Rob Jarratt robert.jarratt at
Tue Apr 7 17:34:38 CDT 2020

> -----Original Message-----
> From: cctalk <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>
> 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. 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?

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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