KSS wrote: ↑
Wed Feb 12, 2020 7:14 pm
This does not make any sense. At least add a fourth pad in the middle, and based on the layouts you've shown every single position can accomodate the 6 hole UNIVERSAL layout shared. Three hole layouts are the worst, as they severely limit the available options. Even adding only the fourth center hole expands the potential candidates greatly.
I have added a fourth pad to each type to make them more suitable for each person's preference.
Why does this matter? Because by adding pads to the layouts you're insulating against shortages and part changes for both present and future. We've all seen popular parts undergo short supply due to sudden popularity and we've also seen even long time available parts simply disappear. And where's the rub?? There is almost NO down side to implementing these additional holes. Perhaps a few cents more per PCB, and literally only a few cents IME. Aside from that, it's ALL good.
Next, do you really think multi-turn pots are necessary for setting gate level? Unless high quality multi-turns are provided they will have more backlash and take a long time to set. With no real gain to offset these two downsides.
I think you've mis-read or misinterpreted my post - "I have stuck to my intention and added six upright side-adjust single-turn trimmers type 3362M
And to again re-iterate, the six hole pattern used with bourns upright multi-turns and bourns flat leave the adjusting hole/point in the same position.
I trust 4-hole is sufficiently universal. To add another row would mean stretching the part outline to avoid screen-printing over some of the pads. It's already quite crowded in many parts of the board.
Finally, it's generally bad practice to use only two pins of a trim. Connecting the third pad ensures there will always be at least some resistance at that point in the schematic. It can't go open circuit. Without this , it can. And with the ever poorer qualkity and specs of todays trimpots, this open circuit result is to be expected over a trims life. So this becomes a support issue too. Because it will usually go intermittent first.
Any pad that appears not to be connected is either connected to ground or +15V (just like every other component that appear not to show tracks on all pads), it's just that it doesn't show the two inner planes on the four-layer board in the design view - check out Q31, Q36, SK11 and C7 in the second shot below. Some ICs obviously have unused pins but everything else is connected to something.
I appreciate your having taken a look at what I wrote before. Hopefully these further comments will help you to see the value in implementing more pads in each trim position. And connecting the 'loose end' traces.
Thank you for your critical review. I trust all the above is now satisfactory.
Here are updated views of the two types:
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