwise it is likely to burn
out. It is, therefore, a power transistor.
When the adaptor signal is logic '0' the LED stops glowing, the phototransistor
switches off and the load current stops flowing through the power transistor.
The diode D2 has been included as a protective device to ensure that no high
voltage spikes can build up across the transistors.
Can you think of any advantages that transistorised outputs have over relay contact
outputs?
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Solid state outputs:
• have no moving parts and, therefore, do not require the same level of
maintenance
• are likely to last longer
• do not produce sparks
• are silent in operation.
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TRIAC OUTPUTS ________________________________________________________________________________________
This is the third type of output interface to be considered. The silent,
maintenance free operation of the d.c. transistorised circuit can also be
obtained for use in switching a.c. loads. The same output circuit cannot be
used because the transistor will not operate an a.c. load. For these loads a
different circuit which incorporates thyristors, or more commonly triacs, is
used.
FIGURE 5 shows a circuit diagram which may be suitable for handling a.c.
loads.
Examine the circuit diagram carefully and note any new component symbols
or different methods of connecting components.
FIG. 5
Refer to the point in FIGURE 5 where the logic signal from the interface
adaptor enters the circuit. The symbol for IC1 is that which is often used for a
R2
OP 1 C1
R 3
R1
D1
Output
Output
IC1
Logic signal
0 V
Opto-triac 1
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buffer amplifier. The circuit of FIGURE 4 used a buffer amplifier but the two
symbols are different. The buffer of FIGURE 4 was an inverter whereas the
buffer of FIGURE 5 is a non-inverter. This device gives out the same logic
level as it receives at its input but a current amplification is possible through
the buffer.
We shall now consider the design of FIGURE 5. You will be expected to
calculate the values of some of the components. Notice that in this design the
two LEDs are not connected in series. Each LED is itself in series with a
current limiting resistor; R1 in series with the indicating LED (D1) and R2 in
series with the LED inside the opto device. These two circuit branches are
connected in parallel across the output of the buffer amplifier. The buffer
accepts the low current logic signal from the output adaptor and provides a
larger current to supply the LEDs. The logic levels at the buffer input and
output are the same i.e. logic 'l' in gives logic 'l' out.
When a logic 'l' (+5 V) is available at the buffer output, current will be
supplied to the LEDs.
You should, by now, be confident about being able to calculate the values of
components R1 and R2 if certain other required or assumed values are
specified. (In other lessons Ohm's law has been used for this purpose.)
Consider first the branch with the indicating LED.
Assume that a suitable value of current through this LED is 10 mA and that
while carrying this current the voltage drop across the LED is 2.4 volts. (This
is shown by FIGURE 6 overleaf.)
What value of resistance should R1 have?
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To answer this question work through steps 1 – 3 below.
FIG. 6
Step 1. How much voltage needs to be dropped across R1?
Step 2. This value of voltage (V1) is dropped while _____ mA of current is
flowing through R1.
Step 3. Use these values of voltage and current to determine the resistance of
R1.
Ω
The value of resistance calculated will not be a standard preferred value.
Choose a suitable resistor from the range given below.
180 Ω, 220 Ω, 270 Ω, 330 Ω, 390 Ω
Chosen value = Ω
Solutions given on page 29.
R V
I1 1= = = =
R1
Logic '1'
0 V
V1
2.4 V
+5 V
10 mA
1
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What value of resistance should R2 have?
In this case we will assume that 2 mA of current will be sufficient and that the
LED inside the opto device will drop 1.6 volts when this current is flowing.
Provide your own sketch of the R2 branch and then calculate the value of R2 using the same method as for R1.
Step 1.
Step 2.
Step 3.
Choose a preferred value from:
1 kΩ, 1.2 kΩ, 1.8 kΩ, 2.2 kΩ, 2.7 kΩ
Chosen value = Ω
Solution given on page 29.
Well, having got some of the design calculations out of the way we can go
back to the diagram of FIGURE 5 to consider the rest of the circuit.
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Look at the opto device. This symbol is different from the optotransistor
covered in Lesson 2. It is, in fact, the symbol for an opto-triac.
OPTO-TRIAC OPERATION
Remembering back to Lesson 2, the phototransistor operation was briefly
explained as follows.
When switched on by the LED light beam a current was allowed to flow into
the collector terminal and out of the emitter. This device can only allow
current to flow in one direction and can therefore only be used for d.c. loads.
The opto-triac, as you might imagine, again requires the light from the LED to
enable conduction through the triac component. The difference here is that the
current through the triac is alternating i.e. changing direction at regular
intervals of time. The operation of the triac must allow conduction in both
directions but at different points in time and only when the LED is carrying
current. The symbol for the triac shows two arrows to signify conduction in
both directions through what is called an inverse parallel pair.
In the circuit of FIGURE 5 the current to the external load flows through the
triac when the LED is conducting. Sometimes the external load requires more
current than the opto-triac can safely carry. When this is the case the circuit
would be changed so that the opto-triac controls (i.e. turns on and off) a power
triac. The power triac should then be the device carrying the external load
current.
