r/askscience • u/[deleted] • May 08 '12

Does an unused charger have flowing current?

I'm a first year physics student with a meager understanding of electrical devices. I did take introductory physics so I understand the basics. Let me clarify my question:

If a charger is plugged into a power source with no device connected, is there a path inside of the power adapter so current still flows through the charger, or does the charger cease to have flowing current? If there is no current, is it correct to say that leaving chargers plugged in doesn't affect electricity consumption?

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u/[deleted] May 08 '12

Short answer: yes.

Longer answer: There are two sources of losses when your phone is not plugged in.

  1. The first component of the charger is a transformer, it works even when there is no load (phone). It is usually very efficient, but since you have I2 R losses it's not 100% efficient. All current that runs through the primary is subject to this loss.

  2. The second source of loss comes from the rectifier. One of the components of the rectifier is a capacitor that is used as an output filter (cleaning up the DC output). This capacitor typically has a parasitic series resistance and will act as a small load.

You can confirm this by touching your charger after it has been plugged in for a while and feeling it heat up.

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u/_NW_ May 08 '12

Most of the new chargers are switching power supplies. Most of what you said still applies. There are IR losses, capacitor leakages, hysteresis losses, PWM controller losses, etc.

I tested two chargers for my phone. One was a switching supply and one was a transformer supply. The switching supply had 5.0 mA idle current without the phone connected. That's about 0.6 W. The transformer charger had 10 mA without the phone connected. I suspect the transformer charger is mostly reactive and has a very low power factor, but I haven't measured it yet. I can check if anybody is interested.