On Wed, Oct 15, 2025 at 12:47 AM Rob Jarratt via cctalk < [email protected]> wrote:
> > > > -----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. > TI is one of the worst for this.Especially if the chip implements an industry standard or is compatible with some other chip. In those cases, you barely get enough to understand. I've had to many times in the past hunt down an industry standard or get the datasheet for the part it's compatible with. DEC was really paranoid about single sourced parts, so just about everything is something that's widely used in the industry, and so has many suppliers... You might try that if you're having trouble understanding the datasheet for the exact part. Warner > > > > 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. > > >
