> Greetings,
>
> I'm a relative electronics newbie and I'm doing some experimenting
> with solenoids for a project.  I have some fairly small ones that I'm
> switching from a PIC using an example circuit incorporating a TIP120
> bipolar transistor.  I also have a couple of beefy solenoids that
> draw about 1.6A and have found that they fry the TIP120 even with
> very short pulses (~1ms).  I tried putting two TIP120s in parallel to
> handle the extra current, and when they switch on, they lock.  I
> can't switch them off again unless I disconnect power to the
> solenoid.  I ordered an NEC bipolar part that can handle 100Volts
> 9Amps (my power supply for the solenoids is 50V 5A) and it does the
> same thing.  Is there something that I'm not understanding that would
> cause the additional current through the beefier solenoid to hold the
> transistor open where the smaller solenoid doesn't?  When using the
> TIP120s in parallel or the NEC, they don't fry and work fine after I
> unplug power to the solenoid.  I'm confused...
>
> thanks for any advice,
>
> darren


You DO have a clamp diode across the solenoid, right? Without it, the
voltage on the collector of the transistor spikes to the breakdown voltage
of the transistor when the transistor is turned off.

Harold

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