The point of the Schottky diodes is to provide protection against power reversal occurring when the power connection is backwards — if there’s a shrouded header it’s impossible to plug the ribbon connector in the “other” way, but it can and does happen sometimes that the ribbon cable has been made wrong, or the power bus board, or the connection to the bus board. And of course if it’s an unshrouded header the connection can be backwards there.
With a 10-pin connector, if it’s backwards you have what should be this:
instead being this:
Ground is OK, but you have +12 V where you should have -12 V and vice versa. With the schematic you show the diodes block current flow in the wrong direction so you’re protected.
But with a 16 pin connector connected backwards you have what should be this:
instead being this:
You have:
- -12 V on what should be Gate — if your module doesn’t use the Gate from the header, this isn’t a problem
- Ground on what should be CV — if your module doesn’t use the CV from the header, this isn’t a problem
- Ground on what should be 5 V — not a major problem
- Ground on what should be +12 V — not a major problem
- +12 V on what should be Ground and
- +5 V on what should be Ground and
- CV on what should be Ground — in other words you’ve shorted +12 V, +5 V, and CV together, this is a problem
- Gate on what should be -12 V — could be a problem if Gate is used somewhere in your synth, it will put positive voltages on what should be a negative rail
A Schottky diode on the -12 V pin would protect against positive Gate voltages, but nothing much can save you from the fact a reversed connector will short +12 V, +5 V, and CV together. You definitely don’t want to be putting diodes on what should be the ground rail. Maybe you can use polyswitch fuses or 10Ω resistors — separate ones on each pair of ground pins, then tie them together on the other side? Doesn’t sound like a great idea to me, but maybe.
I guess if I were doing it I’d just cross my fingers, leave out the diodes, and hope I never got the power connection backwards. To protect against shorts with the connector not reversed you could use fuses or 10Ω resistors on the +12 V, +5 V, and -12 V pins, but I figure if I check for shorts before plugging it in the first time, there’s unlikely to be a short at any later time.
Another option would be to use a 10 pin header — with the power reversal diodes — and then add a +5 V regulator to your PCB design and power the digital stuff with that. That’s really what I do, but then I do it in Kosmo format which only uses 10 pin headers anyway — my synth power supply and bus boards have no provision for +5 V.
The 10 µF capacitors are for bypass and there definitely should be one to ground on each of the +12 V, +5 V, and -12 V rails.
