what can you say about this?

What can you say about this?

Kirchoff’s Law

PN-Junction

---

---

+ ++ +

+ +

---

---

+ ++ +

+ +

---

+

+

+

---

+

++

---

+

+

+

---

+

+

+

P-region

+++

-

+++

----

-

+++

-

+++

----

-

+++

-

+++

----

-

+++

-

+++

----

-

N-region

Depletion Layer

PN-Junction

Depletion Layer

Anode Cathode

Diode Symbol

A K

Biasing the Diode - Forward

P-type N-type

VR

-

-

-

-

-

-

-

-

-

+

+

+

+

+

+

+

+

+

+

+

+

-

-

-

+

+

+

-

-

-

-

-

-

-

-

-

-

-

-

+

+

+

+

+

+

+

+

+

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

-+

-+

-+

-+

-+

-+

-+

-+

-+

-+

-+

+-

+-

+-

+-

+-

+ + + + +

+-

+-

+-

+-

+-

+-

+-

+-

+-

+-

N

P

N

emitter

base

collector

+++++

+-

+-

+-

+-

+-

+-

+-

+-

+-

+-

+-

+-

+-

+-

+-

-----+++++-----

- -+

-+ +

-+

-+

+-

+-

+ +-

+--

- -- - -

Transistor Operation

N

P

N

E

B

C

- - - - - - - - -- - - - - - - - -- - - - - - - - -- - - - - - - - -- - - - - - - - -- - - - - - - - -- - - - - - - - -- - - - - - - - -- - - - - - - - -

- - - - - - - - -- - - - - - - - -- - - - - - - - -- - - - - - - - -- - - - - - - - -- - - - - - - - -

------------

Transistor Operation …

N

P

N

E

B

C

- - - - - - - - -- - - - - - - - -- - - - - - - - -- - - - - - - - -- - - - - - - - -- - - - - - - - -- - - - - - - - -- - - - - - - - -- - - - - - - - -

- - - - - - - - -- - - - - - - - -- - - - - - - - -- - - - - - - - -- - - - - - - - -- - - - - - - - -

------------

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

E+00

1.00

E+00

2.00

E+00

3.00

E+00

4.00

E+00

5.00

E+00

6.00

E+00

-1.00E-02

-9.00E-03

-8.00E-03

-7.00E-03

-6.00E-03

-5.00E-03

-4.00E-03

-3.00E-03

-2.00E-03

-1.00E-03

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 !