I do have a snubber diode in the circuit, and when the TIP120s are in  
parallel, they don't burn up... they work fine when I switch back to  
using the smaller solenoid.


On May 26, 2006, at 12:01 AM, Bob Axtell wrote:

> 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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