5 Apr
2020
5 Apr
'20
4:12 p.m.
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:
https://rjarratt.files.wordpress.com/2020/04/h7270-control-pulse-width-modul
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:
https://rjarratt.files.wordpress.com/2020/04/h7270-control-pulse-width-modul
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).