I have been designing a project that requires a 5V for some components and 12V for others. I am planning to leave this plugged in 24/7 so I want it pretty efficient. I know they don't waste that much energy, but I still want to make it as energy efficient as possible even if I don't notice a difference in my energy bill.

[A.K.A. This is for a friend that read somewhere wall warts waste tons of energy, but if it makes them happy, I might as well do it. I have seen this: How much power is really wasted by a wall wart? but my friend isn't happy with that. I just want a power efficient source. Also, this isn't related to How can I reduce power consumption of my device? because that is saving energy in theory.]

I have a couple of options to use:

  • Buy a 5V and a 12V adapter. Connect the grounds.
  • Buy a 12V adapter and use a voltage regulator and a cap.

Which one is better? Do some adapters/adapter types waste less energy?

Another problem is I might need 1/2A for the 12V circuit which is only going to be used 5% of the time. Does the higher amp converters use more energy when idle? Suggestions? I don't want to use relays because they would be dangerous. I would need less than .5A for the 5V one. Do I need to pick out adapters?


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    \$\begingroup\$ I'm not sure how we can help here. It's your friend that needs convincing and we can't fight his opinion with reason unless we're sure that it was formed from reason. The best you can do is to try to compare the wasted power to engineering expenses that would be caused by deeper research into power conservation field and price of components. What's the use of complex power supply that will not repay itself within its projected lifetime by saving power? Also much will depend on exact unit, so it may be difficult to provide exact answer. \$\endgroup\$ – AndrejaKo Mar 11 '13 at 19:50
  • \$\begingroup\$ Tell them to buy a Kill a Watt and measure them. Then he can have some better idea of what's going on. \$\endgroup\$ – Gustavo Litovsky Mar 11 '13 at 20:15
  • \$\begingroup\$ If you're really concerned with standby-state power consumption, either unplug the device when not using it or incorporate the PSU into your product with a physical on/off switch inline. Standby power = 0. Failing these options, I would follow AndrejaKo's recommendations: is it really worth the effort/cost to save potentially less than a watt or so? \$\endgroup\$ – helloworld922 Mar 11 '13 at 20:21
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    \$\begingroup\$ @GustavoLitovsky I doubt a Kill-A-Watt is useful at adapter power levels. The specs show a high max current error of 1%, and there's no auto-ranging. It's still a great gadget for higher power levels. \$\endgroup\$ – Adam Lawrence Mar 11 '13 at 20:38

I would go with a single mains-to-12V adapater, preferably one that's a switching model.

The switcher will waste less energy generating the +12V rail than the linear, and can regulate its output without dummy loads / burning power unnecessarily.

For the 5V rail, consider building a 12V-to-5V buck converter if the load current justifies it.

("Less than 0.5A" makes the decision less than clear - how much less than 0.5A? If it's 100 microamps, use a linear regulator. If it's 0.49A, you're probably better off with a buck.)


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