02 Wireless charging in household items - construction and wallplug efficiency

RE: 02 Wireless charging in household items - construction and wallplug efficiency

michaelkellett — Mon, 16 Jun 2014 07:28:43 GMT

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).

RE: 02 Wireless charging in household items - construction and wallplug efficiency

janisalnis — Sun, 15 Jun 2014 14:30:03 GMT

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.

RE: 02 Wireless charging in household items - construction and wallplug efficiency

michaelkellett — Sat, 14 Jun 2014 11:03:10 GMT

The torch doesn't look to me as if it really shuts down at all when the load is removed. The small reduction in power drain may be simply due to the removal of the charging load increasing the impedance of the coil.

If the EU regs are in force then the torch is non-compliant - is it CE marked ?

Oh, and how confident are you about the power meter accuracy at very low powers like these ?