onsdag 8 april 2020 skrev Rob Jarratt <robert.jarr...@ntlworld.com>: > I will look at all the suggestions, particularly of a failure on the > secondary side. Something must have burned up, because there was a distinct > burning smell after the initial failure, although I have never been able to > see any physical damage to anything, despite looking many times. > > Aha. Don’t think I seen you writing about that before, or did you? It might be very hard to find the source some times. Even just a small burn will give quite some smell. Check ALL semiconductors very carefully.
> > > But the thing that really puzzles me is that, after correcting the probes > to include the D19 anode, there doesn’t seem to be anything that would > cause D19 to trigger. Am I reading the trace wrong? > > It is very hard to tell from the traces what is going on since the resolution is too low. Use a faster timebase. 5 or 10 microseconds. Find out if you can trigger on something that happen only when it stops. Like channel 2 negative slope. /Mattis > > > Thanks > > > > Rob > > > > *From:* Mattis Lind <mattisl...@gmail.com> > *Sent:* 08 April 2020 07:42 > *To:* r...@jarratt.me.uk; Rob Jarratt <robert.jarr...@ntlworld.com>; > General Discussion: On-Topic and Off-Topic Posts <cctalk@classiccmp.org> > *Subject:* Re: VAXmate PSU > > > > > > > > Den ons 8 apr. 2020 kl 00:34 skrev Rob Jarratt via cctalk < > cctalk@classiccmp.org>: > > > > > -----Original Message----- > > From: cctalk <cctalk-boun...@classiccmp.org> On Behalf Of Brent Hilpert > via > > cctalk > > Sent: 06 April 2020 21:07 > > To: General Discussion: On-Topic and Off-Topic Posts > <cctalk@classiccmp.org> > > Subject: Re: VAXmate PSU > > > > On 2020-Apr-05, at 11:12 PM, Rob Jarratt wrote: > > >> > > >>> I have obtained a scope trace as you suggest. R32 is still lifted so > > >>> the > > >>> UC3842 is powered by the bench PSU, but I am using the full 240VAC > > >>> (no variac). The channels are: > > >>> 1. Ch1. 555 timer. > > >>> 2. Ch2. D19 Anode > > >>> 3. Ch3. D19 Gate. > > >>> 4. Ch4. Q1 Source. > > > > > > Sorry, that looks like a cut and paste error, here is the link to the > > > scope picture > > > https://rjarratt.files.wordpress.com/2020/04/h7270-primary-scr-trigger > > > .png > > > > > > I used a 100ms timebase for the capture and then "zoomed in" a bit > > > > > > You would need to zoom in far more to see what's going on when the SCR > > triggers, to cover just a few cycles around the trigger time. > > > > Once an SCR has been triggerred, the gate becomes a voltage/current > supply, a > > diode drop above 0. > > You see this on your trace in that after triggerring the gate sits at > something +V > > above 0. > > The spike you see may just be an artifact of the internal SCR trigger > action. > > I presume you see some increased current draw from your bench supply for > the > > 3842 after the SCR triggers. > > > > What's up with channel 2? Above you say it's D19 anode which is 3842 Vcc > but > > it shows on the trace as just noise around 0V. > > > > I would still suggest that you scope the state of the secondary-side > crowbar - > > the gate of Q2, and base of Q4. > > Should be simple to do, before trying to remove or disconnect the main > > transformer. > > Oh dear! After Brent's question about D19 anode, I realise that the probe > was connected to the cathode! I have now done it again with the probe > connected to the anode. I have taken two images of the same capture, one at > low resolution to show the overall behaviour > > https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-1.png > > And one zoomed in to show what happens when the SCR shuts down. > > https://rjarratt.files.wordpress.com/2020/04/primary-side- > shutdown-detail-2. > png > > The channels are the same as before, namely: > Ch1. 555 timer. > Ch2. D19 Anode (now corrected as it was previously the cathode!) > Ch3. D19 Gate. > Ch4. Q1 Source. > > I got an earlier trace which showed the D19 anode at 9V, which is under the > Undervoltage Lockout threshold, but I have not been able to repeat it. > > I don't fully understand the debate about using the variac. > > > > I am not going to debate this either since I know what I have been doing > for years and it works perfectly well for me. I have fixed the bigger PSUs > in a VAX 11/750 (one broken switch transistor and multiple broken output > rectifiers). PSU in NORD-10/S (most carbon composition resistors had gone > out of spec). PSUs in many smaller machines as well. > > > > I prefer to work in circuits where I can fiddle around without the danger > of getting killed all the time. Regardless of use of HV differential probe > it can be