Application Note

VISHAY SEMICONDUCTORS
www.vishay.com
Optical Sensors
Application Note
Application of Optical Sensors - Transmissive
by Jim Toal
Vishay is a leading manufacturer of optical sensors. These
sensors integrate an infrared emitter and photo detector in a
single package. The most common types of optical sensors
are transmissive and reflective sensors. Both types detect
the presence of an object without any mechanical or
electrical contact. The output signal of the sensor is used to
control various functions of an application.
Transmissive sensors, also called interrupter sensors,
incorporate an infrared emitter and photo detector that face
each other as shown in fig. 1. When an object is located
between the emitter and detector in the sensing path, it
interrupts or breaks the optical beam of the emitter. The light
energy reaching the detector changes. This change in light
energy or photo current is used to affect an event in the
application.
Reflective sensors incorporate an infrared emitter and photo
detector adjacent to each other as shown in fig. 2. When an
object is in the sensing area, the emitted light is reflected
back towards the photo detector, the amount of light energy
reaching the detector increases. This change in light energy
or photo current is similarly used as input signal in the
application.
This application note describes the proper use of Vishay’s
transmissive sensors. It describes several factors that must
be considered when using a transmissive sensor. Vishay
has a broad portfolio of transmissive sensors in leaded and
surface mount packages with various gap and aperture
sizes. One is just right for your application. Should you have
any design questions, Vishay’s Application Engineers are
ready to assist you.
Reflective Object
Object
Object
TCPT1300X01
Single
Aperture
TCUT1300X01
Double
Aperture
Fig. 2
DATASHEET PARAMETER VALUES
The datasheets of each sensor include the absolute
maximum ratings, and electrical and optical characteristics.
The absolute maximum ratings of the emitter, detector and
the sensor combined are provided. Maximum values for
parameters like reverse and forward voltage, collector
current, power dissipation, and ambient and storage
temperatures are defined. The transmissive sensors must
be operated within these limits. In practice, applications
should be designed so that there is large margin between
the operating conditions and the absolute maximum ratings.
The electrical and optical characteristics indicate the
performance of the sensor under specific operating
conditions. Generally, the minimum and/or maximum values
are provided. These values are guaranteed and are tested
during the manufacture of the sensor. Typical values, while
sometimes provided, should only be used as a guide in the
design process. They may or may not be tested during the
manufacturing process and are not guaranteed. Table 2 at
the end of this note provides the symbol, parameter and
definition of data found in transmissive sensor datasheets.
Document Number: 81452
1
For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
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Revision: 25-Oct-13
APPLICATION NOTE
Fig. 1
Application Note
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Vishay Semiconductors
Application of Optical Sensors - Transmissive
COLLECTOR CURRENT
110
I Crel - Relative Collector Current
If an object is moving towards the aperture, the light will be
blocked and the collector current decreases, fig. 3. If the
object is moving away from the aperture, the light will not be
blocked and the collector current increases. Both scenarios
are commonly found in transmissive sensor applications.
The resolution of the transmissive sensor depends on the
aperture, the light sensitive area of the detector and the
direction of movement. To increase the performance of the
sensor, the object should not be infrared translucent. For
linear position sensing applications, only the collector
current range from 90 % to 10 % can be used to avoid false
detects.
0
100
A = 1 mm
90
80
70
s
60
50
40
30
20
10
0
- 0.5 - 0.4 - 0.3 - 0.2 - 0.1 0
0.1 0.2 0.3 0.4 0.5
s - Displacement (mm)
96 12005
Fig. 4 - Relative Collector Current vs. Displacement
s
1.00
110
0.75
0.50
0.25
0.00
- 1.5
13658
- 1.0
- 0.5 0.0
0.5
1.0
1.5
s - Displacement (mm)
I Crel - Relative Collector Current
I Crel - Relative Collector Current
1.25
The medium or object to be sensed plus its tolerances
normally defines the minimum gap size. Assembly
tolerances and other factors relating to the type of medium
will add to this gap size. The ideal gap size accounts for all
of these factors and is no larger than necessary. This
ensures the optimum current transfer ratio for the system. If
the gap size is too big, stray ambient light may interfere with
the signal, emitted light will diffuse and the current transfer
ratio will not be optimum.
