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Diodes, Triacs, Thermistors, Opto-isolators, Phototransistors
Raj GhoshPrateek Shah
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Diodes
• Diodes are semiconductor devices consisting of two terminals
– Junction Diodes
– Zener Diodes
• A diode is created when a p-type semiconductor is joined with and
n-type semiconductor by the addition of thermal energy.
• On joining, the positive carriers diffuse into the n-type region and vice versa, to create a depletion region within the diode.
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Diodes
• An internal electric field forms in the depletion layer which acts as a
barrier to diffusion of the majority carriers across it.
• Size of the depletion layer determines current flow across the diode.
np
Depletion Region
Majority carriers Majority carriers
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Diodes
• Forward biased configuration when positive side of battery is connected to
p-type semiconductor
• The majority carriers are forced towards the junction and the depletion
region decreases
• Current is sustained by the majority carriers.
Vo
Vo-V
Potential BarrierForward Biased
npif
depletion regionshrinks
V
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Diodes
• Reverse biased configuration when positive side of battery is connected to n-
type semiconductor
• The majority carriers are forced away from the junction which increases the
depletion region
• A small reverse current or leakage current is sustained by the minority carriers
• At sufficiently high voltages, a sudden increase in reverse current is observed
(Zener effect).
np
Reverse Biased
depletion regionincreases
irV
Vo
Vo+V
Potential Barrier
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Diodes
V
I
conductionregion
non-conductionregion
Ideal Curve
The following graphs show the current flow through a diode as a function of
voltage across its terminals in forward biased configuration.
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Junction Diode Applications
• Radio Demodulation
• Power conversion
• Over-voltage Protection
• Logic Gates
• Current Steering
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Zener Diodes
• A Zener diode is a type of diode that permits
current to flow in the forward direction like a
normal diode, but also in the reverse direction if
the voltage is larger than the breakdown voltage
known as "Zener voltage“.
• They are usually used in the reverse biased
configuration, and operate near their breakdown
voltage.
• Zener diodes are specifically designed so that
the breakdown voltage is low.
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Zener Diode Applications
• Zener diodes are used as “Voltage Regulators” and placed in parallel
across a load to be regulated.
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TRIACs
• TRIAC (TRIode for Alternating Current) is a three
terminal switch which can conduct current in either
direction.
• It is triggered by a positive or negative voltage applied to
the Gate (G).
• Once triggered, it continues to conduct current, till the
current falls below a threshold value.
• A1 and A2 are the current carrying terminals.
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TRIAC operations
• It is a 5 layer device equivalent to two silicon-controlled rectifiers (thyristors)
• Region between the main terminals are parallel switches (PNPN and NPNP)
TRIAC structure TRIAC Characteristic Curve
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TRIAC Characteristic Curve
• 1st quadrant - A2 is (+) with respect to A1
• VDRM is the break-over voltage of the Triac
and the highest voltage that can be blocked
• IRDM is the leakage current of the Triac when VDRM is applied to A1 and A2
• IRDM is several orders of magnitude smaller than the “on” rating
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TRIAC Applications
High Power TRIACS
• Switch for AC circuits, allowing the control of very large power flows with
milliampere - scale control currents
• Eliminate Mechanical wear in a Relay
Low Power TRIACS
• light dimmers
• speed controls for electric fans and other electric motors
• modern computerized control circuits
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Opto-Isolator
• Opto-coupler
• Photocoupler
• PhotoMOS
• Used to transmit current
between terminals without
an electrical connection
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Functioning
• Electric signal converted to optical signal and back
• Usually an LED along with a photo-diode is used to achieve this
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Configuration Modes
• Photovoltaic mode– Diode acts as current source– Output current and voltage depend on load
impedance, light intensity
• Photoconductive mode– Diode connected to supply voltage– Magnitude of current directly proportional to
light intensity
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Applications
• Cut down ground loops– Current between terminals supposed to be at
same potential but actually are not
• Block voltage spikes
• Isolate low current control circuitry from power supplies, high-current circuitry
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Opto-isolators in digital logic
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Phototransistors
• Also photodiodes
• Convert light into current or voltage
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Functioning
• Similar to regular transistor with light acting as base/gate
• PN junction creates electron-hole pair when photon strikes
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PIN structure
• Sometimes used instead of PN junction
• Intrinsic layer between P and N layers• Increases area where electron-hole pair can be
generated
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Modes of Operation
• Photovoltaic– Photocurrent restricted– Voltage builds up – device forward-biased
• Photoconductive– Diode reverse biased– Decreases junction capacitance– Faster but noisier
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Applications
• Measuring light intensity– CD players– Smoke detectors
• PIN diodes (fast)– Optical communication– Light regulation
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Thermistor
• “Thermal Resistor”• Resistance changes with temperature
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Types
• Positive Temperature Coefficient (PTC)– Resistance rises with temperature– Also known as “posistors”
• Negative Temperature Coefficient (NTC)– Resistance decreases as temperature rises
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Functioning
• NTC Thermistors– Semiconductor cast chip– Increasing temperature increases conductivity
• PTC Thermistors– Mostly switching – resistance rises at
particular temperature– Ferroelectric material - Curie temperature
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Applications
• Current-limiting devices for circuit protection –resistive heating (PTC)
• Heating in temperature-controlled ovens (PTC)• Current limiting devices in power supply circuits
(NTC)• Monitor temperatures (NTC)
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Questions