It has taken me ages to get back to this, but I think I have hit an
I reassembled the PSU after the PWM started working again and tested it with
a dummy load and using two 100W light bulbs in series on the input side to
limit the current. However, it did not work. I traced this to the 7812
regulator that supplies Vstart (PSU Sheet 1) not getting enough voltage to
run, about 3.7VDC. In the same conditions the working one gets about 10 or
11VDC. I also noticed that on the good PSU the 100W bulbs pulsed and were
fairly dim, but on the bad PSU they were brighter and glowed steadily.
This made me suspect that the startup transformer may have been damaged. So
I checked the output of the startup transformer. On the bad PSU this was
about 2.4VAC, while on the working one it was about 12VAC. I lifted all 4 of
the diodes connected to the startup transformer and they all tested OK.
Sadly, I think this means the transformer must have been damaged. I have the
facility to do a ring test on the transformer, but don't want to remove it
unless there is a need to. Is there any other failure mode that I am not
considering? I didn't check the inputs to the transformer, so something
could be short on the input side and pulling the input to the startup
transformer down. It is worth reminding anyone reading this, that the 7812
on PSU Sheet 1 did fail with a short to ground.
I have included a couple of minor responses to Brent's last email to me
From: Brent Hilpert <bhilpert(a)shaw.ca>
Sent: 26 May 2023 17:24
To: rob(a)jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
Subject: Re: [cctalk] Rainbow H7842 PSU Fault
On 2023-May-25, at 1:43 PM, Rob Jarratt via cctalk wrote:
> This evening I went to check Vstart for any oscillation. However, all of
sudden, the current draw is down to 85mA and PWM has
started working. I
am at a loss to explain it. I wondered if there might be a dry joint, but
tried a few light taps and shakes and it continues to
work. Perhaps your
of some debris causing a short might explain it,
otherwise I just don't
Operation with only VStart+12 places the circuitry into an unspecified
operating region - a region outside of the design intentions. In part,
semiconductor junctions and portions of circuitry are
their normal/designed-for state. It is not surprising
that you are seeing
odd/unpredictable behaviour under this operating environment, nor is it
surprising that it's different than the 'good' supply under the same
So why was it in shutdown earlier the other day but not now? :
Who knows - it's operating in an unspecified region. Perhaps the room
temperature is 2 degrees higher. That's a serious point, not phase-of-the-
Well, the weather here has indeed got a bit warmer.
When you supplied the proper startup environment with both Vstart+12 and
Vstart-12 both the bad and good unit behaved as expected for the design.
Why is the VStart+12 current draw higher when it was in shutdown versus
when the PWM controller IC is pulsing? :
Because in shutdown the 'Chopper Driver' transistor (PSU Sheet 2) is held
hard ON (conducting) (see datasheet).
Holding this transistor ON subtracts it's off-state current (~ 17mA) but
it's on-state base current (~ 37mA) and it's
on-state collector current (~
73mA), for a net up-to ~ 93mA increase (may be less dependant on duty
cycle of PWM), to the Vstart+12 current.
Thanks for explaining that, it makes more sense now. I need to learn to look
at other parts of the schematic beyond the immediate thing I am interested
in because I keep missing things like this.
There remain two unexplained things here:
- Where was that unusual current-sense voltage that sends it into
shutdown coming from?
(I provided one potential explanation earlier, but it remains
at this time).
- The 51-ohm current-sense resistor in the -12V supply vs the mode
of operation of
the -12 supply remains unexplained/non-sensical. At the max current
you mentioned (150mA),
the V drop across that R would be >7V (!), which makes no sense. If
had it in hand, I'd be
double-checking the drawing of that current-sense circuit around the
51-ohm R as a start.
Agreed that this does not make a lot of sense. I have checked the actual
value of the resistor (in circuit) and it matches the schematic. I have also
checked the schematic against the actual circuit and it really does seem
like the schematic is correct.
But this is not to say that either of these has anything to do with the
were/are dealing with, they're just things that
aren't understood at this
Either or both could be pursued out of curiosity or
for the sake of
> I am thinking I may put it back together and test with a light bulb in