>     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).

It looks like you need more base drive to turn the transistor on
harder, 10k is too high. Probably 470R would do it as I suspect the
TIP41 has relatvely low gain. This would give about 9mA base
drive. You wont need the resistor to ground for turning off the
transistor as this is done pretty well when the PIC output goes low.
Hope this does the trick.

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/text: 025 334 069
eMail:  brent.brown@clear.net.nz

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