Sunday, February 22, 2009

No Vampire Cellphone Charger

Ok, this one's too easy, so I suspect it's already being done; or else there is some market impediment that I'm overlooking. Please discuss in the comments if you are more savvy than me.

Cell phones chargers, like most chargers that draw from a 120-Volt wall plug, require a boxy-looking transformer to step the voltage down to a digestible level before feeding it to the device. Unfortunately, these transformers induce current and use power even if the device is not attached to the charger! So when you leave your phone charger or laptop charger in the wall, it draws a steady 1 or 2 Watts ALL THE TIME! Popular names for this are vampire power, standby power, or phantom power.

The idea here is to put a simple, mechanical switch where the phone plugs onto the charger. When the phone is attached, it closes the switch. Power from the wall can reach the transformer, be stepped down, and then charge the phone. When the phone is removed, the "secondary" coil is of course opened, but now the switch opens the "primary" coil as well; meaning that the wall can no longer feed the transformer and waste power.

A single cellphone charger could save around 10 kWh per year this way. With millions of users, this is a high potential for energy efficiency. Added manufacturing costs would be the added switch and length of wire. The same function could also be accomplished with a smart charging chip to sense active vs. vampire power levels. One would have to analyze which is more cost effective: a mechanical or electronic shutoff.

CATEGORY: Energy / Technology
IDEATION: February 16, 2009.


  1. I think the issue is cost versus reward. My last electric bill, I paid $0.115 per kWh... so getting the better charger might only save me a buck over a year. Implementing the change to the charger in a reliable manner would probably cost less than $1 per unit to manufacture... but most people wouldn't differentiate at this point. It sucks, but until legislation comes out for this sort of thing or energy costs go way up, there's not much incentive. For devices that are more expensive, though, I could see something like this happening as a green marketing effort.

    Thanks for doing this, Dave! I love seeing all of these!

  2. I was just looking at this again today - I bet you could do this with a capacitor and a relay. The DC current to the phone could trigger the relay (or diode, depending on the current needs), so that when you unplugged your phone, the relay cuts off the main power. The capacitor would store power waiting for the phone to be connected again, at which point the relay would switch back on. Hmm...

  3. Jon - that sounds brilliante! I'm not super familiar with diodes and relays, but yeah, if you can do it without moving parts, that would increase reliability and decrease cost. nice!

  4. Hi! Do you know any company that manufactures/sells transformers used in cellphone chargers?

    I'd appreciate any information you can provide.

    Thank you so much!

  5. Hello Anonymous - sorry, i don't personally know of anybody, but if any commenters from the ole' internet want to weigh in - i'll leave the thread open on the blog. - Dave

  6. the relay cuts off the main power. The capacitor would store power waiting for the phone to be connected again, at which point the relay would switch back on. Hmm...

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  7. AT&T has just come out with a "no vampire power" cell phone charger: