[cctalk] Re: Rainbow H7842 PSU Fault

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. 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. 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)? 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 line via a 2k7 resistor to mimic both the positive and negative startup supplies on sheet 1. Comparing (no pun intended) what appears on the positive and negative power supply pins of the comparators between the working and non-working power supplies under these conditions might reveal something. If the two behave similarly, it may be time to focus on whether the comparators in the faulty PSU are behaving correctly. However, I would be wary of attempting to make tests around any of the comparators with the mains supply connected to the PSU in case of inadvertently disabling a protection circuit which could be detecting and protecting against a real overload. This would probably lead to something going bang :-( To expand a little further, I got the 45uA by applying Ohm's law to the 75k resistor with 3.4V across it. All the other resistances in series would be negligable compared to 75k. This seems reasonable given what Brent was saying about variation in diode characteristics when the current through them is very low. Regards, Peter Coghlan.