[sdiy] Converting rack mount to DC input?

Thu Jul 9 17:56:56 CEST 2026

You could use an internal relay for the switch-over. If there is power on the DC connector -> switch over. Otherwise keep the AC rectifier connected.

Ben

> On 9 Jul 2026, at 16:06, cheater cheater <cheater00social at gmail.com> wrote:
> 
> In order to provide the bridging functionality it has to have at least 8 pins.
> 
> On Thu, Jul 9, 2026 at 1:41 PM Ben Stuyts <ben at stuyts.nl> 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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>> 
> 