What I need to test & verify is the current sense on the power supply. =20
Does it set a flag if the current  is outside of the limits?  If it is=20
outside, what is the actual trip point?

The problem is that I need to be able to vary the current through a load=20
that is not referenced to ground.  It is reference to the +12v with the=20
low side varying between 0 and 12v.

My specifications for accuracy are not the real problem.  I can measure=20
the real current with enough precision but I need a way to vary the load=20
repeatability.

-Phil-

On 3/3/2012 1:50 PM, Spehro Pefhany wrote:
> 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 300m=
A
>> 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 fixe=
d
> LEDs and/or resistors to test at your limit values?
>
> --sp
>
>
>
>
>
>
> -
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--=20
--------------------------------
Long ago when men cursed and beat the
ground with sticks,  it was called witchcraft..

Today, it's called golf.

--=20
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