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In this blog entry I have examined the 5W wireless kit performance with changing load. Load was an adjustable 50 Ohm wire-wound resistor in series with a current meter.
5V output voltage of the board was measured with an oscilloscope. Ripple from 175 kHz switching frequency on the scope was below 100 mVpp.
Noticed that the RX 5V current is nicely hardware limited to 1 A. When the load resistor is smaller than 5 Ohm the voltage drops, but current remains constant at 1A.
At 1A the RX chip gets hot, but one can still keep finger on it. Maximum transferred power is as specified by the Würth Elektronik datasheet 5V˟1A=5W.
A very nice feature is that RX chip is automatically adjusting amount of energy taken from the receiving coil to keep the voltage regulated to exactly 5V, see the video included at the end. This allows to misalign the RX-TX coils within ca 1 cm. Within a couple of seconds the output voltage stepwise increases until it reaches 5 V. If the separation is too large then a red status LED on the TX board starts to blink.
Below is measured wall-plug efficiency with the Conrad Energy meter 3000. This is combined efficiency including also losses in the AC adapter supplied in the kit.
See that the maximum wallplug efficiency of the system is 50 % around 0.5 A load current.
When the receiver is not shut down and is just producing 5V without any load, the system consumes 1 W from the wallplug. So for real world applications is recommended to engage the “enable” pin to shut down the system once the battery of a gadget is charged full.
For curiosity I measured the efficiency of just the AC adapter by loading it with a variable load resistor. Measurements are shown in the table below. Average efficiency of the AC wallplug adaptor was around 65%.
In previous blog post there was a discussion that AC power meter might not be precise enough. So I decided to power wireless power kit from a DC lab power supply with a digital panel meter showing voltage and current.
Picture above shows a measurement when current through the load resistor was set to 0.3A. Power at the load was 5*0.3=1.5W. Consumed power was 5V*0.5A=2.5W giving efficiency around 60% (54%@200mA , 64% at 400 mA, 67% at 1A). The consumed current increased when the coils became misaligned or separated.
When no load was present the board was producing 5V and consuming 145 mA from 5V yielding 0.7 W idle power consumption.
On sleep mode (with removed RX module) the consumption was 30 mA from 5V giving 0.15 W stand-by power dissipation.
In summary, the wireless power kit should provide 5V up to 1A for powering some electronic gadget. Output looks well protected against overvoltage, but undervoltage condition can appear easily if the coils become misaligned. So one really needs a backup battery.
Next blog entry: 05 Galvanically isolated USB with 5V supplied from the wireless power kit