Author Topic: Transistor - what resistor to base?  (Read 1266 times)

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Offline Razor ConceptsTopic starter

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Transistor - what resistor to base?
« on: April 06, 2009, 06:02:31 PM »
I've been using transistors to switch on some LEDs. But what I never figured out was the resistor on base to the trigger. Is there some way to calculate that resistance for the load I want to trigger? Can I just use no resistor at all? I've tried no resister with a 2222 but I fried one that way, so I started using resistors from 250 to 1k ohms randomly... and so far they are working fine.

Offline Webbot

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Re: Transistor - what resistor to base?
« Reply #1 on: April 06, 2009, 06:53:31 PM »
You need to know the current passing through the collecter/emitter ie through your LED and the gain provided by your transistor. A typical value may be a gain of 50. This means that to provide a current of 50ma thru your LED you would need 1ma at the base.
There is a voltage drop typically of 0.6v between the base and emitter so if you are feeding it a 5V signal from your controller then the remaining voltage drop is 5v - 0.6v = 4.4v.
Using R=V / I  then R = 4.4V / 1ma = 4.4k ohms

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Offline sofcrafts

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Re: Transistor - what resistor to base?
« Reply #2 on: April 08, 2009, 04:08:11 PM »
I suggest you use a lower resistor value than  calculated above. This will ensure the transistor saturates--. Transistor dc gains drop as collector current increases.

For example from the 2N2222 data sheet:

If the  collector current of 150mA , you can get a Vcesat of 300mV with 15mA of base current. This makes the base resistor about 300 ohms  if the drive is a 5volt signal. Hopefully the control  input can suppply 15 mA.

 In above case  I would suggest say a 1K resitor-- a balance between saturation and control input loading.



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