Re: my "proportional/derivative" led driver? comments please!

On Fri, 17 Apr 2009 13:38:10 -0700, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Fri, 17 Apr 2009 12:32:44 -0700 (PDT), acannell@xxxxxxxxx wrote:

i am new to PID theory and circuit design, and I threw this together
over a few days.

http://i242.photobucket.com/albums/ff197/acannell/leddriverpid.jpg

the goal is to create a voltage controlled current driver for an LED,
where the current is directly proportional to the voltage, i.e. 100mV
= 100mA, and the driver is useful up to at least 50khz, including
square waves.

what mystifies me is how well adding the 1nF cap worked. without it
there was about 15% overshoot (instead of rising to 100mV, it rose to
about 113mV).

i tried everything i could think of to make a derivative section, but
none of it worked as well as that cap.

so, is this circuit close to being good? what have I done wrong?

Asa

That upper opamp circuit is very strange and arguably unstable.

John

Looks like a twist (mis-connect?) of the Howland current source/"pump"

...Jim Thompson
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