Brent Hilpert bhilpert at
Mon Apr 6 15:06:30 CDT 2020

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
> 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.

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