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A friend mentioned that his home router had blown its 12V PSU recently and had to be replaced.

That got me wondering - Servers and routers can have isolated dual PSUs for exactly this condition, but domestic gear is un-redundent.

  • Lazy way - simply connect the positive and negative leads from two identical wall-wart PSUs together and into the same 12V input. The failure modes if one PSU fries could easily take out the second PSU, so this is of limited practical use.
  • External Cutover box - with multiple inputs it takes power from one until that fails, then a relay clicks over to use a second input.
  • Internal Cutover - same as above but integrated into the device in question

I am aware of AC multibox-like devices that have two input leads and do have a relay that takes power from a second input if the first supply goes toward 0V, but these are relatively expensive.

A UPS does not help here - that's still in series with the PSU.

Why do we not see dual ultra-low voltage PSUs for appliances? Is it purely the additional component cost?

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  • \$\begingroup\$ I'm excluding server PSUs that happen to have chunky -48V DC inputs from a telco data center environment - that's not common in most homes. \$\endgroup\$
    – Criggie
    Feb 13, 2022 at 22:12
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    \$\begingroup\$ You could design a dual power supply, with single mains cord and single DC plug out. It would just be bigger and more expensive. Besides if one supply fails, how would you know half of it is damaged and still connected to mains? What if one of the supplies has a failure mode to increase the output voltage? \$\endgroup\$
    – Justme
    Feb 13, 2022 at 22:26
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    \$\begingroup\$ We do see them (but it's not as simple as just paralleling them). The laptop I'm typing on is supplied by both a battery and an AC/DC adapter. There's all the necessary arbitration and switching on its motherboard. \$\endgroup\$
    – user16324
    Feb 13, 2022 at 22:37
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    \$\begingroup\$ ”simply connect the positive and negative leads from two wall-wart PSUs together and into the same input” This assumes that neither PSU will try to clamp the other. Could be the case, but without knowing it, it’s a recipe for 1-2 broken PSUs, complete with magic smoke. \$\endgroup\$
    – winny
    Feb 13, 2022 at 22:39

3 Answers 3

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Connecting two (or more) power supplies in parallel has been discussed numerous times here, and the answer is pretty much the same in all cases.

Unless the PSUs are designed to share a load, you'll probably end up with one PSU supplying all the current while the other one is basically off - supplying no current to the load.

Say your load needs a nominal voltage 12 V. If one PSU's set point is 12.05 V, and the other PSU's set point is 11.95 V, what's going to happen? The PSU with the higher set point is going to supply all the current, and might try to back-feed the PSU that's at the lower voltage.

Diode ORing can help with the backfeed problem, but does nothing about the fact that the two PSU's will not share the current equally.

What you need to do here is to control the two PSU's through a common control loop. This is basically what high current SMPS's (hundreds of amps) do, except that they use multiple switching power trains to provide the high current needed, with a common control loop for all the power trains.

With this type of architecture, you can have some built in fault tolerance (for example, you only need m of the available n power trains to supply the needed current), or could have a spare power train available.

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Why do we not see dual ultra-low voltage PSUs for appliances? Is it purely the additional component cost?

Yes.

And, you can achieve automatic cutover plus isolation (so one supply failure does not take out both supplies) with two diodes. The arrangement is called diode-OR.

https://en.wikipedia.org/wiki/Diode-or_circuit

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As the other answers have said, it's quite possible to create a simple dual redundant power supply using diodes.

But if a manufacturer was to do that, then

  • The power supply would be more expensive
  • The power supply would be bigger
  • The energy efficiency would be worse, with one supply doing nothing most of the time

For domestic electronics that are built to a price, it's not really justifiable.

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