At 08:14 AM 5/26/2006 -0700, you wrote:

>On May 26, 2006, at 7:15 AM, Sergey Dryga wrote:
> >
> > Darren,
> > I would use a MOSFET to drive a solenoid, bipolar transistors
> > require to much
> > current to drive them.  There is a lot of logic MOSFETs that will
> > do the job,
> > e.g. NTE2980
>
>I'll try a MOSFET and see what happens.  I'm really curious though to
>understand why I'm getting this locking on behavior.  From what I've
>read so far about transistors it doesn't make sense.  The circuit
>couldn't be simpler AFAICT.

Yes, it's best to get to the bottom of these things so they don't bite
later.

Inductive loads can result in SOA (safe operating area) violations of the
switching device. The TIP120 is actually a pretty wimpy transistor (5A) for
this kind of service.

How are you driving the base? If from a 5V PIC, hopefully with no more than
about 400 ohms in series.

Also, you may need to pay some attention to the ground return current to make
sure it isn't getting into other stuff. The problem is actually worse with
MOSFETs because they tend to switch much faster, and a teeny bit of inductance
can lead to failures.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->>Test equipment, parts OLED displys http://search.ebay.com/_W0QQsassZspeff


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