> -----Original Message-----
> From: Brent Hilpert via cctalk <[email protected]>
> Sent: 14 October 2025 23:14
> To: General Discussion: On-Topic and Off-Topic Posts ([email protected])
> <[email protected]>
> Cc: Brent Hilpert <[email protected]>
> Subject: [cctalk] Re: Rainbow H7842 PSU
>
> 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.
I have a Texas Instruments datasheet that does explain it, but much further
down in the Application section. I have to say that many datasheets assume you
already know an awful lot about the devices and how they work, which is
definitely not the case for someone like me.
>
> 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.
