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'[EE]: get 33mA safe from PIC pin'
2002\01\19@103420 by Roman Black

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Hi, i'm not sure if this is useful to anyone,
but I had an app that needed to drive a low-sat
transistor from a PIC pin. The transistor (NPN)
base needs 30mA, so I checked how much a PIC pin
can source if helped by a simple pull-up resistor.
I got an OK result, 33mA into the transistor base
with the actual PIC output never exceeding 25mA
(OK for 12C509). It saved me another transistor
and resistor.

+5v ------------------------------
           |
           R 180ohm      load (2.5A)
           |              |
           |              |
           | 68ohm        C
   PIC -------R----D----B     NPN
                  + -     E
                          |
                          |
gnd ------------------------------

PIC LO:
* PIC pin is 0.8v sink 23mA
* Base current 0mA (OFF)

PIC HI:
* PIC pin is 3.5v source 25mA
* Base current 33mA (ON)
 * 25mA from PIC
 * 8mA from R (180 ohm)
* diode is needed on transistor base to ensure
turnoff.

The BD203 transisor i'm using is switching
2.5A ok with 33mA base current, and much lower
CE on voltage than any darlington. Yes I probably
could have used a FET but I have BD203s and
they are very solid performers, especially for
$0.35 US each. :o)
-Roman

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2002\01\21@023933 by Vasile Surducan

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I'm agree with solution. Atmel 8051 clones need this trick
all the time for large currents.
Vasile


On Sun, 20 Jan 2002, Roman Black wrote:

{Quote hidden}

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