Please explain the circuit.

1) Can somebody explain me why we are using tip 122 darlington transistor in this circuit. http://www.ecs.umass.edu/ece/m5/tutorials/tip122_transistor_tutorial.html

2) suppose i do not what to use use tip 122 in the circuit rather make my own darlington transistor so what voltage gain transistors should i buy for this.

the stated goal in the link is to control a device whose power requirements exceeds the 2N3904

that answers your first question.

in order to answer your second question, you have to list what you want to power.

I would offer that because you are asking that question, you would be much better off using the TIP120 if the power requirements exceed the 2n3904, or read up on transistors and learn how to calculate the values needed.

2) suppose i do not what to use use tip 122 in the circuit rather make my own darlington transistor so what voltage gain transistors should i buy for this.

Total irrelevant as the final gain is the product of the two individual gains. In making your own you need one high current transistor, these normally have small gains sometimes as low as 10. And one low current transistor these have typical gain of 200. Giving a gain of 2000 or so.

i want to control a 12v dc fan with arduino uno.

zammikao:
i want to control a 12v dc fan with arduino uno.

So you need to know what current this draws before you can make any sensible decision.

so what voltage gain transistors should i buy for this.

First à BJT transistor has no voltage gain but only current gain. To have à voltage gain a BJT must be associated with resistors.

How to calculate your min current gain for the Darlington ? As Grumpy say you must know how you need current in the load -> I1. You know that an I/O can not deliver much than 40 mA --> It is ABSOLUTE MAXIMUM RATING. 20 mA maximum for an Arduino I/O is a good value.

so Gmin = I1/20mA.

The 12v fan draws 0.15 amp current. One more query this circuit is to control 12 v fan but i could not understand how it controls it. There is no resistor associated in the collector side so how does the pwm output from arduino controls the fan which is supplied with 12v in one side.

It's best you look for some instructions on the web for basic transistor switching such as: http://techhouse.brown.edu/~dmorris/projects/tutorials/transistor.switches.pdf

There is no resistor associated in the collector side

Why should there be?

so how does the pwm output from arduino controls the fan which is supplied with 12v in one side.

By turning the fan on and off very quickly, the ratio of on to off times controls the speed. See:- http://www.thebox.myzen.co.uk/Tutorial/PWM.html

The fan is the “resistor” as well as an inductor (hence the diode)…

An WHY the TIP120? Because it is about as generic as the 2N2222A and the 2N3904... meaning you can pick one at nearly every Radio Shack retail store... (or what ever similar store exists where you are) So you can be assured that not a lot of thought went into selecting the TIP120 other than... it can carry more current than a small signal switching transistor and you can get in your car or on your bike and pick one up "now".

Sometimes the choice of a component is not the result of a calculation but the availability in the developer drawers. :grin: I agree with you for this application the TIP120 is oversized