Electronic – Constant current circuit with transistor


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I want to build a constant current circuit of 317 mA using using a TIP 42C PNP transistor.

I am able to provide constant base current of 3.17 mA and the minimum value of beta of the transistor is 30.

I took beta as 100 for safety and still my collector current varies according to the load resistance connected to it.

Please tell me what I did wrong and how to make the circuit work.

Best Answer

A simple PNP transistor fed with a constant base current is not going to function as what most people would describe as a constant current source. If beta is 30, the output current will be 95 mA and not 317 mA. Beta varies with temperature and saturation levels so it cannot be relied upon to be constant in a proper design.

A design that I have used a lot is here: -

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Taken from my answer here. Supporting text from that answer: -

Choose R1 to be low in value hence it won't drop too much voltage in sourcing current to the load. If you chose 1 ohm then to get 300 mA through the load, Vin would be 0.3 volts and the drop across R1 would also be 0.3 volts.

There are a variety of ideas about current sources here.