Wilbert Knol wrote:
> >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
There is a common misconception that air has good dielectric
(insulation) properties.
For example, the PTFE glove our wives slip over the iron is good for a
few kV provided it is not punctured, although it is only several thou
thick. Air cannot do this.
Boards with suitable dielectics easily outperform air gaps cut into
boards - provided that all surface contaminats are properly removed at
installation, and subsequently during maintenance.
Ian ZS6BTE
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