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Author Topic: Ugh, how to determine resistor for PNP transistor Base?  (Read 1933 times)

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Offline corrado33Topic starter

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Ugh, how to determine resistor for PNP transistor Base?
« on: July 14, 2011, 04:57:11 PM »
I feel stupid.   :(

I, for the life of me, don't understand this stuff.

So I need a resistor on the base pin of a PNP transistor to protect it yada yada yada.  I've read that a good approximation for what you should use would be to make the base current 5 times the value(or from a different site 30% more) that would just saturate the transistor.  On the datasheet I just find a base-emitter and collector-emitter saturation voltage.  Here's the datasheet for the pnp I'm using.
http://www.fairchildsemi.com/ds/PN%2FPN3645.pdf

Ok as I'm writing this I'm also researching, so I found this equation to calculate the resistor.
R1 = Supply Voltage / ( Maximum Current Required / Minimum HFE * saturation current )

The only thing I don't know is the Minimum HFE.  I just don't get it.  There is a bunch of Hfe ratings there, but none at 5V and 40 mA...  I'm just confused....
 :(
« Last Edit: July 14, 2011, 05:10:26 PM by corrado33 »

Offline newInRobotics

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Re: Ugh, how to determine resistor for PNP transistor Base?
« Reply #1 on: July 15, 2011, 05:04:03 AM »
           Vc x hFE
  Rb = -----------
             5 x Ic

I think thats the one You need :)

Source: Transistor Circuits

Edit: If You use transistor as a switch - use min hFE in Your equation (in Your case it is 20).
« Last Edit: July 15, 2011, 05:14:47 AM by newInRobotics »
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Offline Archangel

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Re: Ugh, how to determine resistor for PNP transistor Base?
« Reply #2 on: July 17, 2011, 03:25:10 AM »
Or buy "DIGITAL" Transistors like the FJN4301RTA (Mouser 512-FJN4301RTA), which have the resistors built inside them and save the board space.

 


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