Darren Gibbs wrote:
> 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
>   
Are you protecting the TIP120's with any transient protection? when the 
solenoid switches, the
TIP120 will see a spike of hundreds of volts, unless a diode/zener/TVS 
is not installed from the
TIP120 oputput to GND to kill the spike.

Otherwise, I'd suspect that the TIP120 is burning up due to not being 
heatsinked. remember that
the TIP120 is a darlington transistor, and has a large drop across it, 
probably 1V. At 5A, the TIP120
must dissipate AT LEAST 5 watts; it cannot do it without a heat sink.

--Bob
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