15 Oct
2025
15 Oct
'25
6:41 a.m.
-----Original Message-----
From: Brent Hilpert via cctalk <cctalk(a)classiccmp.org>
Sent: 14 October 2025 23:14
To: General Discussion: On-Topic and Off-Topic Posts (cctalk(a)classiccmp.org)
<cctalk(a)classiccmp.org>
Cc: Brent Hilpert <bhilpert(a)shaw.ca>
Subject: [cctalk] Re: Rainbow H7842 PSU
On 2025Oct 14,, at 1:42 PM, Rob Jarratt <robert.jarratt(a)ntlworld.com> wrote:
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.
-----Original Message-----
From: Brent Hilpert via cctalk <cctalk(a)classiccmp.org>
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
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.