DIY "Redundant" DC power supply?

[Dave Corder]

Hey all,

Has anyone ever used a DC automatic transfer switch, like this or this, to provide some level of redundancy to something that has only a single DC input (like a low-end/SOHO switch or router that doesn't have dual power supplies)?

For some background,

I have a small wall-mount rack for my network core, consisting of:

* Dell R210ii running OPNSense
* Brocade ICX6610-48p w/ dual PSUs (most of the network)
* Hasivo 5-port 2.5Gbps RJ45 PoE + 2x10Gbps SFP+ switch (2.5Gbps Wifi6 APs)
* Comcast XB7 gateway in bridge mode

These are all plugged into a 8-port PDU (it was an APC9211, but I replaced its guts with a modern ESP32 microcontroller + an 8-way relay board, running ESPHome), which is in turn plugged into a rackmount APC UPS.

Never one to leave a working setup along, I was thinking about the PDU situation earlier. If I ever want to update the firmware on the ESP32 in the PDU, that will cause a reset, which will cause the relays to flap and all the devices to reset (and my internet connection to drop for a few minutes).

With the Brocade, this is easy to avoid - plug each PSU into a different UPS/PDU. For my OPNSense box, that's fairly straightforward as well - I can upgrade it to something that has dual PSUs as well. For the Comcast XB7 and Hasivo, though, they each have only a single DC input. If I used two DC adapters for each, wired through one of those switches I linked above, that would in theory allow me to connect each one to two different PDUs and achieve some level of redundancy and allow me to power cycle a PDU without breaking my internet connection (even if they do ultimately come from the same AC source).

Seems like this would work in theory...just wondering if anyone has tried anything like this before, and how it went.

 

[TheGuyDanish]

The way I see it, the simplest approach would probably be to get something like a Mean Well ERDN20-12 (assuming your DC stuff is all 12v and your load is at most 20A) since that could take care of two inputs and a single output (with some signalling for monitoring)

The other approach is to decouple the ESP32 from the actual control of the relays. You could introduce an SR (set-reset) latch between the ESP32 and the relay. That way, so long as power to the SR latch isn't interrupted, the ESP32 can reset all it wants without the equipment going down. So long as it doesn't put the reset line high. Two TI SN74LS279A's could be a good choice for something like that.

If the goal is a compact device, I think I favor the second option more, myself, since it's really minimal changes to get to your goal. But if I were to make something from scratch for myself. A 1U project box, two nice, high quality power supplies, a fan or two and the ERDN20 could be a very sexy solution.

 

[Dave Corder]

The way I see it, the simplest approach would probably be to get something like a Mean Well ERDN20-12 (assuming your DC stuff is all 12v and your load is at most 20A) since that could take care of two inputs and a single output (with some signalling for monitoring)

This item I linked to (https://www.amazon.com/Automatic-Tranfer-Switch-Without-Transfer/dp/B0CB7DFFKP) would appear to be functionally the same as the ERDN20-12. But it's basically the same price as the ERDN20, and I'd trust something from Mean Well more than something from some random seller on Amazon/AliExpress...

Unfortunately, not all of my stuff is 12v DC - the Xfinity XB7 is, but the Hasivo switch is 52V DC

The other approach is to decouple the ESP32 from the actual control of the relays. You could introduce an SR (set-reset) latch between the ESP32 and the relay. That way, so long as power to the SR latch isn't interrupted, the ESP32 can reset all it wants without the equipment going down. So long as it doesn't put the reset line high. Two TI SN74LS279A's could be a good choice for something like that.

That seems like a good idea, and something I've already considered, but unfortunately I am currently using an all-in-one board that ticks all my boxes (Kincony KC868-A8, which has Ethernet, as one of my other requirements is for this setup to not rely on WiFi), and when I last looked I couldn't find any similar boards that also had latching capabilities, and I didn't want to do a fully DIY setup (I'm open to suggestions, though, if someone knows of anything like the KC868-A8 but with latching abilities).

If the goal is a compact device, I think I favor the second option more, myself, since it's really minimal changes to get to your goal. But if I were to make something from scratch for myself. A 1U project box, two nice, high quality power supplies, a fan or two and the ERDN20 could be a very sexy solution.

My main goal is to avoid issues if the ESP32 has to reset, so If I could find a suitable board for the second option, I agree, that would definitely be easier, but also just having redundant power is a nice-to-have as well, so I'm leaning towards the first option.

Appreciate the input!

 

[Mithril]

The absolutely lowest tech way would be a "diode OR" with 2 low drop diodes (one for each power supply) rated for at least 2x the expected amps (safety margin). They will dissipate v_drop times amps in watts. You could also look for "ideal diode" modules or make one yourself, essentially it's 2 mosfets back to back (or a single one used directionally using the internal diode to block reverse current) with an op-amp or similar that compares the voltage drop across and enables or disables the mosfets accordingly. MUCH lower drop, however more expensive and either a DIY or buy a module thing (should be ~5 bucks for something rated up to 30A). Since the v_drop is much lower theres less power lost and less heat to dissipate.

There are also "UPS" mosfet OR circuits, with a similar control over which mosfet pair is on usually with one input being the primary.

I've done the diode OR for lower power things 1a (12w) or less is easy, get a Schottky diode as they tend to drop .4v or less. You may only need to put that on your backup DC as well, depending.

I've also seen people going analog where they have a diode or diode OR (diode on each) and use a relay to bridge out whichever is primary. the diode means you never lose power entirely and in theory reduces internal arcing since the current still has a path.