When many electronic devices sleep, the current is very small. However, once they are activated or send signals, the current consumed will jump several orders of magnitude. The application case shared this time is to measure the current of Amazon echo, an Amazon smart speaker, under different working conditions. The next test method can be applied to almost all mobile devices or battery powered IOT devices.
Precision resistance measurement
If the current of electronic equipment varies greatly in different working modes and changes rapidly, measuring them will be a great challenge. Usually, these devices only consume microamperes of current in the sleep state, and then suddenly switch to the working state, they will consume milliamperes or even amperes of current. Because the sleep current itself is very small and the range of switching mode is very large, it is not suitable to use DC current clamp to measure. The more common method is to measure the voltage drop at both ends of a small precision resistor.
On the one hand, it should be large enough to ensure that the voltage drop can be effectively measured; on the other hand, it should not be too large to cause the power supply voltage of the equipment to drop too much, which will affect the normal work.
In this case, the Amazon echo smart speaker is powered by 15V, with a rated current of 1.5A. According to the rule of thumb, when the current is maximum, the voltage drop does not exceed 1% of the supply voltage. In this example, we insert a 0.11 Ω resistor into the echo power supply line, and the 1.5A current has a voltage drop of 165mv. It should not affect the work of the equipment, and we do not have to worry about this precision The power consumption of the close resistor is too high and the resistor is heated. Theoretically, smaller resistance can be used for precision measurement, but if possible, larger resistance can reduce the noise interference, and the signal-to-noise ratio of measurement results is higher.
At the same time, we measure the waveform of the supply voltage to calculate the power. The key point is to measure the voltage at the end closest to the electrical equipment, so as to eliminate the influence of the insertion resistance and get the real power consumption of the equipment.
Unit conversion and resolution
With PicoScope’s “custom probe” function, the voltage at both ends of the insertion resistor can be recalibrated to the current value. Then a simple formula P = V * I is used to get the power waveform in watts.
It is necessary to consider the resolution of the oscilloscope again. The maximum current we may measure is 1.5A, and the corresponding voltage is + 165mv. Basically, the better range is ± 200mV, and the corresponding measurable current range is ± 1.82A.
Tip: in this case, the maximum voltage value we measured is 165mv, while the range is ± 200mV, that is to say, we only use less than half of the range, which is obviously a waste. A better way is to set the measuring range to ± 100mV, and then add a bias of 100mV to get a measuring range of 0-200mv, which can further improve the resolution of the result, that is, 0.11ma (which is roughly equivalent to the accuracy of four and a half digits of a digital multimeter)
If a higher resolution is needed, the resolution enhancement mode can be turned on, which will bring a certain loss of sampling rate. Of course, if the maximum current is less than 1.5A in other test scenarios, a smaller range can be set accordingly, and the minimum range can reach ± 10mV, so that the microampere level current can be measured.
Figure 2 rescaling the pressure drop signal using PicoScope’s custom probe function
Differential measurement and fast sampling
The position of the precision resistor can be determined according to the test requirements, to see whether it is necessary to measure the current of the whole system or a part of it, such as the processor or WiFi module. Usually, a tested object has several test points to choose from. In this application case, the effect of PicoScope 4444’s differential input channel is very ideal, because each channel can be connected to both ends of a precision resistor, and the positive and negative poles can be arbitrarily connected to the side of the power supply or the side of the power circuit, so there is no need to worry about the short circuit or the formation of a ground ring. These mistakes are easy to happen when using the traditional desktop oscilloscope.
The current consumed by the device, such as the current consumed by the microcontroller or memory, may change rapidly at the rate of microseconds. In order to catch these fast changing values, the test instrument needs to have enough bandwidth and sampling rate. Picscope 4444 has a bandwidth of 20MHz and a sampling rate of 400ms / s. It is an ideal choice for measuring fast changing current.
Tip: when you try to measure the fast changing current waveform of the processor, memory or WiFi module, you can temporarily remove the decoupling capacitor, so that you can see the current pulse peak and measure the rise time.
Figure 3 DC and ripple of transformer output
Power supply of Amazon echo smart speaker
Before measuring the power consumption of echo in different modes, let’s take a quick look at the output waveform of switching power supply when echo is not connected. We use channel a to measure the supply voltage, DC coupling, and get the voltage value of 15.36v; use channel B to measure the supply voltage, the difference is using AC coupling, and get the ripple of 150mV and the frequency of 86hz.
After the echo is connected, the DC and ripple voltage are basically the same (15.36v and 135.5mv respectively), but the ripple frequency rises from 86hz to 19.64khz, that is to say, the power module will skip many pulse cycles when it is no-load to improve efficiency.
Figure 4 measure the voltage and current of echo, picscope 4444 is an ideal choice
Play loud music
Next, we set echo to the mode of playing music at maximum volume. We measured the power supply voltage, current and power consumption. The maximum current is only 771mv, which means that the power supply capacity of 1.5A is enough to drive the device. At the corresponding time of peak current, the 15V supply voltage decreased by 216mv (1.4%). We also set up automatic measurement of average power (3.383w) and peak power (11.58w)
Because Amazon echo smart speaker is designed in such a way that it can keep open and receive commands at any time, it is necessary to measure its power when it is idle waiting for commands. So we put echo in standby mode and let it play music at a lower volume (Volume3).
The green line in the figure is a current waveform captured in full bandwidth, which is difficult to recognize due to switching noise. The red line is that we have done 10Hz low-pass software filtering for the green waveform, so that we can clearly see the envelope.
In the first 11.6 seconds, we can see that echo is in standby mode and only consumes 114ma (1.71w)
If the efficiency of the power supply module is estimated to be 85% for one year, echo will use 18 kwh. According to 60 cents per kilowatt hour, it will use 10 yuan and 80 cents per year. (editor in chief: the choice of running a family)
In the area between the scales, the power jump is because echo detects a voice command and transmits it through WiFi. Surprisingly, when playing music at a lower volume, the power consumption is almost the same as that of standby (117ma, only 3mA more).
The reason may be that echo is always recording in order to accurately recognize voice commands. Recently, there have been news reports about hidden dangers of echo monitoring, or because of official design considerations or being easy to crack. In short, this has become a topic of public concern.
Figure 5 current waveform and envelope of standby wake up play music
So, the conclusion is that if you let echo stay on standby or play background music quietly, the electricity cost is almost the same after a year, so please enjoy the warmth of the music from your back.