If you put in a FET you still need to juggle the resistors.  For a FET
start by swapping the two resistors  You will need some resistor to GND
to stop the solenoid from firing while the PIC boots.

I think your TIP41 is fine with better drive.

Sherpa Doug

> -----Original Message-----
> From: Tom Messenger [mailto:krist@THEGRID.NET]
> Sent: Monday, July 23, 2001 5:24 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE]: How to control a 24V solenoid with a PIC and a
> transistor?
>
>
> 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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>
>

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