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 a
> 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 I have
> tried a few light taps and shakes and it continues to work. Perhaps your idea
> of some debris causing a short might explain it, otherwise I just don't know.
Operation with only VStart+12 places the circuitry into an unspecified
operating region - a region outside of the design intentions. In part, several
semiconductor junctions and portions of circuitry are polarised opposite to
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 operating
environment.
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-moon
satire.
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 adds
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.
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 unknown
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 I
had it in hand, I'd be
double-checking the drawing of that current-sense circuit around the
51-ohm R as a start.
But this is not to say that either of these has anything to do with the fault
you were/are dealing with, they're just things that aren't understood at this
point. Either or both could be pursued out of curiosity or for the sake of
completeness.
> I am thinking I may put it back together and test with a light bulb in series.
