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Transistor Resistor Help please

sergioq

Oct 29, 2013
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Oct 29, 2013
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Hello All,

Am new to electronics and playing around is all. I bought an NPN transistor from radioshack, but when I look up any online resistor calculators none of the fields they ask for, are on the data sheet for the transistor.

Therefore not sure what kind of resistor to but for the base.

Am assuming that perhaps they're using different terminolgy, thought I'd reach out for some help.

Also, really just want to use this for a switch to light up an LED or two. Not even sure if it's too much or too little for a 6V battery supply.

Many thanks,

Sergio

TIP120 Darlington Transistor
Power Dissipation 65W
Collector-emitter voltage 60V
Collector-base coltage 60V
Emitter base V 5V
Collector current (cont) 5.0A
Collecor curretn peak 8.0A
Base current 120mA
Unclamped inductove load energy 50mj
 

BobK

Jan 5, 2010
7,682
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7,682
That transistor is overkill for lighting a few LEDs, but you could use it anyway.

This is a darlington transistor, which has two transistors connected together in a way to achieve very high gain. To determine the base resistor accurately, you would need to know the gain (beta) of the transistor. But it is so high with a darlington that it really does not matter that much.

I would just use a 10K resistor to start with. That would supply about 480uA of base current. The gain (from a datasheet) is a minimum of 2500, so that would be enough base current to saturate it at 5A of collector current, which is way more than you need unless you are lighting some very powerful LEDs.

Bob
 

duke37

Jan 9, 2011
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A Darlington transistor needs more voltage across it than a simple npn transistor. With two leds in series and the drop across the Darlington, you may be very close to the limit when using 6V. Depending on the led voltage.
 

Miguel Lopez

Jan 25, 2012
255
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Jan 25, 2012
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He can connect the LEDs in parallel. With such transistor, there will be no problem with the current increase.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
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He can connect the LEDs in parallel. With such transistor, there will be no problem with the current increase.
But watch how they share (or don't share) the current.

In this case, each should have their own resistor.
 
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