At 05:52 PM 7/23/01 -0300, you wrote:
>Hello,
>
>    I'm trying to turn a 24V solenoid (electro-magnet) with an NPN
>transistor (TIP41) wired to a PIC. Here's the circuit:
>
>
>                   24V-----------+
>                             solenoid (with reverse diode)
>                     C-----------+
>    PIC----10K--+----B
>                |    E
>               1K    |
>                |    |
>               GND  GND
>
>    It's not working. It seens the transistor have a much larger resistance
>(when on) than the solenoid (about 5 ohms). When I disconect the solenoid,
>the multimeter shows 24V ok when PIC pin is high. With the solenoid, it
>shows below 1V (more like a hundered millivolt).
>
>    Please, someone can help me?

Sure. Try a little Ohm's law first. Desired collector current will be 24V/5
ohms = 4.8AMPS.

4.8AMPS divided by beta of transistor is how much base current you need. In
saturation, a "rule of thumb" is beta of 20; try optimisically with beta of
50... 4.8AMPS/50 = 96mAMPS

Pic at real 5 volts: (5-.6)/10k = 440 microamps.

Remove NPN, put in FET. Don't worry, be happy.
Tom M.

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