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Constant Current Source again!

GraemeC said:
Hi people

I've posted on this before and I've never really got it resolved.

I need to create an adjustable constant current drive that can supply
between 150-300mA to drive an experimental LED. I only have a 5v
supply to play with.

I think there is some confusion out there with regards the term
"constant current source"

If you look at wikipedia "constant current source" and look on the
discussion page you will see a heated debate going on.

What I mean is a source that will provide the same current regardless
of the supply voltage and load resistance(obviously within reason). I
would expect to change the value of the current by changing ratios of

I guess have two questions. Does anyone have a design that meets my
spec for a constant current source and could it be modified to meet my
design criteria.

I know we can do this!


Rather than torment you with ascii schematics, let's talk our way
through this. Connect the gate of a N-fet to the output of an opamp. At
the source of this fet, connect a resistor to ground. Since the fet is
a source follower, we have not done anything other than buffer the
output of the opamp, which is driving a resistor. Tap the connection
where the source of the fet intersects the resistor, and connect this
to the negative input of the opamp. Connect the positive input of the
opamp to a voltage reference of your choice. This voltage now appears
across the resistor. The current in the resistor is V/R, and it flows
into the drain of the N-fet.

If you need accurancy, note that offset of the opamp adds to the
voltage reference. Perhaps a Max400.