[cctalk] Re: Rainbow H7842 PSU Fault
> > Yes, that is certainly the problem. Presumably more current is flowing > through the current sense resistor than should be. > Do you mean when the power supply is connected to the mains here? If we are still talking about the test case when you are applying an external supply to Vstart only, there should be almost zero current flowing through the 51 Ohm current sense resistor because the chopper is not operating because it has no HV supply. We can calculate the current flowing through the current sense resistor using Ohm's law given it's resistance and the voltage across it so we don't need to presume. > > 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, > My feeling is that (in the test case with external Vstart only) the -12V rectifier diode is being caused to conduct by the forward bias voltage applied to it from Vstart/V2 (should that be Vz I wonder?) via the 16k9 resistor, the 75k resistor and the wound components. The current available to flow through the diode is limited to some microamps by the 75k resistor. As Brent said, this is very low on the diode's characteristic curve and there are likely to be variations between the voltages dropped across different diodes. If diode was not present in the circuit, approx 4V (which IIRC you measured at E1b non-inverting input) should appear across the points where the diode would have been. It is more the characteristic of the diode which is determining how much voltage is appearing across itself than anything else. The other components involved have little say about what voltage appears across the diode. They just allow a small current to flow through the diode to forward bias it and the diode gets to decide how much voltage then appears across itself, depending on the magnitude of the current flowing through it when forward biased and it's characteristic curve. > > 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. > 0.08V across a 51 Ohm resistor suggests that approx 1.6mA is flowing through it from Ohm's law. 0.01V across the same resistor suggests that approx 200 uA flowing through it. I assume this is with the power supply connected to the mains, not using the external supply to energise Vstart only as I can't see where 1.6mA would come from in the latter case. 1.6mA being drawn from the -12V supply is not suggestive of an overload unless it's capacity is very limited indeed. Perhaps this is the normal expected loading? 200uA seems very low indeed but perhaps this is to be expected? If the -12V line is not being produced, the load cannot draw any current from it. I don't think that either of these conditions should trigger an overload response from the PSU. (Are the 0.08V and and 0.01V voltages appearing in the same direction or is one of them positive and the other one negative?) (Does the resistor measure approximately 50 Ohms in both cases with the power switched off)? > > 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? > Sorry, this is not very clear to me and I already get confused enough when dealing with negative supply rails. The -12V output being higher could refer to it's voltage being more positive with respect to 0V or it's voltage being more negative with respect to 0V. In any case, I'm not sure I recall what the voltages measured on the -12V lines of the working and non-working power supplies were with them connected to the mains. I think I recall they were +0.6V and +0.4V respectively when power was applied to Vstart only. I think it is likely that most of what we are seeing here is to be expected and is explainable either by the test conditions created by applying power to Vstart only or by conditions created by the presence of the fault and not necessarily because we are close to the location of the fault. I think the next step might be to check what is happening with the comparators. Are their positive power supply pins connected to Vstart or V2 (Vz?) or somewhere else? Are their negative supply pins connected to 0V or maybe to the -12V line or maybe somewhere else? Are they getting suitable volages on both supply pins that will enable them to operate under startup conditions? If either the positive or negative supply to one or more of them is missing or wildly incorrect at startup, they cannot be expected to do their jobs properly. It might be worth doing some more testing without a mains supply connected but with an external supply for Vstart and also an external negative supply going to the -12V
[cctalk] Re: Rainbow H7842 PSU Fault
> 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.
