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Will this work (optocoupler over shunt)?
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ArthurDent:
spec - "Attached below is the schematic for a bulb current monitor with a voltage overhead of 50mV."

Even less voltage drop than the previous circuits but is the optocoupler and the second 12VDC supply necessary? Seems like the two halves could be combined with the comparator driving Q4
spec:

--- Quote from: ArthurDent on December 06, 2018, 03:39:22 am ---spec - "Attached below is the schematic for a bulb current monitor with a voltage overhead of 50mV."

Even less voltage drop than the previous circuits but is the optocoupler and the second 12VDC supply necessary? Seems like the two halves could be combined with the comparator driving Q4

--- End quote ---
Of course, but that is not what the OP has asked for. We can only assume that the OP has a reason for having two isolated supplies. The resolution would be to ask him. :)

Another aspect of simplifying the design is to ask what the relay is for. Maybe that could be eliminated too.
spec:

--- Quote from: ArthurDent on December 06, 2018, 03:39:22 am ---spec - "Attached below is the schematic for a bulb current monitor with a voltage overhead of 50mV."

Even less voltage drop than the previous circuits but is the optocoupler and the second 12VDC supply necessary? Seems like the two halves could be combined with the comparator driving Q4

--- End quote ---

AD, is this what you had in mind >>
ArthurDent:
spec - "AD, is this what you had in mind >>"

That is what I had in mind and I couldn't see why there would need to be 2 supplies. The only question I have with the circuit is the input connections on the unused comparator. I think connecting the unused inputs directly to ground is recommended as in section 8.1 of this application note.

http://www.ti.com/lit/ds/symlink/lm393-n.pdf
spec:

--- Quote from: ArthurDent on December 06, 2018, 02:27:44 pm ---spec - "AD, is this what you had in mind >>"

That is what I had in mind and I couldn't see why there would need to be 2 supplies. The only question I have with the circuit is the input connections on the unused comparator. I think connecting the unused inputs directly to ground is recommended as in section 8.1 of this application note.

http://www.ti.com/lit/ds/symlink/lm393-n.pdf

--- End quote ---
Well well well- I have been doing it wrong for all these years! Thanks for the info, but I am not too happy with tying both unused inputs to 0V because, it seems to me, that then the comparator would be in an undefined state and likely to oscillate (the inputs are still active at 0V input).

UPDATE #1 2018_12_06 I just sent a report to TI about the recommendation to tie both inputs of unused comparators to 0V- be interesting to see there response, if it's from and applications engineer that is. 
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