> >I don't like the idea of most of the HT appearing across the filament
> >transformer secondary...
<snip>
> or the bottom (B-minus). If we keep chassis potential as the voltage
> reference, the only difference in the second case is only that the B-
> minus end of the resistor spikes to a high *negative* voltage.
OK, I'm convinced. The negative side of the cap stack will go B volts
below ground potential.
> As far as I can see, there isn't any difference in the grid-cathode
> region around the tube, is there?
Agreed.
>Use a glitch resistor in B+ to also trigger an opto SCR like the H11C4.
> >They are good for 7.5 kV of isolation and don't cost much.
<snip>
> latching problem. The only thing that worries me about DIL opto-couplers
> at high voltages is the idea of several kV between pins only 0.3in apart
> - or less than 0.25in between edges of pads. Does anyone here have
> experience of such high voltages across short spans of fibreglass PC
> board? It sounds like it may be less than the UL minimum creepage
> distance, so do you have to slit the board or what?
Good point. The board could be a problem.
The only useful data I could find is in Reference Data for Engineers, by
Edward Jordan (ISBN 0-672-21563-2) 7th Ed. The table on page 5-31
suggests .2 inch per kV for 'uncoated boards' at sealevel - 10,000 ft.
If you live above 10,000 ft, it's one inch per kV :-)
There is another figure: .12 inch per kV for 'conformal coated boards' ,
all attitudes.
Jordan doesn't say what coating and what board material
(G10/G11/phenol etc).
Using the worst case first figure, the 0.3 DIL spacing would only be
good for 1.25 kV (pad clearance of .25 inch). Not good enough.
Perhaps the best option would be to slot the board on a band saw, and
drop the DIL package in the gap with flattened leads, surface mounted
(as done in UHF pre-amps with flat pack MOSFETS etc).
Optek and Bedford make LED/photo transistor pairs in a 14 mm long
wire ended tube that are rated at 10 kV breakdown (OPI1264A, Farnell
catalogue).
This might be a better choice for high side sensing, instead of the DIL
packaged H11C4 optical SCR.
Wilbert, ZL2BSJ
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