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Thread: Protecting a BJT
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face BY : David VanHorn email (remove spam text)



At 07:45 PM 11/3/00 +1100, Roman Black wrote:
>Kresho @ Advanced Technika wrote:
>>
>> Thanks for all the replies.
>>
>> Yes, I've got a flyback diode across the transistor.

I missed this comment previously.

This won't cut it, unless the diode is a high power zener, with a knee
somewhat lower than VCEmax.   For a simple diode, it needs to be in
paralell with the motor.

Picture what happens at turn-off. The voltage on the bottom of the motor is
going to rise as the magnetic field collapses. Then it's going to force
current through the easiest path, which is likely putting your transistor
into avalanche mode. Depending on the energy stored in the rotor and the
magnetic field, the transistor barbecues.


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