🚀 go-pugleaf

John Popelish <jpopelish@rica.net> wrote in message news:<3CC805D4.D1FD10C3@rica.net>...
> Adam Seychell wrote:
> (snip)
> > the output takes several seconds to settle after power is applied, as
> > if the LED is heating up and stabilizing. I suspect the LED is
> > changing its radiation pattern with temperature, thus altering
> > relative
> > amounts of light to the feedback and measuring photodiode.
> > Currently a mirror is used to reflect part of the light to the
> > feedback photodiode. The two photodiodes therefore receive light from
> > two different
> > spots of the LED. Your idea of using a beamsplitter should get around
> > this
> > problem because the same light path will be directed to both
> > photodiodes.
> > 
> > As for the circuit I'm using all 100ppm/K components and photodiodes
> > are run in shortcircuit mode (i.e across the inputs of a high
> > impeadance op-amp).
> 
> Are you including an integrator in the brightness feedback loop, to
> force the measuring diode to produce a preset amount of signal or just
> a fixed gain, to reduce the variations?


John,

Yes, the feedback is an integrator type so the DC gain is equal to 
op-amp open loop gain. Speed is not a problem so I've whacked a 
largish 0.1uF across it for the moment. As far as I can tell all the 
electricals seem to be working. It is the physical stability that is 
causing me a headache. Everything is in aluminium blocks and bolted 
down so it not exactly going anywhere.  It seems Tony has the 
right idea, build two identical units and measure ratio between 
them. Its back to the drawing board.