Transistor current limit circuit power handling

Posted on Feb 5, 2014

Designing a simple current limiter, which charges a large (4. 7mF) capacitor with a charge current of (roughly) 500mA from a supply voltage from about 10-20V - see the below circuit. My dilemma is that I already have a bunch of MMBT2222A and it would be nice to use this part without another line item. Whilst it can happily push 500mA through t

Transistor current limit circuit power handling
Click here to download the full size of the above Circuit.

he collector, it only has a power rating of 350mW, which will be massively exceeded when the capacitor is first charged as the voltage across the collector will be about 10V and hence the power will be about 5W for the first 200ms or so. In the circuit above, I have added an extra 2N2222A in parallel (with the idea of adding more as required) with the existing one, but I feel this strategy if fraught with danger - mismatched gains will cause unequal currents and defeat the whole purpose of the exercise.

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