Electronic – Transistor current limit circuit – power handling


I am 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.

Transistor current limit with extra transistor

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 the 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.

So, is there a clever way to achieve this circuit? Or, do I just have to bite my tongue and put a transistor in place with a better power rating?

Best Answer

Use a small emitter resistor to counter difference in transistor gain like this:


simulate this circuit – Schematic created using CircuitLab

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