At 04:17 PM 3/3/2012, you wrote:
>    I am trying to build a controllable resistive load for a test system
>and I was hoping that someone has solved this problem before.
>
>    The over all design is meant to drive four parallel LEDs & resistor
>pairs and to detect if any of them are open or shorted. The driving
>circuit has a 12v supply with a current sense and the low side is an FET
>that is PWM controlled to Gnd.
>
>    I need to design a variable load that can sink between 20mA and 300mA
>and will emulate the LED/Res load.  This means that the resistance would
>be 60 to 600 Ohms.
>
>    I have looked at the programmable resistance chips like the MCP4012
>or the AD5246 but the operating current range is too small.
>
>    I have also looked at commercially available ones but they are very
>expensive and meant for carrying 15Amps.
>
>    Cost is not a significant factor since I only need to build about
>three of these but I would hate to waste couple of thousand dollars when
>a couple of hundreds (or less) would do.
>
>    Any suggestions would be appreciated.
>
>-Phil-

What are your specifications for accuracy? The LED + resistor pairs
do not behave quite like a resistor. Are there multiple LEDs in series
in each pair (seems like you'd want to do  that with 12V available
and the max single junction voltage is only a few volts.

Could you use signal relays (either electromechanical or MOSFET) and fixed
LEDs and/or resistors to test at your limit values?

--sp






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