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Re: Transistor Saturation
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electronics forum Guru Wannabe

Joined: 20 Sep 2005
Posts: 133

PostPosted: Thu Aug 03, 2006 2:38 pm    Post subject: Re: Transistor Saturation Reply with quote

Bart wrote:
I've got a bunch of general purpose NPN transistors and I want to turn on
and off 15Vdc with 5Vdc from logic chips like binary counters or different
logic gates (XOR, NAND,etc). From what I read about these transistors I
guess I want them to go into saturation which is maximum CE conduction. Is
there a rule of thumb to turn a transistor all the way on or do I go to the
datasheet for each particular transistor (which still sometimes confuses me
and not sure which callout is the designation for base volt/amp for
saturation). I've read that .7V base voltage above emitter voltage will do
it, I've also read that I need 5 volts above collector voltage will do it.
Any guidance is appreciated,

Using the minimum Beta of the transistor (from the datasheet),
calculate the required base current needed to pull the collector
voltage down to the voltage at the base. Assuming a common emitter
configuration, the voltage at the collector is equal to the (applied)
collector voltage minus the IR drop in the collector resistor. Then
multiply this current by 10, for a 10x overdrive factor as it is called
in the Art of Electronics. Knowing the base voltage (5v in this case)
and the required base current, you can calculate the needed base
resistor to provide this current.
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