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What can you say about this?. Kirchoff’s Law. PN-Junction. Depletion Layer. +. +. -. -. -. -. +. +. -. -. +. +. -. +. +. -. -. +. -. -. +. -. +. +. -. -. +. +. +. -. +. -. -. +. -. -. -. +. +. +. -. -. +. +. +. +. -. -. -. +. -. -. -. +. +. - PowerPoint PPT Presentation
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What can you say about this?
Kirchoff’s Law
PN-Junction
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P-region
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N-region
Depletion Layer
PN-Junction
Depletion Layer
Anode Cathode
Diode Symbol
A K
Biasing the Diode - Forward
P-type N-type
VR
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A K
P-regionN-region
Biasing the Diode - Forward
P-region
N-region
Characteristics
Diodes
Will this work?
• Almost shorting the supply
• High current in the loop.
DiodesWill this work? Approach
• Assume diode drop to be ≈ 0.7V.
• Rest of the voltage should be across resistor.
• R limits the current• Choose R value such that
diodes current is within its ratings.
• Wait there is more!
Diodes
Caveat• The resistor power rating
should also be considered.
• Power rating of resistor should be at least max. drop (Vr) time max current (I).
LEDs
How to drive an LED? Always remember• LED drop is ≈ 2V• LED current can be from
10mA to 20mA.• Adjust resistor value
based on intensity required.
Interfacing LED with µC
Quiz• What is the value of
VCC?– ≈3V
• What is the voltage at PA.1? – ≈3V
• Will the circuit work? – Yes
Alternative Arrangement
Sourcing Sinking
Now are we there?I know there
are 52 IO on µC. I’m going to
connect 52 LEDs to the µC
But!!
Why didn’t that work?
What more do we
need to know?
Well! Lets Look at
the Data sheet
µC Datasheet
2mA, 4mA and 8mA pad drive for digital communication,
up to four pads can be configuredWith an 18-mA pad drive
for high-current applications.
What should we do?
Boost the current from the µC. Use a…
Transistor
p
n
n
emitter
base
collector
n
p
p
Diffusion in Transistor
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emitter
base
collector
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Transistor Operation
N
P
N
E
B
C
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Transistor Operation …
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IC
IE
IB
Quiz
• What type of device is a transistor?– Active– Current operated
• What do you mean by current operated?– It is changes in input current that produces
changes in output current.
Transistor Characteristics
0.00
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0.00E+00
CE Output Characteristics
IB = 0uA
IB = 1uA
IB = 2uA
IB = 3uA
IB = 4uA
IB = 5uA
Collector Emitter Voltage (VCE)
Co
llec
tor
Cu
rren
t (I
C)
Cutoff
saturation
Transistor (BC108) Data sheet
Interfacing LED with µC revisited…
Interfacing LED with µC revisited…
Interfacing LED with µC revisited…
Interfacing LED with µC revisited…
Interfacing LED with µC revisited…
But, How can we design this?
• WKT, transistor has to operate in saturation and cut-off regions.
• In saturation, VCE ≈ 0.2-0.4V and LED drop of 2V.
• Calculate value of RC for Ic ≈ 15mA to 20mA.
• Calculate IB with β=100.• Calculate
Rb = (Vcc-VBE) / IB
Finally !
