> 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 to > +0.6V. > > > > 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 the 75k > 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.
