[sdiy] Converting rack mount to DC input?

Thu Jul 9 16:08:04 CEST 2026

On Thu, Jul 9, 2026 at 2:10 PM S Ridley via Synth-diy
<synth-diy at synth-diy.org> wrote:
>
>
> Molex Mini-Fit Jr?
>

Isn't that the same connector family the self-combusting 12V HPWR
connector comes from? I.e. smaller pins than the pcie 8-pin?

>
> On Thu, 9 Jul 2026 at 12:43, Ben Stuyts via Synth-diy <synth-diy at synth-diy.org> wrote:
>>
>> Perhaps a 4 or 5 pin XLR connector is an option? They are not that expensive, and lots of ready-made cables are available.
>>
>> Ben
>>
>> > On 9 Jul 2026, at 10:18, cheater cheater via Synth-diy <synth-diy at synth-diy.org> wrote:
>> >
>> > One way to have DC input, but retain the capability of using the
>> > internal power supply, is to have a 4P2T switch for configuration.
>> >
>> > It seems that 4P2T switches are super expensive, especially if they're
>> > supposed to withstand, say, 1A.
>> >
>> > So I started looking around. TLDR: classic pcie 8-pin power connectors
>> > are probably the best, and also the cheapest.
>> >
>> > My first thought was to look at PCIE x1 slots. The cheapest advanced
>> > connector on earth, for 0.4 Euro at unit price, you get a connector
>> > that handles 1.1A per pin as per eg this document, page 6, 4.4
>> >
>> > https://cdn.amphenol-cs.com/media/wysiwyg/files/documentation/gs-12-233.pdf
>> >
>> >> 4.4 CONTACT CURRENT RATING
>> >> 1.1 amp per contact minimum per EIA-364—70, method 2 and PCI Express Connector High Speed Electrical Test Procedure. The temperature rise shall not exceed 30 degree C. Ambient condition is still air at 25°C.
>> >
>> > The connector has 36 pins, so while I don't think it would handle 36A,
>> > I'm sure it would handle something like 2A, especially if you share
>> > pins. You can even leave pins empty to prevent shorts during
>> > insertion.
>> >
>> > You could put the connector out the back, and either insert a plug-in
>> > "card" (really just a small edge connector with wires soldered on)
>> > that provides DC, or a pass-through "card" that shorts some pins
>> > together to carry power from the internal power supply. And now
>> > instead of $20-30 per unit, this costs $1 per unit. My main question
>> > is how I would fix the connector, but maybe a simple screw hole in the
>> > connector that mates with a threaded hole in the case could do the
>> > trick.
>> >
>> > A pcie x1 port is 25mm long, so it can fit upright in the back of a 1U
>> > rack unit, which is 45mm, so it doesn't take up much space either.
>> >
>> > The cheapest one that can be found at Mouser currently and can be
>> > bought in low volumes is roughly 0.4 Euro.
>> >
>> > https://www.mouser.at/ProductDetail/Amphenol-FCI/10018783-10200TLF?qs=V%252BXmToedwojeZUI4fPwmPA%3D%3D
>> >
>> > Qty.  Unit Price  Ext. Price
>> > 1     € 0,439     € 0,44
>> > 10    € 0,372     € 3,72
>> > 25    € 0,332     € 8,30
>> > 100   € 0,316     € 31,60
>> >
>> > By making the connector require a dummy plug to connect the internal
>> > power supply into the circuit, it makes it impossible to connect both
>> > DC power and mains AC, so that makes the design intrinsically safe
>> > without using switches.
>> >
>> > Alternatively to a PCIE connector I could use some panel mount plug
>> > with 8 pins, have 4 pins for DC input, and have the other 4 pins carry
>> > power from the internal power supply, and similarly use a shorting
>> > plug to use the internal power supply.
>> >
>> > Circular DIN connectors start at about 8 Euro per pair of socket and plug.
>> > Circular metric aren't better.
>> > MIL Spec connectors seem to be cheaper. But the mouser search sucks,
>> > so I can't find them by number of pins.
>> >
>> > PCIE 8-pin power connectors are real, real cheap - 10 cents a piece.
>> > They're latching (no need for screws and stuff). There doesn't seem to
>> > be a panel mount version, but one could mount a through-hole connector
>> > to a pcb, and have the pcb have screw holes for mounting to the rear
>> > panel. They handle high power - 150W meaning 3A per pin (half the pins
>> > are return pins). Seems like a winner to me. They're also tiny so they
>> > will easily fit in the back of a 1U unit even vertically. The biggest
>> > pain here might be making the rectangular hole and having it look any
>> > good. might be a case for custom die, maybe a small steel job that
>> > uses two bolts to screw together the two cutting parts through the
>> > sheet metal.
>> >
>> > "Pin and socket" connectors are the same kind of thing, just not
>> > specifically PCIE 8-pin. Still cheap at roughly 20 cents a piece...
>> > useful alternative if more than 4 rails are necessary.
>> >
>> > D-sub connectors carry up to 3A... and are very cheap and can do panel
>> > mount of some sort. But I'll be damned if I use one of those cursed
>> > things for power.
>> >
>> > Automotive connectors seem inexpensive too, but I haven't really
>> > looked into them much other than a quick parametric search.
>> >
>> > I wonder what everyone thinks of this.
>> >
>> > On Tue, Jul 7, 2026 at 6:39 AM cheater cheater
>> > <cheater00social at gmail.com> wrote:
>> >>
>> >> Don't you think this may have had something to do with you
>> >> distributing AC power, rather than DC with 0V potential?
>> >>
>> >> On Sat, Jul 4, 2026 at 11:33 PM Mattias Rickardsson <mr at analogue.org> wrote:
>> >>>
>> >>> Den lör 4 juli 2026 14:23Florian Anwander via Synth-diy <synth-diy at synth-diy.org> skrev:
>> >>>>
>> >>>> Maybe, it's a stupid thought, but...: could it be that a device relies on beeing galvanical separated from other devices. I this case the common supply might clash with the devices concept.
>> >>>
>> >>>
>> >>> This reminds me of a somewhat similar issue I had many years ago:
>> >>>
>> >>> I wanted a Korg MS-20 and a Korg KR-55 to share one external power puck. (Swedish-sold old Korgs had a big external power transformer to step down from 220 V AC to 16 V AC, which is then what the instrument then accepts via a plug with 2 flat prongs that doesn't seem roadworthy with European electrical standards.)
>> >>>
>> >>> So I made a short splitter Y cable on the 16 V AC side and attached both instruments and turned them on. But then I connected an audio cable between them (or audio cables from both units to the same mixer) and I had a blown fuse! It was apparently not okay to connect those two grounds together when running off the same AC supply.
>> >>>
>> >>> /mr
>> >>>
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>>
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