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I want to modify this circuit below so the current can be trimmed.

The problem is the variations in base-emitter voltage of the NPN transistor. According to datasheets, it is between 0.55V and 0.7V with a collector-emitter current of 2mA. I need to trim the current through the LEDs to 0.35A, using a potentiometer of 100 ohms or greater.

How could I modify this circuit? The supply will be well regulated.

schematic

simulate this circuit – Schematic created using CircuitLab

Thanks

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  • \$\begingroup\$ Why not use a current mirror? \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 6 '13 at 13:28
  • \$\begingroup\$ Thanks - but unless I have the wrong idea, wouldn't a current mirror essentially double the power the circuit uses? I'll have an array of these circuits, so the extra power would be very significant \$\endgroup\$ – CL22 Oct 6 '13 at 13:49
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    \$\begingroup\$ Hah, yes. That bit slipped my mind. Try a diode in there then. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 6 '13 at 13:51
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    \$\begingroup\$ The part number you used is an N-channel MOSFET, the symbol you used is a P-channel MOSFET and I think you swapped drain and source. \$\endgroup\$ – jippie Oct 6 '13 at 14:28
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Do you have enough headroom to increase the current sensing resistor slightly an drive the BJT from a potmeter in parallel?

schematic

Of course you could optimize R3 to a series of resistors with ratios 270:100:180, where 270 is near ground, 100 is your pot and 180 at source.

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You could achieve a superior result making an adjustable current source following this circuit design. Replace the R1 and R2 resistors in the following with a 100K pot to allow for adjustability.

enter image description here

The U3 component is a shunt regulator that produces a fixed drop of 1.25V when biased at current as supplied through R0. This shunt reference part can be substituted with other similar parts that would have a 1.2 or 1.25V rating.

With a 100K pot set at 72K/28K as shown the current in the output LEDs will be ~350mA.

Note that I have changed the current sensing resistor from your original 2 ohms to 1 ohm.

The opamp shown is a 1.2MHz bandwidth device with rail-to-rail inputs and outputs. It may be replaced with another part number with similar characteristics.

The shunt regulator can be shared with multiple LED current sinks. If you go over three or four shared circuits it would be necessary to decrease the value of R0 so that the bias current of the shunt regulator is increased. It is a good design practice to have the current through the shunt regulator to be 5->10 times larger than the current through the pot (i.e. R1 and R2 divider).

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