Hi, Russel

> You MAY be able to current sense at lower voltage drops using a long
tailed
> pair using only two (or possibly three) transistors.

    I am not quite sure that I could trust the match between the transistor
to form a differential amplifier and get good results. It might be worth a
try together with a "current mirror" circuit. A little more complex but
might work.

> Also something like an LM358 (dual opamp) or LM339 (quad comparator) is
> quite cheap - probably around $US0.20 in volume, maybe less.)

    I have some problems with that option. The environment I will be working
with has very noisy supply and voltage can go easily up to 80v ! I would
have to power the opamp from a "shunt" power supply referenced to the
positive rail because I have to use high side driver transistor and have not
control on ground return ! Cheap op-amps do not like to work too close to
the supply rails. Simple op-amps would be too much trouble in my opinion.

> Having a transistor partially turned on with external bias would also
allow
> lower voltage drop across the sense resistor but complicates the circuit
> somewhat.

    That is exactly what I am trying to accomplish right now. It should work
but I am still trying to sort out some problems. I decided to use it using
the RDS-ON off the FET together with the shunt resistor and there are some
nasty problems when the FET is off or there is no load. As a good side
effect I should be able to show when there is a short circuit or detect an
open load condition. I should have it working by the end of next week and I
will get the results back here. It is the least complicated solution, even
with the added parts to bias the transistor.

Best regards,
Alexandre Guimaraes

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