13 Oct
2025
13 Oct
'25
11:26 a.m.
However, I am thinking that I
shouldn't be too concerned about the AC OK signal because it is clear that
the 5V output is out of spec being at around 4.3V, so I think I probably
need to fix the 5V output and then perhaps the AC OK will sort itself out,
or at least I can fix AC OK once the outputs are actually valid. As far as I
can tell the Power OK and AC OK signals don't influence the operation of the
PSU.
Is the +12V line similarly low?
Given that I do get an output of 4.3V, I would have thought that the
thyristor was not being switched on.
Yes.
I have measured Raw 5V and it too is around the same voltage as the 5V
output. In fact both now measure about 4.4V. I checked the resistance
between the junction of the +5V rectifiers and the +5V output and it is
about 0.1R, so I don't think there is a break in continuity.
After Brent's explaination, it looks like I was barking up the wrong tree
regarding a break in continuity. However, 0.1R might still be a little on
the high side. At 5A, this resistance would drop 0.5V. On the other hand,
the PSU ought to be able to sense and cope with this sort of drop and raise
the drive to compensate for it.
What sort if resistance is there between the junction of the +12V rectifiers
and the +12V output? It ought to be similar or maybe slightly higher than
in the +5V case.
I hadn't spotted the capacitor on the 12V Pulse output. The rectifier output
for 12V looks like this:
https://rjarratt.wordpress.com/wp-content/uploads/2025/10/h7842-12v-seconda…
This looks a little bit larger than 12/5 times the +5V rectifier output.
and 12V Pulse looks like this:
https://rjarratt.wordpress.com/wp-content/uploads/2025/10/h7842-12v-pulse.p….
Strange that this one seems to be going nearly a volt negative.
I wonder is the +12V rectifier output going similarly negative but we can't
see it with the scope set to 50V/division? If so, that might cast suspicion
on the lower diode in the FE16C +12V rectifier package, or I might be barking
up the wrong tree again.
It is odd that 5V pulse is getting nicely filtered and smoothed by a 470R
resistor in series and whatever capacitor is across it but 12V pulse is not
getting smoothed at all by the much larger 48k7 resistor in series and the 4n7
capacitor across it. If the 4n7 capacitor is open circuit, this might cause
E3c to assert shutdown for the duration of the peak of the pulses on the
12V pulse line and end up lowering the output voltages on average. However,
I'm not quite sure what E3b would have to say about this - I can't quite
figure the logic here. I think maybe E3b is supposed to latch the shutdown
condition and this doesn't seem to be happening.
Or could it be that the 4n7 capacitor shown in the schematic across 12V pulse
is supposed to be shown across 5V pulse and there is no capacitor across
12V pulse?
Vstart is 11.3V and Vz is 6.4V. I have not been able to find where Vz comes
from on the schematic either, if I elaborate the KiCad schematics further I
will try to find its source. The supply to E1, E2 and E3 is 11.3V,
referenced to the GND output, the GND pin on E1 is -12.9V and the GND pins
on E2 and E3 are -0.7V
As Brent mentioned in another post, Vz seems too high. This probably needs
to be sorted out before anything else. Vz being high at E1a might explain
why Power OK is not being asserted.
E3 GND pin being at -0.7V might explain what is pulling 12V pulse negative.
As Brent said, it would be good to find whether there is good reason for
this -0.7V.
It may be worth checking the resistors around the inputs to E4 and Vref,
especially the two 500R resistors.
I think Vref (E4 pin 16) is supposed to be 5.1V - it would be worth
checking this voltage too.
Regards,
Peter.