29 Apr
2023
29 Apr
'23
12:30 p.m.
It looks like the conditions in the control and
overcurrent sensing parts
of the
working and the non-working PSUs are different so some
further
investigation
may be required as to why this is the case.
Yes, that is certainly the problem. Presumably more current is flowing
through the current sense resistor than should be. I thought that perhaps
the higher positive voltage might be enough to switch on the rectifier diode
while the lower positive voltage on the good PSU won't, but it seems to
switch on in both cases because I measured a voltage across the current
sense resistor in both cases. I measured 0.08V on the bad PSU and 0.01 on
the good PSU. So this would explain why an over current is being detected.
What I don't now get is why the -12V output is higher on the bad PSU because
if there is something shorted or a capacitor with high current leakage on
the -12V output, that would surely result in a lower voltage on the -12V
output, not a higher one?
>
> I hadn't noticed the connection to -12V on the non-inverting input of
E1b.
However, I
don't think this can account for the 0.6V because in both
the working and non-working PSUs the non-inverting input to E1b is 4V.
See further down.
>
> However,
> when I was checking this, I noticed that I must have made a mistake,
> because the -12V output actually measures +0.4V on the working PSU
> (not zero as I first thought), but the non-working PSU measures +0.6V.
> Also, I noticed that the -12V output on the working PSU rises more
> slowly to +0.4V than on the non-working PSU where it rises more quickly +0.6V.
the 75k
I still don't understand where this +ve voltage on the -12V output can
come from though? Whether on the working PSU or the non-working PSU.
The 4V at the non-inverting input of E1b is linked to the -12V line via resistor and from there via a smoothing choke and the
chopper transformer
secondary to the -12V line rectifier diode connected to pin 6 and on to
ground via the parallel diode/resistor combination. This should result in
a
current of about 45 microamps flowing to ground
through those components
mentioned.
This in turn should cause the rectifier to be forward biased and drop
approximately 0.6V across it which is characteristic of silicon diodes.
Does that make sense?
It took me a while to understand this, but yes I get that now.
(It's rather confusing in that the -12V line rectifier is on the ground
side of
the
chopper transformer secondary instead of on the supply
side where it would
more usually be encountered. It's electrically all the same wherever it
is
put
as the components involved are in series anyway.)
It ought to be possible to measure the same 0.6V across the diode to
confirm
this is where it is being dropped (and to measure the
remaining 3.4V of
the
4V at E1b across the 75k resistor). As to why it is
only 0.4V on the
working
power supply, I haven't thought that far ahead yet
:-)
On the bad PSU I measure a 0.48V drop across the rectifier diode (the one
directly connected to the transformer, not the one in parallel with the
current sense resistor), on the good PSU I measure a 0.4V drop.
I think the +0.6V on the -12V line is explainable and to be expected under
the
test conditions described. It looks like there could
be something wrong
in the
control circuitry which is preventing the power supply
from starting up.
This might also account for the difference between the 0.4V and 0.6V.
Brent's suggestions for checking the condition around the comparators and
how they are supplied with power are good ones. I haven't made any
further
suggestions because I don't have any right now :-)
I had forgotten about Brent's suggestion. Still need to check this.
Regards,
Peter Coghlan.