> I am driving a 24VDC motor, draws < 1A, with a TIP121 that pulls down the
> motor's negative side. I switch this motor on and off regularly (about 10
> seconds on, 30 seconds off). I have found that the transistor becomes
short
> circuit after some time (days or weeks) and then self destructs due to the
> continuous conduction and overheating. Has anyone experienced this or know
a
> way to protect the BJT from this?

This is probably happening because the transistor is getting abused.  Do you
have proper flyback protection, since the motor is an inductive load?  Is
the flyback diode fast enough?  In this case it seems a FET is more
appropriate.  At 1A and 24V max you can easily get a FET with a very small
on resistance which will result in a much smaller voltage drop than the BJT.
The right FET can do this without even a heat sink  -  assuming you drive it
properly.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, olin@cognivis.com, http://www.cognivis.com

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