Working with transistors - Part 1

This is a quick summary of the info. that I've collected while learning how to use transistor.

Transistor is made up of 2 diodes (see below pic.). Before talking about transistor, let's talk about diode first.

Diode

Forward Voltage Drop

There is a forward voltage drop when a diode is fully turned on, this forward voltage drop is generally within the 0.6V ~ 1V range depending on the material used to build the diode. A germanium-based diode might has a lower forward voltage drop of around 0.3V. The type of diode also has some importance in defining the forward voltage drop; light-emitting diodes can have a much larger VF, while Schottky diodes are designed specifically to have a much lower-than-usual forward voltage.

Excerpt from Vishay's Datasheet for 1N4001 thru 1N4007

Excerpt from Vishay's Datasheet for 1N4001 thru 1N4007

The forward voltage measured with a digital multimeter (0.553V)

Aside from forward voltage drop, there are 2 other important parameters to observe when selecting a Diode. They are: Maximum repetitive peak reverse voltage, and Maximum average forward rectified current.

Excerpt from Vishay's Datasheet for 1N4001 thru 1N4007

Maximum repetitive peak reverse voltage

This is about how much voltage can the diode take if the voltage is connected backward.

Maximum average forward rectified current

How much current can flow through the diode without frying it.

Light Emitting Diode (LED)

LED has much higher forward voltage that regular diodes. In the case of the green LED that I use, it has a forward voltage of 1.442V.

Transistor

As mentioned earlier in this post, the diagrams below shows how the NPN and the PNP transistors are made using diodes.

How transistor works - using NPN transistor as example.

Basically, transistor can be seemed as an electron valve or variable resistor. In the case of an electron valve, if we forward bias the junction between base and emitter, a small amount of electron will flow from Emitter to Base, but a larger amount of electron will flow from Emitter to Collector.

In the case of a variable resistor, when there is no current between Base and Emitter, the resistance between Collector and Emitter is very large (hence there is little or no current between Collector and Emitter). When there is a big current between Base and Emitter, the resistance between Collector and Emitter is very small (hence there are more current between Collector and Emitter).

As for how large the Emitter to Collector electron flow would be (the gain factor), it's stated as HFE in the datasheet.

2N3904

2N2222

Sample Application

Assuming a LED that consumes 100mA needs to be driven by a MCU pin that could only source 20mA.

- LED: 3V, 100mA

- MCU pin: 3.3V, 20mA

- Transistor: 2N2222 (the measured Base - Emitter forward voltage is around 0.665V, for ease of calculation, it's rounded to 0.6V)

Assuming we want to drive the LED using 5mA. The R1 value would be (3.3V - 0.6V) / 5mA = 540 Ohm. If R1 value is lowered to 330, the current between Emitter - Base is (3.3V -0.6V) / 330 = 8mA.

Now that we have worked out the configuration for Base, it's time to work out the configuration for Collector.

VCE(sat) is the voltage drop between Collector - Emitter when it's fully turned on.

For the Collector end, there is a 2.7V that needs to be taken care of by a limiting resistor (6V - 3V - 0.3V = 2.7V).

R2 = 2.7V / 100mA (LED's Power Consumption) = 27 Ohm.

The power dissipation on R2 is 2.7V x 100mA = 270mW which is over 1/4W; therefore, R2 needs to be of 1/2W to handle the power.

The diagram below shows the actual measured IC and IB and the ones calculated according to the formulas above.

References

Lesson 48 - Transistors
http://electronicstheory.com/COURSES/ELECTRONICS/e101-48.htm

Transistors
https://learn.sparkfun.com/tutorials/transistors

SparkFun According to Pete 7-5-11: Diodes and Transistors <- Very Good!! Main source of this post.
https://www.youtube.com/watch?v=w9cd7B5QRRo

SparkFun According to Pete # 36: Transistor Biasing Configurations
https://www.youtube.com/watch?v=t0UOSIUve9E

SparkFun According to Pete #37: Transistor Biasing Configurations Part 2
https://www.youtube.com/watch?v=IkabRft5Sdk&spfreload=10

Diodes
https://learn.sparkfun.com/tutorials/diodes/real-diode-characteristics

PN2222A NPN General Purpose Amplifier Datasheet
https://www.fairchildsemi.com/datasheets/PN/PN2222A.pdf

2N3904 Datasheet
https://www.sparkfun.com/datasheets/Components/2N3904.pdf

Common emitter
https://en.m.wikipedia.org/wiki/Common_emitter

Common collector
https://en.m.wikipedia.org/wiki/Common_collector

Common base
https://en.m.wikipedia.org/wiki/Common_base

ARDUINO PIN CURRENT LIMITATIONS
http://playground.arduino.cc/Main/ArduinoPinCurrentLimitations