8 May
2023
8 May
'23
12:03 p.m.
-----Original Message-----
From: Brent Hilpert <bhilpert(a)shaw.ca>
Sent: 07 May 2023 23:53
To: rob(a)jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Subject: Re: [cctalk] Rainbow H7842 PSU Fault
On 2023-May-07, at 1:54 AM, Rob Jarratt via cctalk wrote:
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.
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.
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.