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  • Author Author: janisalnis
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02 Wireless charging in household items - construction and wallplug efficiency

janisalnis

Previous blog entry: 01 First experiences and plans for Wireless Power Challenge

Next blog entry: 03 Searching for Electro-Magnetic Interference (EMI)

 

Wireless charging with inductive coupling has been around in our homes some time.

One application is linked to medical equipment. Electrical toothbrushes remove plaque more efficiently than conventional ones and are intended to be rinsed with water or disinfected. Electrical toothbrush tipically contains a 1.2V NiCd and is charged by putting it into a stand containing wireless charger. The chargers of the toothbrushes are also hermetically sealed. Cost is from 24 EUR and upwards.


Philips toothbrush

image

  • It is from year 2000.
  • On the transmitter indicated power consumption 3W.
  • Oscilloscope picks up quite nice sine wave with T= 19 us, frequency 52 kHz.
  • No automatic transmitter shutdown. Transmitter continues to oscillate also when toothbrush is removed.

 

Braun Oral-B toothbrush

image

  • From year 2010.
  • Wireless charging of 1.2 V NiCd. One can find youtube videos how to replace the NiCd.
  • Oscilloscope picks up quite nice sine wave with T=30 us, frequency=33 kHz.
  • No automatic transmitter shutdown. Transmitter continues to oscillate also when toothbrush is removed.

 

 

Pocket Flashlight Torch

image

Another application for wireless charging is a pocket torch. They are used in garages, basements and construction sites where moisture and dirt could be a problem and wireless charging prevents from contact oxidation and allows to hermetically seal the torch. It can be easily grabbed and put back in holder while being in the darkness.

 

  • Bought in Swedish home-repair shop for 12 EUR.
  • 9 white LEDs with lenses. One dim blue LED indicates charging.
  • Three AAA rechargeable batteries. Specified 440 min burn time at 13 lm.
  • Automatic transmitter shutdown  when flashlight is removed. Automatic charging start when torch inserted. Coil is wound by normal magnet wire, many turns, without magnetic shielding. Concentric with recever coil. Frequency 27 kHz. Can be seen on oscilloscope if the probe is near the coil.

 

 

Curiosity was driving me to examine it inside.

image

Construction is held together with screws and hermetic seals made with rubber rings. Transmitter coil is driven in transformer-less circuit from 220 V. On the board is rectifier, 10uFx250V and a few other components. Coil has only 2 wires and is driven by something looking like a single transistor. A42 B331 written on it. Searched but could not find a datasheet. Probably it is something more smart. Because circuit has an automatic shut-down when flashlight is removed.

 

 

Samsung Galaxy S 4 Qi Wireless Charging Pad

It is the most recent application up to date. One can find for reviews in the Internet. Transformation is done by replacing the original Samsung battery and cover. It is the same standard as used in Texas Instruments wireless power kit that will be used in the upcoming blog posts.

 

 

 

Examining Wireless charging wallplug efficiency

 

Let’s examine wallplug power efficiency with a Voltcraft Energy Monitor 3000.

image

We will measure active power dissipated. Device displays also reactive power that is tipically about two times higher, but we are not paying for it.

 

Philips toothbrush

  • idle without toothbrush   2W
  • while charging also   2W

 

Braun toothbrush

  • idle, toothbrush removed   0.7W
  • when charging   0.7W


ClasOhlson Torch

  • idle, torch removed   0.9W
  • when charging   1.1W

 

Texas Instruments QI wireless power kit

  • wallplug power used without receiver board    0.0W
  • with Würth Electronics LED load board    2.0W

 

Excellent! Shows that TI Qi automatic shutdown is working nicely.

In summary, only the TI Qi meets the EU regulation stating that standby consumption should be <0.5W.

 

 

Next blog entry: 03 Searching for Electro-Magnetic Interference (EMI)



  • Any active device will respond quite differently to a change in AC voltage or being powered by DC, devices with power factor correction will respond very differently to AC or DC supplies. The only (practical) way to measure these things accurately is to use synchronous ADCs measuring current and voltage and then to process the data. You can use a cheap micro with a 12 bit ADC (ST Discovery board with STM32F4xx series micro would do well) but you need to be very careful about the interface to the mains. (16 bit ADCs would be better but are likley to cost more and add to the complexity).

     

    MK

  • It is a nice surprise for me how much activity the post has triggered. I am not specialist in the field of power measurements and discussion is very inspiring.

     

    Thanks to Michael pointing out that 1 mA at 230 V corresponds to 0.23 W. So digitalization of current sets the power sensitivity limit.

    This allows to think about some reverse engineering. 1 mA to max 13 A gives 13000 steps corresponding to 14 bits of ADC resolution.

     

    Actually switching power supplies have a lot of reactive current consumption. Total [VA] value is almost 2 times more than active [W]. Current resolution 1mA is for the total current.  0.23 W would be a limit for the total power, but for active power resolution would be ca half of it. That close to 0.1W.

     

    I also have an old style  variable transformer with a sliding contact allowing to change AC voltage between 0... 250V.

    With decreasing voltage the current will increase and this should allow to average out the current digitalization quantization step limitation.

     

    Second approach would be to use 200V DC power supply (as all switching power supplies have a 230V rectifier and a filter capacitor) and precisely measure the current consumption with a multimeter.

     

    As a third approach I have an old electromechanical power meter. Could try measure time it takes to complete n-circles.

  • in reply to mcb1

    The proper analysers multiply current and voltage in real time and also work out phase angle and thus power factor - of course the steady state current isn't the whole story but the EUP directive (Energy Using Products) is concerned with steady state power in idle and active operation.

    The PA1000 gives you harmonics as well which is essential for CE or similar approval.

     

    Probably much safer to use a current sensor (rather than a sense resistor) for experimental work (no need to break the mains wiring).

     

    MK

  • in reply to michaelkellett

    But doesn't tell you as much

    Well it means you have to do some calculations, but there is no reason why you can't read peak (assuming your meter allows it).

    Thus you have average current, peak current, and if you use an oscilloscope, you could even get the current in the first cycle.

     

    We have a similar device at work to measure the instantaneous currents for various equipment.

    Some are extremely high, in the order of 40Amps or more ...hence we don't want more than one on a single breaker.

     

    That's why we know these devices are not very accurate.

     

    Mark

  • in reply to mcb1

    But doesn't tell you as much image

     

    But for a quick garbage/not_garbage test quite good enough I would think.

     

    MK