The components shown in the dotted box do not form part of the load circuit
but are often provided for the protection of the semiconductor triac. The R3 in
series with C1 forms a snubber circuit which will suppress unwanted switching
interference and the varistor will suppress a.c. transients. The values of these
components depend upon the size and type of load being switched.
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If you have understood this lesson then attempt the following Self-Assessment
Questions.
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NOTES ________________________________________________________________________________________
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SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________
1. Identify, by naming, each of the circuit diagram symbols shown in
FIGURE 7.
FIG. 7
(a) (b)
(c) (d)
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2. State the characteristics of the signals obtained from an output interface
adaptor.
3. Why is it necessary for a PLC to have output interface circuitry?
4. State typical contact ratings for relay output circuits.
5. What kinds of load can be supplied from a volt-free relay output?
6. Transistorised outputs are designed to handle certain types of load. What
are the characteristics of such loads?
7. What is the purpose of using a non-inverting buffer?
8. A manufacturer's leaflet states that the output circuitry of a PLC employs
power triacs. What types of load can this PLC drive and for what types
is it unsuitable?
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ANSWERS TO SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________
1. (a) INVERTER or INVERTING BUFFER
(b) DIODE
(c) RELAY WITH ONE SET OF N/O CONTACTS
(d) OPTO TRIAC
2. Logic levels of voltage are available from the interface adaptor i.e. a logic
'l' being +5 V and a logic '0' being 0 V. The current available is very
limited and should be kept to a low value of around 1 mA.
3. The purpose of the output interface circuitry is to take in the logic level
signals and use these to enable control over much larger power loads.
The PLC itself cannot control these power loads directly because it is
limited to microelectronic values of power.
4. The exact ratings of any output device can only be obtained by reference
to the manufacturer's specifications. However, typical values for output
relays would be 2 A @ 24 V d.c. and 2 A @ 240 V a.c.
5. Virtually any kind of load can be supplied from volt-free relay contacts so
long as the ratings provided by the manufacturer are observed. It would
be very dangerous to exceed the values of voltage and current for either
a.c. or d.c. loads – more information on this point will be covered in
Lesson 4.
6. Transistorised outputs are designed to drive low voltage d.c. loads only.
Typically loads requiring around 1A @ 24V d.c. would be quite
acceptable. This covers indicator lighting panels and the pneumatic
solenoid valves mentioned in Lesson 3.
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7. A non-inverting buffer accepts a logic signal at its input and gives the
same level of logic signal at its output. It does not change the level of the
signal in any way. Buffers do have the advantage of drawing in a very
small current from the circuit which feeds it whilst having the capacity to
provide a much larger current at its output. In the circuit of FIGURE 5
the buffer will take in less than 1 mA whilst providing 12 mA output (10
mA for one LED and 2 mA for OP1).
8. Power triac outputs are specifically designed for control of a.c. loads at
voltages and currents dictated by the size and rating of the triac used and
the insulation and ventilation properties of its enclosure.
Triacs are not designed to operate d.c. loads of any type.
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ANSWERS TO QUESTIONS ON PAGES 14 AND 15 ________________________________________________________________________________________
1. Following the assumptions previously made a value of between 2 V and
2.4 V would be acceptable.
2. R1 diverts some of the current which flows out of LED (D1) around the
LED which is inside OP1. This is because the LED is carrying 10 mA (in
this case) but the LED in OP1 does not require this much current.
3. The voltage drop across the two LEDs in series would be a total of about
4 volts. 5 volts is being applied but only 4 volts is being dropped. The
difference in voltage, i.e. 5 V – 4 V = 1 V needs to be dropped across
the resistor R2.
The actual voltage drop across R2 will be:
In practice this would be acceptable because when the 10 mA flows into
the output of IC1 its actual '0' voltage will rise to about 0.4 V.
V I R
V
V
= × = ×
=
=
10 56
560
0 56
mA
mV
V
Ω
.
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ANSWERS TO QUESTIONS ON PAGE 19 ________________________________________________________________________________________
Step 1. 5 V – 2.4 V = 2.6 V
Step 2. 10 mA of current
Step 3.
Chosen value = 270 Ω
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ANSWERS TO QUESTIONS ON PAGE 20 ________________________________________________________________________________________
Step 1. 5 V – 1.6 V = 3.4 V
Step 2. Current = 2 mA
Step 3.
Chosen value = 1.8 kΩ
R V
I2 3 4 3400
1700= = = = . V
2 mA mV
2 mA Ω
2 6 2600 260
. V 10 mA
mV 10 mA
= = Ω
29
Teesside University Open Learning (Engineering)
© Teesside University 2011
________________________________________________________________________________________
SUMMARY ________________________________________________________________________________________
This lesson commenced by looking at the reasons for, and problems associated
with, output interface circuits. An analysis of types of output circuit made it
possible to examine important points relevant to the user and to see how
manufacturers cater for the users' needs.
The actual circuits used for analysis were varied and not representative of any
one manufacturer. Components readily available and commonly used have
been chosen and assembled in diagram form to indicate the range of
possibilities available to a designer. Understanding design limitations enables
prospective users to realise the reasons for particular ratings. Lesson 4
concludes the present work on interfaces but more will be covered later when
analogue inputs are analysed.
30
Teesside University Open Learning (Engineering)
© Teesside University 2011
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