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| John Swift:
Hi Mad Jack & Tinkering_Guy , not many people are used to thinking in terms of constant current supplies usually constant current sources are hidden inside analogue IC's - replacing the resistor tail , in long tail pairs in op amp's for instance . If you don't want to build a switch mode power supply to power L.E.D lights in a car constant current supplies to 3 or 4 L.E.D.s in series will keep the light constant as the battery voltage varies between 12 to 15 volts or so , and variation in the forward volt drop of the L.E.D.s if one L.E.D. should go short circuit the current would not change in the others the 2N3819 n-channel F.E.T. is unusual because it is symetrical - the drain and source leads can be interchanged !! the 2 F.E.T.s I tested with the drain connected to + supply and the source and gate connected to 0 v the first F.E.T. the current was 8.4 mA at 4v and 9.1 mA at 20v - not bad for one component the second F.E.T the current was 9.0 mA at 4v and 9.9 mA at 20v a resistor in the source lead will reduce the the current - negative bias on the gate turns the F.E.T off starting of with the 2 terminal bi colour LED I did think of using 2 current limiters and 2 diodes to control alternate half cycles of an ac voltage . but with a 3 terminal bi colour L.E.D., I added 2 more diodes in inverse parallel with the L.E.D.s, realised it looked a bit like a bridge and then I only need one current control circuit I expect the 2N2222 was originally made in the sixties for the defence or computing industry made in vast quantitys , the price would fall making it an attractive choice for many applications |
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