A = 0.5 mm
90
80
s
70
60
50
40
30
20
10
0.1 0.2 0.3 0.4 0.5
s - Displacement (mm)
96 12006
Fig. 5 - Relative Collector Current vs. Displacement
110
I Crel - Relative Collector Current
APPLICATION NOTE
Most of Vishay’s transmissive sensors use an aperture to
focus the light onto a single plain and direction. Smaller
apertures are intended to give better resolution which will
result in a steeper sloped IC. A single channel transmissive
sensor has one emitter and detector pointing at each other
while a dual channel sensor will have one emitter and two
detectors, and apertures, fig. 4, 5, 6. Single channel sensors
are used to detect the presence or absence of an object and
to detect speed. Dual channel sensors are commonly used
to detect direction and speed using quadrature encoding.
0
0
- 0.5 - 0.4 - 0.3 - 0.2 - 0.1 0
Fig. 3
APERTURE, GAP SIZE
100
100
0
A = 0.25 mm
90
80
s
70
60
50
40
30
20
10
0
- 0.5 - 0.4 - 0.3 - 0.2 - 0.1
96 12007
0
0.1 0.2 0.3 0.4 0.5
s - Displacement (mm)
Fig. 6 - Relative Collector Current vs. Displacement
Document Number: 81452
2
For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
Revision: 25-Oct-13
Application Note
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Vishay Semiconductors
Application of Optical Sensors - Transmissive
DARK CURRENT
10 000
Ie rel; Φe rel
1.2
IF = 20 mA
0.8
0.4
0
-10 0 10
VCE = 10 V
IF = 0 A
50
100
140
T amb - Ambient Temperature (°C)
94 7993
Fig. 8 - Relative Radiant Intensity/Power vs. Ambient Temperature
1000
2.0
10
1
96 11875
10 20 30 40 50 60 70 80 90 100
Tamb - Ambient Temperature (°C)
Fig. 7 - Collector Dark Current vs. Ambient Temperature
TEMPERATURE
Photo transistors and infrared emitting diodes are
temperature dependent. As temperature increases, the light
and dark current increases while emitter output decreases.
Radiant intensity of the emitter decreases by -0.7 %/°C
while the sensitivity of the phototransistor increases by
+1 %/°C. So, an increase in the light current of the
phototransistor is off-set by a decrease in the output of the
emitter, fig. 8 and fig. 9. Consequently, the change in the
output of transmissive sensors due to temperature change
is comparatively small at less than 10 % from -25 °C to
+70 °C, fig. 10. Because of this, it is not recommended or
necessary to try to compensate for changes in temperature
in the design of transmissive sensor circuit.
Temperature also plays an important role in determining the
emitter forward current in the application. As an example, if
the maximum forward current at an ambient temperature of
25 °C is 50 mA. As shown in fig. 11, as power dissipation
decreases the forward current must be reduced according
to changes in the ambient temperature. At an ambient
temperature is 85 °C, the maximum current is roughly 20 %
of the value at 25 °C. In practice, the actual current should
include a large safety margin and the lowest possible
current should be used.
Ica rel - Relative Collector Current
100
0
APPLICATION NOTE
1.6
1.8
VCE = 5 V
Ee = 1 mW/cm2
λ = 950 nm
1.6
1.4
1.2
1.0
0.8
0.6
0
94 8239
20
40
60
80
100
Tamb - Ambient Temperature (°C)
Fig. 9 - Relative Collector Current vs. Ambient Temperatur
CTR rel - Relative Current Transfer Ratio
ICEO - Collector Dark Current (nA)
When a phototransistor is placed in the dark, or zero
ambient illumination, and a voltage is applied from collector
to emitter, a certain amount of current will flow. This current
is called the dark current. It consists of the leakage current
of the collector-base junction multiplied by the DC current
gain of the transistor. The presence of this current prevents
the phototransistor from being considered completely “off”
or being an ideal “open switch”. In datasheets, the dark
current is described as being the maximum collector current
permitted to flow at a given collector-emitter voltage. The
dark current is a function of this voltage and temperature,
fig. 7.
