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).

What I'm trying to do is build a solid state relay that has a very low
turn on voltage, very low operating current, and the control current does
not increase with control voltage. Ideally, the SSR would turn on with a
control voltage of 500mV or so, but I don't see that happening. We have no
power supply available other than the control voltage. An MOC3023 fires
the output triac at about 1.1V across the LED with an LED current of 3mA.
The LED right across our control voltage is pretty good until the voltage
goes above 1.1V when, of course, the current increases. So, how can I keep
that current from increasing without losing my low turn-on voltage?

THANKS to all for the great ideas!

Harold


> At 03:10 PM 6/24/2004 -0700, Harold Hallikainen wrote:
>>The two terminal restriction is because I have no power supply available,
>>just the 0-10VDC signal that needs to power the LED.
>
> Ok, you were putting unneeded restrictions in place.
> One option is to power your constant current circuit above the LED, so
> that
> it's Vf isn't subtracted from the operating range.
>
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