VAXmate PSU

[Rob Jarratt]

> -----Original Message-----
> From: cctalk <cctalk-bounces at 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 at 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. 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.

Regards

Rob