>
> As an example of an alternate approach I sketched this up, 100% untested:
>
> http://i.imgur.com/dKXND.jpg
>
> Meets requirements except for turn-off delay. =A0 You could slow down
> the circuit with a cap across R2, forming an RC low pass filter, where
> R=3D(R1||R2||R3).
> --

I acquired a TL431 from an old PSU and experimented with this example
http://i.imgur.com/dKXND.jpg but could not get it to provide any
hysteresis, i.e the turn on level was the same as the turn off level.

I have however, incorporated the TL431  into my previous design.  As
someone mentioned it does provide a very good reference and far
superior to the zener.  Russel mentioned that they only require 500uA
for regulation but these seem to require 10mA.  I will need to read
the manual again and experiment more here.

The circuit checks out ok with an 8 amp resistive load.  Nothing seems
to get hot and the levels once set are reasonable consistent
regardless of the load which appeared to be an issue when using the
zener.  But I am not 100% certain this was due to the zener.  I think
it may have had to do with the supply rails shifting with the large
load.

Thanks for all the assistance.

Cheers Justin

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