Interesting idea! Seems like I could just use a rail to rail op amp with
the LED between output and inverting input. Current sense resistor between
inverting input and ground. Reference voltage on non-inverting input.

Thanks!

Harold


> 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
>
> --
> http://www.piclist.com hint: To leave the PICList
> mailto:piclist-unsubscribe-request@mitvma.mit.edu
>


--
FCC Rules Online at http://www.hallikainen.com

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads