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?