Hello, It has taken me ages to get back to this, but I think I have hit an insurmountable problem.
I reassembled the PSU after the PWM started working again and tested it with a dummy load and using two 100W light bulbs in series on the input side to limit the current. However, it did not work. I traced this to the 7812 regulator that supplies Vstart (PSU Sheet 1) not getting enough voltage to run, about 3.7VDC. In the same conditions the working one gets about 10 or 11VDC. I also noticed that on the good PSU the 100W bulbs pulsed and were fairly dim, but on the bad PSU they were brighter and glowed steadily. This made me suspect that the startup transformer may have been damaged. So I checked the output of the startup transformer. On the bad PSU this was about 2.4VAC, while on the working one it was about 12VAC. I lifted all 4 of the diodes connected to the startup transformer and they all tested OK. Sadly, I think this means the transformer must have been damaged. I have the facility to do a ring test on the transformer, but don't want to remove it unless there is a need to. Is there any other failure mode that I am not considering? I didn't check the inputs to the transformer, so something could be short on the input side and pulling the input to the startup transformer down. It is worth reminding anyone reading this, that the 7812 on PSU Sheet 1 did fail with a short to ground. I have included a couple of minor responses to Brent's last email to me below. Regards Rob > -----Original Message----- > From: Brent Hilpert <[email protected]> > Sent: 26 May 2023 17:24 > To: [email protected]; General Discussion: On-Topic and Off-Topic Posts > <[email protected]> > Subject: Re: [cctalk] Rainbow H7842 PSU Fault > > On 2023-May-25, at 1:43 PM, Rob Jarratt via cctalk wrote: > > > > This evening I went to check Vstart for any oscillation. However, all of a > sudden, the current draw is down to 85mA and PWM has started working. I > am at a loss to explain it. I wondered if there might be a dry joint, but I have > tried a few light taps and shakes and it continues to work. Perhaps your idea > of some debris causing a short might explain it, otherwise I just don't know. > > 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. Well, the weather here has indeed got a bit warmer. > > 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. > Thanks for explaining that, it makes more sense now. I need to learn to look at other parts of the schematic beyond the immediate thing I am interested in because I keep missing things like this. > 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. Agreed that this does not make a lot of sense. I have checked the actual value of the resistor (in circuit) and it matches the schematic. I have also checked the schematic against the actual circuit and it really does seem like the schematic is correct. > > 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. > > > > I am thinking I may put it back together and test with a light bulb in series. >
