Guest zaid Posted May 31, 2014 Report Share Posted May 31, 2014 i am looking for helpi would like to design a circuit that gives continious 20 mA DC output using battery source Quote Link to comment Share on other sites More sharing options...
audioguru Posted May 31, 2014 Report Share Posted May 31, 2014 You did not ask for constant current as the battery voltage runs down or when the load resistance is changed.If you use a 9V battery that actually measures 9.0V then a load resistance of (9V/20mA=) 450 ohms draws 20.0mA.There are many constant current source circuits in Google. Most use an opamp driving an emitter-follower NPN transistor, a current sense resistor and a voltage reference.I would use an LM10 IC that is an opamp plus an adjustable voltage reference in an 8-pins case plus an NPN transistor. Quote Link to comment Share on other sites More sharing options...
Guest zaid Posted May 31, 2014 Report Share Posted May 31, 2014 thanks a lot for your commentsi am looking for that amount of output current on a variable load between 1k-5k Quote Link to comment Share on other sites More sharing options...
Hero999 Posted May 31, 2014 Report Share Posted May 31, 2014 What are ytou trying to do?Quite often a resistor will do.A simple constant current source can be built using an LM317 but it has a high drop-out voltage, typically 2.75V at 20mA. Quote Link to comment Share on other sites More sharing options...
audioguru Posted May 31, 2014 Report Share Posted May 31, 2014 i am looking for that amount of output current on a variable load between 1k-5kThen Ohm's Law says that the constant current source needs to produce a voltage of (1k x 20mA=) 20V to (5k x 20mA=) 100V. Where will you find a battery that is 100V when it is almost dead? Quote Link to comment Share on other sites More sharing options...
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