Hi Rob, > > Thanks for the various replies. Here are my responses and further > observations. > > I was asked if I had a load on the PSU while bench testing it. The answer to > that is no. >
I initially thought that the +0.6V on the -12V line might have been arriving there via a load which was connected to both the -12V line and a positive supply derived from the +15V you are applying from the bench power supply. It seems that this cannot be the case as there is no load connected and all the positive outputs from the PSU are at zero anyway. An the internal to the PSU path from Vstart to the -12V line is a much more likely explaination for this +0.6V. > > I did check the two diodes around the -12V current sensing resistor (PSU > Sheet 3), but as was pointed out the current sensing resistor is very low > value (51 ohms). The two diodes measure the same, in circuit, on the working > and non-working PSUs. I suppose I may need to lift them to check them > properly. > I don't think there is any need to check these further. I was barking up the wrong tree whe I suggested suspecting them. > > I am not sure about the comment suggesting that the bench test conditions > could result an overcurrent being sensed on the -12V output. This is because > I use the same bench test conditions on both the working and non-working > PSUs, and on the working one an overcurrent is not sensed. > 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. > > 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'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 :-) 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 :-) Regards, Peter Coghlan.