1.5
1.4
VCE = 5 V
1.3
I F = 20 mA
1.2
d = 0.3 mm
1.1
1.0
0.9
0.8
0.7
0.6
0.5
- 30 - 20 -10 0 10 20 30 40 50 60 70 80
96 11913
Tamb - Ambient Temperature (°C)
Fig. 10 - Relative Current Transfer Ratio vs. Ambient Temperature
Document Number: 81452
3
For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
Revision: 25-Oct-13
Application Note
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Vishay Semiconductors
Application of Optical Sensors - Transmissive
where the forward voltage of the emitter, VF, typically 1.2 V,
is subtracted from the supply voltage, VCC, and divided by
the forward current. Again, design in safety margin between
actual operating conditions and the absolute maximum
ratings. Intensities that are too high will reduce response
time and potentially accelerate degradation. However, since
the emitter points directly at the detector over a small gap,
forward currents for transmissive sensors are typically low.
I F - Forward Current (mA)
100
80
60
40
SWITCHING TIMES
20
0
- 25
15213
0
25
50
75
100
Tamb - Ambient Temperature (°C)
Fig. 11 - Forward Current vs. Ambient Temperature
AMBIENT LIGHT
Ambient light can impair the sensitivity of the transmissive
sensor though its effect is reduced compared to reflective
sensors because of relative small gap sizes and the in-line
nature of the emitter and detector. Steady light falling
directly on the detector may saturate the phototransistor. If
an object intended to block the light path does not block the
direct ambient light, the phototransistor may remain
saturated and no signal will be generated. Varying ambient
light results in incorrect signals and missed detections. In
many applications, it is difficult to precisely determine the
ambient light and its effects. Therefore, the influence of
ambient light must be minimized by using optical filters,
inspired mechanical design and, if necessary, AC operation.
Most of Vishay’s transmissive sensors are molded from
epoxy that blocks visible light. Still, a large portion of
sunlight is in the infrared. Locate or house the sensor so it is
recessed to eliminate direct light. Pulsed operation can be
helpful in some applications. AC operation is the most
effective protection against ambient light.
APPLICATION NOTE
EMITTER INTENSITY
Emitter intensity depends largely on the forward current, IF.
The absolute maximum forward current is found in the
datasheet. For some of Vishay’s transmissive sensors, the
maximum forward current is 25 mA at an ambient
temperature of 25 °C. If the forward current is too low, the
optical output of the emitter will not be stable. A current
limiting resistor is required. Without it, the current of the
diode is theoretically limitless and the diode will burn out.
The value of the current limiting resistor is calculated using
the formula:
The speed of response of a phototransistor is dominated by
the capacitance of the collector-base junction and the value
of the load resistance. A phototransistor takes a certain
amount of time to respond to sudden changes in light
intensity. The response time is usually expressed by the rise
time and fall time of the detector. If the light source driving
the phototransistor is not intense enough to cause optical
saturation, characterized by the storage of excess amounts
of charge carriers in the base region, rise time equals fall
time. Transmissive sensors are generally saturated when an
object is not present so fall time is larger than rise time. The
selection of the load resistor, RL, will also determine the
amount of current-to-voltage conversion in the circuit.
Reducing the value of RL may result in a faster response
time at the expense of a smaller voltage signal.
DEGRADATION
End-users purchasing a transmissive sensor want an
accurate estimate of how long the sensor will last. Many
will have minimum life requirements. Unlike most traditional
light sources, infrared emitting diodes do not fail
catastrophically. Instead, the light output degrades over
time, fig. 12. Therefore the useful life of a transmissive
sensor can be defined by the time when it fails to provide
sufficient light for the intended application. Infrared and
visible light emitting diode life is often quoted to be
100 000 hours but this is based on the average life span
of a single, 5 mm epoxy encapsulated emitter. Vishay’s
reflective sensors also have a single emitter that is epoxy
encapsulated. With some similarity, average life span can be
considered comparable. As a rule-of-thumb, plan for 30 %
degradation of the emitter over the life time of the sensor.
