Brent Hilpert bhilpert at
Sun Apr 5 18:12:20 CDT 2020

On 2020-Apr-05, at 2:53 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.
> The picture is here:
> ator.png
> You are correct, I have mixed up the values with R30 and R31, the correct
> value is 15K. I have updated the schematic, and rearranged it to look
> diagrammatically more like a sample diagram in the TI datasheet for the
> UC3842. The updated schematic is here:
> ator.png

The links for pic and schematic don't seem to be correct.
I picked up a modified 15K+15K schematic from the blog web page but haven't found the scope pic.

Keep in mind at a low scope timebase resolution (long timebase) (as per your previous scope pics), a brief spike on R13 may not show up in the scope display (too brief for the discrete per pixel display interval, but it could have charged C18 enough where it will linger longer and show up on the display. DSOs can present some vagaries like this. If you haven't done so, examine the traces at a higher timebase resolution (it could have been recorded in the sampling, just not being seen).

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