Is it possible to detect when a device is consuming power in a stand-by or nominally off state? For example if I leave my charger plugged in, detect that a small amount of current is being drawn and stop the flow. What about turning it back on? For example, plugging in my phone it could detect the demand and turn the flow back on.

How could you create a device which could turn an outlet on or off depending on the load?

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    \$\begingroup\$ I think you might be confused as to what phantom power is. en.wikipedia.org/wiki/Phantom_power \$\endgroup\$ – Kellenjb Apr 26 '11 at 19:58
  • \$\begingroup\$ I am referring to the current drawn by devices when switched off. Phantom power, stand-by power whichever term you prefer. But in the context of electrical conservation, not microphones. \$\endgroup\$ – aaronfarr Apr 26 '11 at 20:01
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    \$\begingroup\$ Slightly off topic to the actual question, but based off of the numbers provided by Tricklestar, it would take about 1,800 years for the power strip to pay for itself (not accounting for inflation and what not) \$\endgroup\$ – Kellenjb Apr 26 '11 at 21:00
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    \$\begingroup\$ Boy the more I read about those power strips the more I think its just a few companies that are trying to make money and not real science. The wikipedia page is full of sales jargon but not a single bit of science. It even says " Video games often use power when they are turned off, but the standby power can be further reduced if the correct options are set." I am sure they mean video game consoles, but its very clearly not what they said. \$\endgroup\$ – Kellenjb Apr 26 '11 at 21:06
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    \$\begingroup\$ I think the term you were looking for was vampire power - please edit, very confusing for people for whom phantom power has a distinct meaning. \$\endgroup\$ – overslacked Apr 26 '11 at 22:13

I think you're laboring under a false assumption. Where there are power strips that turn outlets off to save power based on a 'control' outlet, the 'control' outlet is always live. It will always be consuming its 'phantom power'/'vampire power' (cue the Twilight fangirls!). All of the outlets that are slaved to the 'control' outlet will be off until the 'control' outlet has enough current flowing through it to trip the circuit. As others have suggested, this is the only reasonable way to do it - you can't be continually switching something on and off just to test whether it's on.

I have such a power strip for my home entertainment system - older solutions were to run power through your receiver via a built-in pass through. When the receiver is off, the passthrough is off, when on, it's on. I can't say for certain whether it's saving me any money but it definitely means fewer absurd 'off' lights on the electronics (seriously, a light that's on when the device is off? Crazy).

If you want to measure this 'phantom'/'vampire' power, get a Kill-a-watt. It measures power draw of whatever is plugged into it. I haven't used one yet but many people rave about them, and Lady Ada hacked one to put an Xbee unit in it for datalogging purposes, so they're at least popular.


You wouldn't be able to totally isolate the flow, because once you did that, there'd be nothing to detect: plugging something in to an 'off' power bar doesn't lead to any power flow that could be detected.

There'd have to be some sort of cycle-on, cycle-off mode of operation - turn the AC on, measure the power, make a decision to stay on or shut off, and repeat.

Measurement is tricky as well - you may well need instrumentation-grade current metering to detect the low current flow, which can also withstand the 15A of a usual circuit without frying.

Some components may draw larger amounts of initial power to get going, before they settle into a quiescent state. Many components hiccup on and off by themselves. The detector will have to deal with all of these variables as part of its logic.

Also, the control circuitry (and AC controls) will themselves consume power. You have to figure if what you can potentially save is worth the power budget needed to save it.

Finally, whatever you have plugged into this power bar will need to be able to withstand numerous perpetual turn-on and turn-off cycles. This is quite stressful for power supplies in general (due to inrush current).

  • \$\begingroup\$ Thanks for your response. In power bars which have a "control" outlet, how does it know when the power it turned on for that outlet? Specially what kinds of components? \$\endgroup\$ – aaronfarr Apr 29 '11 at 20:10

Use a Kill-a-Watt to measure it.


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