[mythtv-users] Desk Top Power

[Simon Hobson]

Michael Watson <michael at thewatsonfamily.id.au> wrote:

> Why build it, when you can find them at around the same cost of a suitably rated relay.
> When you cant buy a device to do what you want (at reasonable prices), then you look at building your own....

You ask this on the mailing list of a project which is all about making something that (to a large extent) you can buy off the shelf ! Sometimes it's not about the end result, part of the enjoyment is the journey there.



Daryl McDonald <darylangela at gmail.com> wrote:

> The relay idea interests me IIUC the power of the PC holds the relay in the closed position feeding a power bar with my wallwarts plugged in, and opens when the PC shuts down?

Exactly



jrh <jharbestonus at gmail.com> wrote:

> Regarding power imbalance or however it is called, is this still an issue for power supplies built today?

Yes I would say for a typical PC PSU.
The multiple outputs typically come from multiple secondary windings and rectifiers from one transformer. The feedback control operates on the primary output - it's only possible to accurately regulate one output without secondary regulators. The other rails will more or less follow the main one - the relationship between output voltages depends on the ratio of the number of turns on the transformer, and the voltage drop in the rectifier diodes and windings.
If you keep the load on the primary output constant, then varying the load on one of the secondary outputs will vary the voltage on that output - increase load and the voltage will droop, reduce it and the voltage will rise.

> This is interesting to me, and could be an exercise in supplying whole house 5v/12v outlets!(Looking at all the wall warts I have in this room that are either 5 or 12 volts).

As others have said, a "whole house" low voltage DC system would be impractical - you'd either use a lot of copper or have too much volt drop. Best you could do would be to use an intermediate voltage (eg 48V as used ina  lot of telecoms and data centre equipment) and regulate it down with DC-DC converters. But those DC-DC converters won't be much different in efficiency to a wall wart.

But for a "whole corner of the room" system, it's worth considering. Where our local LUG meets, they used to share with a makerspace. They had a big "power anything" universal USB supply sat in the middle of the bench. Apparently it's "interesting" getting the right gubbins in to make all devices believe it's a high power supply - Apple do one thing, Samsung do something else, and so on - and if you don't get it right then some devices will just assume it's a low power socket and not fast charge.