RL = (VCC - VF)/IF
Document Number: 81452
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For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
Revision: 25-Oct-13
Application Note
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Vishay Semiconductors
Application of Optical Sensors - Transmissive
Relative IC / IF (CTR)
1.6
Operating
conditions
IF = 50 mA
Ta = 40 °C
1.2
0.8
0.4
0
100
0
1000
10000
Life Test Hours
15208
Fig. 12
The three main causes of degradation are:
• A loss of efficiency caused by mechanical stress
deforming the crystal structure
• A loss of optical coupling caused by delamination
between epoxy and chip
• A loss of efficiency caused by thermal stress on the crystal
structure
The rate of degradation or aging is affected by:
• Chip technology: GaAs and GaAIAs Double Hetero (DH)
technologies result in lower rates, while GaAIAs and
GaAIAs/GaAs technologies result in higher rates of aging
• Package technology: metal can packaging technologies
result in lower rates, and epoxy packaging technologies
result in higher rates of aging
• Chip size: The smaller the chip, the higher the current
density. A higher current density results in faster aging
There are a number of ways to minimize emitter degradation
or aging. First, minimize the junction temperature. As long as
the junction temperature, TJ, is kept below 100 °C, heating
of the pn-junction will cause no significant degradation. To
reduce junction temperature, minimize the forward current
and the ambient temperature. Second, in applications
where there is temperature cycling, keep the forward current
for the corresponding temperature well below that shown in
fig. 11. This is especially important since degradation due to
mechanical stress and delamination is potentially greater in
epoxy-based sensors. Transmissive sensor datasheets
include a curve showing Total Power Dissipation vs.
Ambient Temperature. Use this curve as a guide to minimize
degradation.
Vishay features state-of-the-art chip technologies and high
quality standards in the assembly process resulting in low
degradation rate of our sensor components.
TABLE 2
PARAMETER
VR
Reverse voltage
IF
Forward Current
IC
Tamb
Tstg
VF
Gap
Aperture
Forward
surge current
Power dissipation
Junction temperature
Collector emitter
voltage
Emitter collector
voltage
Collector current
Ambient Temperature
Storage Temperature
Forward voltage
ICEO
Collector dark
current
VCEsat
Collector emitter
saturation voltage
tr
Rise time
tf
Fall time
IFSM
PV
TJ
VCEO
VECO
APPLICATION NOTE
SYMBOL
DEFINITION
The maximum permissible applied voltage to the anode of the LED such that the current flows in the
reverse direction
The direct or continuous current flowing in the forward direction of a diode, from the anode to the
cathode
Distance from emitter face (or post) to detector face
The opening in the detector post that admits light
The maximum permissible surge or pulse current allowed for a specified temperature and period in the
forward direction
The maximum power that is consumed by the collector junction of a phototransistor
The spatial mean value of the collector junction temperature during operation
The positive voltage applied to the collector of a phototransistor with the emitter at a reference potential
and open base
The positive voltage applied to the emitter of a phototransistor with the collector at a reference potential
and open base
The current that flows to the collector junction of a phototransistor
The maximum permissible ambient temperature
The maximum permissible storage temperature without an applied voltage
The voltage drop across the diode in the forward direction when a specified forward current is applied
The current leakage of the phototransistor when a specified bias voltage is applied so that the polarity
of the collector is positive and that of the emitter is negative on condition that the illumination of the
sensor is zero
The continuous voltage between the collector and emitter when the detector is in its “ON” state as
measured with the Kodak neutral test card, white side
Amount of time it takes the output voltage to go from 10 % of the lower specified value to 90 % of the
upper specified value
The time required for the output voltage to go from 90 % of the upper specified value to 10 % of the
lower specified value
Document Number: 81452
5
For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
Revision: 25-Oct-13
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