> -----Original Message-----
> From: Brent Hilpert <bhilp...@shaw.ca>
> Sent: 07 May 2023 23:53
> To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
> <cctalk@classiccmp.org>
> Subject: Re: [cctalk] Rainbow H7842 PSU Fault
>
> On 2023-May-07, at 1:54 AM, Rob Jarratt via cctalk wrote:
> > The comments about the tolerance of the 7812 were right, it doesn’t
> appear to be an issue with the replacement 7812 regulator because when I
> tried using the bench PSU to feed exactly 12V to the circuit from the output
> of
> the 7812 the comparator still gave the wrong result. It was still wrong if I
> applied only 11V
> >
> > I then looked at the value of Vz on the good and bad PSUs, when applying
> 12V to the 7812 output. That was 5.4V in both the good and bad PSUs.
> Where I saw a difference was on the -12V output, it was +0.4V on the good
> PSU and 0.56V on the bad one (the voltage varied so this was an average). I
> checked the voltage drop across the current sense resistor. It is 0.01V on the
> good PSU and 0.08V on the bad PSU, which would explain the higher positive
> voltage on the -12V output and the comparator being turned on.
>
>
> From earlier measurements and the 45uA calc of current through the 51Ω
> sense resistor, the V across the 51Ω ISense-12 resistor should be only 0.002V.
>
> So a question is where is this 0.08V coming from? An unfulfilled -12V supply
> for the E3 power pin might have been an explanation, as extra current might
> be drawn out of the E3d.+in input due to the +in being pulled below
> (negative to) the
> E3 -power pin. But you say that pin is connected to GND, so the source of the
> 0.08V should be sought, some more comprehensive measurements around
> the E3d inputs / ISenseR might help.
I thought I had answered where the Vcc and GND are connected on the comparator.
Vcc is connected to Vstart and GND to GND. I will look again at all the inputs
to check.
>
> From calculations from design/schematic:
> - E3d.+in should = 1/2 the voltage across the 51Ω ISense-12 resistor.
I measure 0.08V across the current sense resistor and indeed I found 0.04V on
the E3d +IN
> - E3d.-in should = 0.033 V.
And that is what I measured (actually 0.034V).
> - The trip condition to send the E3d output high (shutdown) is: V(ISR-12) >
> 0.066V (V across ISense-12 Resistor).
Indeed. The drop across the resistor is too high causing the comparator to turn
off, when it should be on and sinking current to allow the PWN to operate. I
think the comparator is working correctly, the problem seems to be the voltage
drop across the resistor is genuinely too high, but I can't see why.
Under the same test conditions, I measure a 0.01V drop across the resistor in
the good PSU.
>
> Also: Is that ISense-12 resistor really a 51Ω resistor? Or is it 5.1Ω or
> 0.51Ω?
> Again from calculation, 51Ω would imply a current limit of only 1.3mA, which
> is rather low.
It is 51R, I have measured it (in circuit) and the colour coding confirms it.
The spec in the technical manual says the -12V is rated at 150mA max.
Thanks
Rob
PS Will reply to Peter's email later when I have had a chance to understand it
and to try the suggestions.
