Hi-
Thanks for this clear and informative summary of how a transistor works!
Even I could understand it, specially the practical example :-)
David

Dwayne Reid wrote:

> At 09:58 AM 2/11/01 -0500, Byron A Jeff wrote:
> >I've found that lately that using transistors for interfacing is critical.
> >In the past I've used optoisolators and open collector IC's for higher
> >voltage interfaces. But it's clear that understanding how plain old NPN
> >transistors do their work is important.
> >
> >So today's question is how to use an NPN transistor to switch 24V
> >(arbitrarily picked) with a PIC output pin. From what I've read if you
> >do the standard:
> >
> >         +24
> >          |
> >        Load
> >          |
> >          C
> >         /
> >PIC-R--B
> >         \
> >          E
> >          |
> >         GND
> >
> >Then the transisor won't be saturated because the 5V output from the PIC
> >isn't high enough relative to the collector voltage.
>
> Nope - circuit is OK, text under the circuit is wrong.
>
> Here is an easy way to look at the circuit above.  You have to consider the
> voltage between the Base and Emitter terminals of the transistor.  The
> emitter - base junction looks like a diode, with anode at B and cathode at
> E.  In order to make the transistor conduct, you have to feed CURRENT into
> the B-E junction.  If the applied voltage is less than 1 diode drop
> (0.65V), no base current flows and the transistor is OFF.  As the applied
> voltage is increased, the voltage at the B-E junction is clamped at 1 diode
> drop and base current is determined by (voltage at PIC pin) - (voltage at
> B-E junction) all divided by the value of the series base resistor.
>
> Lets pick some real world numbers.  Vbe is ~0.65Vdc.  Vpic_pin is
> 5V.  Resistor is 1k.  Base current is (5-0.65) / 1000 or about 4.35 mA.
>
> The MAXIMUM collector current that can flow through the Collector is the
> base current X Hfe.  If the actual collector current being sunk by the
> transistor above is LESS than the maximum possible current, the transistor
> is saturated.  Note that Hfe varies as Ic varies - if you want to saturate
> a transistor with a fairly high collector current, you really have to look
> at the Hfe curves for that transistor to determine the minimum gain at that
> current and work backwards to find the minimum required base current.  But
> for small transistors like the 2n4401 and 2n4403, you can assume Hfe is
> greater than 100 when Ic is less than 100 mA with no problems.  If you want
> to ensure that the transistor is well into saturation, aim for a base
> current of 2X the minimum calculated base current.
>
> So - the above circuit driving a 100 mA load with a base current of 4.35 mA
> is well saturated.  You could increase the base resistor to 2k2 and still
> be in saturation.
>
> dwayne
>
> Dwayne Reid   <dwayner@planet.eon.net>
> Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
> (780) 489-3199 voice          (780) 487-6397 fax
>
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