On 2025Oct 14,, at 1:42 PM, Rob Jarratt <[email protected]> wrote: >> -----Original Message----- >> From: Brent Hilpert via cctalk <[email protected]> >> >> There’s some confusion here somewhere. >> Those input V's would imply the comp. output should be loZ to Vsupply– pin, >> around –12V; not hiZ, +7.5V. > > Oh my! I have clearly got my understanding the wrong way around, not sure how > I did that because I read the datasheet carefully. Somehow, I got confused. I > re-measured and found 1IN+=5.5V, 1IN-=9.4V, Power OK=6.7V, but GND (on the > 393) is -13V. So as you say Power OK should be -13V and AC OK H would be > asserted. I guess this must mean that the comparator itself is faulty. I have > some 393s, so I will replace it and see what happens.
I don’t think I’ve ever seen a comparator datasheet that explicitly laid out the input-to-output function - contrast with other device datasheets with detailed truth tables galore. The comp. datasheets always seem to assume “everybody knows that”. You can figure it out if you look at some of the example circuits or squint closely at just the right parameters in the specs and graphs or trace the operation through the internal schematic if present. In the absence of that, a lot of people seem to (wrongly) assume that “well, + > – would be 1, so transistor ON”. The other way of looking at it, is it’s the 'same direction' of behaviour as an op amp, but without the upper drive-high output transistor.
