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:

> 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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>
>

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