Society of Robots - Robot Forum
Electronics => Electronics => Topic started by: corrado33 on July 14, 2011, 04:57:11 PM
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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 (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....
:(
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Vc x hFE
Rb = -----------
5 x Ic
I think thats the one You need :)
Source: Transistor Circuits (http://www.kpsec.freeuk.com/trancirc.htm)
Edit: If You use transistor as a switch - use min hFE in Your equation (in Your case it is 20).
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Or buy "DIGITAL" Transistors like the FJN4301RTA (Mouser 512-FJN4301RTA), which have the resistors built inside them and save the board space.