Rob Jarratt robert.jarratt at
Thu Apr 9 03:14:02 CDT 2020

I will repeat it tonight (or tomorrow) with the faster timebase and get the last two cycles. The switching frequency is 100KHz.






From: Mattis Lind <mattislind at> 
Sent: 09 April 2020 08:47
To: rob at; Rob Jarratt <robert.jarratt at>; General Discussion: On-Topic and Off-Topic Posts <cctalk at>
Subject: VAXmate PSU


torsdag 9 april 2020 skrev Rob Jarratt <robert.jarratt at <mailto:robert.jarratt at> >:

I did use a faster timebase, 10us, triggering on the negative edge of channel 2, and it looked identical to this:


I thought the switching frequency was in the range of 50 kHz. But it might be higher. What would be interesting is to see the last two switch cycles in detail. What is the switching frequency?


Admittedly the trace you see was taken on the slower timebase and then zoomed in, but there was no difference, the result was identical.


You can loose detail since the sampling rate is a function of sweep frequency/ time base setting.  






From: Mattis Lind <mattislind at <mailto:mattislind at> > 
Sent: 09 April 2020 05:52
To: rob at <mailto:rob at> ; General Discussion: On-Topic and Off-Topic Posts <cctalk at <mailto:cctalk at> >; Rob Jarratt <robert.jarratt at <mailto:robert.jarratt at> >
Subject: VAXmate PSU


onsdag 8 april 2020 skrev Rob Jarratt <robert.jarratt at <mailto:robert.jarratt at> >:

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. 








From: Mattis Lind < <mailto:mattislind at> mattislind at> 
Sent: 08 April 2020 07:42
To:  <mailto:rob at> rob at; Rob Jarratt < <mailto:robert.jarratt at> robert.jarratt at>; General Discussion: On-Topic and Off-Topic Posts < <mailto:cctalk at> cctalk at>
Subject: Re: VAXmate PSU




Den ons 8 apr. 2020 kl 00:34 skrev Rob Jarratt via cctalk <cctalk at <mailto:cctalk at> >:

> -----Original Message-----
> From: cctalk <cctalk-bounces at <mailto:cctalk-bounces at> > On Behalf Of Brent Hilpert
> cctalk
> Sent: 06 April 2020 21:07
> To: General Discussion: On-Topic and Off-Topic Posts
<cctalk at <mailto:cctalk at> >
> 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
> >
> > .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
> I presume you see some increased current draw from your bench supply for
> 3842 after the SCR triggers.
> What's up with channel 2? Above you say it's D19 anode which is 3842 Vcc
> 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

And one zoomed in to show what happens when the SCR shuts down. <> 

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.




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.



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