dangerous. Running it on 50VAC with a protection transformer do > expose a lot of problems already and you can poke around safely in the PSU. > > I have not yet seen a problem that wasn't seen at low voltage, but I > expect there could be semiconductors that experience breakdown that occur > at lower than specified voltage. > > > > > > However, my > measurements appear to suggest that when I use the variac the SCR triggers > because of what appears to be a genuine overcurrent detected by R13. I > think > this is because the duty cycle at low AC input voltages is 50% (rather than > about 10% or less as per the trace I have just taken), and I measured 2V > across R13, which does seem to be enough to trigger the SCR. When I use > 220VAC, the voltage across R13 does rise to 6V, which should also trigger > the SCR I think, except that the peak last a lot less and so perhaps the > fact that the 6V last for a brief period is insufficient to trigger it? > > > > > > On the issue of duty cycle. If we look at this from the start up > perspective rather than the steady state perspective. At startup there are > no stored energy in the output filter capacitors. The voltage on the output > is thus 0. As soon as the PSU is doing its first switching pulse energy is > transfered as the main switch transistor is cutting off. The energy is > transfered into the capacitor and into the load. The voltage is starting to > increase. > > > > The duty cycle generated by the PWM circuitry is in pure relation to the > voltage error, i.e. the difference between output voltage and reference > voltage. In essence it is a P-regulator. > > > > When there are 0 Volt out the duty cycle will be at the maximum. Nothing > strange about that. But what is maximum duty cycle? It depends on the > circuitry used. The UC3842 can do up to almost 100% duty cycle. However it > may be wise to limit duty cycle in a flyback design so that the transformer > is not saturated. I am not sure if there is some kind of duty cycle > limitation in this circuit though. > > > > So if it can handle 50% duty cycle at startup it should be able to handle > it at any time. Besides it would be incredible weird to design a circuit to > use a 10% duty cycle at its standard operating point and detecting over > current at 50%. Then you have much less head room for load and input > variations. > > > > I am more or less convinced that what you see on the primary side is a > result of some kind of fault on the secondary side. > > A very common problem is short-circuit rectifier diodes on the secondary > side (D12, D11, D21, D22, D23, D24). They can be difficult to measure > correctly in circuit since the resistance of the secondaries of the > transformer is so low. Depending on type you can either desolder them > completely or just lift one end of them. > > > > My experience is that electrolytic capacitors seldom short circuit. They > probably boil and explode instead. Tantalum capacitors often short circuit. > Some of them goes into fire other just stay short circuit. So check for > tantalum capacitors and try to measure them for short circuit. > > > > You have a crowbar on the secondary side. Are you sure that one hasn't > triggered? If you still run on variac you can disable the crowbar circuit > by removing the SCR and ramp up the voltage slowly to see if that makes any > difference. > > > > Breaking the feed back loop: > > > > R23 seems to be in the feedback path. If you lift it and insert a voltage > from a lab supply here you could simulate the output voltage and study the > behaviour of the UC3842 for different feed back voltages. You will see that > it will stay on max duty cycle up until close to the nominal voltage and > over a very small span change to almost no pulse out at all. This is due to > the gain of the circuit. > > > > /Mattis > > > > > I have seen the suggestions to study the waveforms at a much higher > resolution. What I am doing is setting the overall timebase in the 100ms > range so that I can trigger on when the 555 starts to oscillate and capture > the whole period of operation until the SCR triggers. I can then zoom in, > as > can be seen from the trace provided in this email. I hope that is good > enough, or am I missing some problem with doing it this way? > > I would like to follow Mattis's suggestions (and other people have said it > too) to break the feedback loop, but it does look difficult to know how > best > to do it. > > I also understand Brent's suggestion that the gate spike is just the result > of the SCR triggering, rather then the cause of the trigger. I had wondered > if that might be the case. > > Regards > > Rob > >
