> -----Original Message-----
> From: Brent Hilpert via cctalk <[email protected]>
> Sent: 14 October 2025 18:00
> To: General Discussion: On-Topic and Off-Topic Posts <[email protected]>
> Cc: Brent Hilpert <[email protected]>
> Subject: [cctalk] Re: Rainbow H7842 PSU
>
> On 2025Oct 13,, at 2:13 PM, Rob Jarratt <[email protected]> wrote:
> >> On 2025Oct 12,, at 3:28 PM, Rob Jarratt <[email protected]>
> wrote:
> >>>
> >> …
> >>
> >>> I measured AC OK by adding a pull up resistor, but if I am honest, I
> >>> forgot about this in my latest measurements. I have also looked at
> >>> Power OK, knowing that AC OK needs a pull up resistor, and it is
> >>> logically false, so it is definitely saying the power is not OK.
> >>
> >> What is ‘logically false’? 0V or –V?
> >
> > No, its about 7.5V. The comparator +ve input is 6.3V and the -ve input is
> 10.4V, making the comparator output high impedance. I measure about
> +4mV on the gate of the JFET. I think this means the JFET will be conducting
> and so pulling the AC OK H down to 0V, which is what I see when I include a
> pull up resistor on the AC OK H output. What you write below would seem to
> confirm this.
>
> 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.
Thanks
Rob
>
> The output for these comparators is an open-collector NPN transistor with E
> on the Vsupply– pin.
> Behaviour is:
>
> inputs V output output output
> relation trans’r Z logical
> ===== ===== ==== ====
> +in > –in OFF hiZ 1
> +in < –in ON loZ-to-Vsup– 0
>
> Where Vin are a signed voltage.
>
> So for the input V's you mention, what should be occurring is:
>
> E1a.+in = +6.3V
> E1a.–in = +10.4V (these would be saying
> ‘power-is-good')
> ==> +in < –in
> ==> E1a.outTran = ON
> ==> E1a.outV = –12V ~ (because E1.Vsupply– = –12.9V ~, per
> your prior measurement)
> ==> JFET.G < 0V (negative by some number of volts)
> ==> AC-OK = hi-Z = logic.1 = +V with pull-up R.
>
>
> >>> Vstart is 11.3V and Vz is 6.4V. I have not been able to find where
> >>> Vz comes from on the schematic either, if I elaborate the KiCad
> >>> schematics further I will try to find its source.
> >>
> >> TDS = Tony’s schematic
> >> DRM = DEC rainbow manual per Wayne’s ref.
> >>
> >> Vz is in the lower left corner of TDS.pdf.5.
> >> Vz is apparently “V PR” in DRM and should be 5.1V [DRM.pdf.261-7.4.2.1].
> >> Which is to say the zener there is some 5.1V type.
> >> 6.4V is rather high.
> >> I would suggest finding that zener on the board and measure right across
> it.
> >
> > I will locate this diode and measure it and also check the 7812, but it
> > might
> be a couple of days before I get time to do this.
>
> Note the 5.1V comes from the DRM and it’s conceivable DEC adjusted the
> design and put a 6.2V zener in there without updating the DRM.
> IMO it’s something that should be sorted out as Vz is used for a bunch of
> control refs, though not the main Regulation ref.
> Ideally the diode type would be visible.
