At 03:52 PM 6/24/2004 -0700, you wrote:
>Thanks! I'm not seeing how I can get around the LED Vf requirement. What I
>need is a dynamic resistor that sits between the LED and the variable
>supply. The supply varies between 0 and 10V. No matter what the voltage, I
>want the LED current to be 5mA. (recognizing, of course, that I won't get
>that when the power supply voltage is below the LED Vf). This "dynamic
>resistor" should adjust between 0 ohms and some higher resistance to limit
>the LED current to 5mA. Ideally the circuit draws only 5mA (and all of
>that goes through the LED).

You can do this with two active parts (an LM10C and a BJT) plus three
resistor in such a way as to get down to within 60mV or so of the Vf for
minimum supply voltage. Divide the 800mV reference down to 50mV with two
resistors, connect it to the non-inverting input of the op-amp. Connect
the op-amp output to the base of the transistor and feed the voltage
back from an emitter sense resistor to ground to the op-amp inverting input.
The LM10C is powered from the supply voltage, the LED goes from the
collector to the +ve supply voltage. Wasted current is a few hundred uA
typically, and it will work down to Vf + 60mV or so with any color of
LED. You use an emitter resistor to drop 50mV at 5mA, or 10 ohms.

The LM10C is a bit expensive though (1.60 in 100's), the other parts
cost just about nothing.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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