[cctalk] Re: Rainbow H7842 PSU Fault

On 2023-May-25, at 1:43 PM, Rob Jarratt via cctalk wrote: Operation with only VStart+12 places the circuitry into an unspecified operating region - a region outside of the design intentions. In part, several semiconductor junctions and portions of circuitry are polarised opposite to their normal/designed-for state. It is not surprising that you are seeing odd/unpredictable behaviour under this operating environment, nor is it surprising that it's different than the 'good' supply under the same operating environment. So why was it in shutdown earlier the other day but not now? : Who knows - it's operating in an unspecified region. Perhaps the room temperature is 2 degrees higher. That's a serious point, not phase-of-the-moon satire. When you supplied the proper startup environment with both Vstart+12 and Vstart-12 both the bad and good unit behaved as expected for the design. Why is the VStart+12 current draw higher when it was in shutdown versus when the PWM controller IC is pulsing? : Because in shutdown the 'Chopper Driver' transistor (PSU Sheet 2) is held hard ON (conducting) (see datasheet). Holding this transistor ON subtracts it's off-state current (~ 17mA) but adds it's on-state base current (~ 37mA) and it's on-state collector current (~ 73mA), for a net up-to ~ 93mA increase (may be less dependant on duty cycle of PWM), to the Vstart+12 current. There remain two unexplained things here: - Where was that unusual current-sense voltage that sends it into shutdown coming from? (I provided one potential explanation earlier, but it remains unknown at this time). - The 51-ohm current-sense resistor in the -12V supply vs the mode of operation of the -12 supply remains unexplained/non-sensical. At the max current you mentioned (150mA), the V drop across that R would be >7V (!), which makes no sense. If I had it in hand, I'd be double-checking the drawing of that current-sense circuit around the 51-ohm R as a start. But this is not to say that either of these has anything to do with the fault you were/are dealing with, they're just things that aren't understood at this point. Either or both could be pursued out of curiosity or for the sake of completeness.