Today, we present a product that has been in gestation at Yoctopuce for over two years: the Yocto-Watt-V2-C. It took so long because we didn't just put a USB Type C connector on the Yocto-Watt. We've taken the opportunity to add some significant improvements to the product, making it an even more interesting measuring instrument than its predecessor.
The basic function that has made the Yocto-Watt such a success remains unchanged: it's a USB electrical sensor for measuring voltage, current, and power consumption, for both direct and alternating current, with reinforced insulation for measures directly on the mains.
The Yocto-Watt-V2-C looks just like the Yocto-Watt
Sampling frequency
Drawing on 12 years' experience of Yocto-Watt sales, we decided to replace the 16-bit A/D converter with 600 Hz sampling by a 24-bit version at 5.3 kHz. Indeed, 600 Hz sampling is a limiting factor for:
- the frequency range supported for alternating current
- the accuracy of AC measures
- cos Φ (displacement factor) calculation
The Yocto-Watt's 600 Hz synchronous sampling allows it to capture just 10 or 12 samples per period, depending on the mains frequency (50 Hz or 60 Hz). This gives credible measures when averaged over a few dozen periods. But for our customers who work with 400 Hz alternating current, this is not usable. This was the first limitation that we wanted to remove.
Even at 50Hz or 60Hz, in some cases the sampling frequency can be of enormous importance. This is especially the case for recent devices, designed to save energy, which use a switch-mode power supply. Here's a typical current consumption profile for a switch-mode power supply, as a function of sinusoidal voltage variation:
Typical consumption profile of a low-speed switching power supply
To accurately calculate the power consumed by this type of consumer, you need to be able to measure the voltage at the precise instant in the period when the current peak occurs, which the original Yocto-Watt cannot do. It therefore tends to underestimate consumption in such situations, with an error of up to 20% for loads with high harmonic distortion. The new version corrects this default thanks to its 5.3 kHz sampling.
The original Yocto-Watt version is also not always able of calculating the true displacement factor between voltage and current (DPF, also known as cos Φ). It can only estimate the power factor (PF), which doesn't necessarily correspond to cos Φ in the case of loads with high harmonic distortion. The Yocto-Watt-V2-C, on the other hand, has enough samples to estimate both quantities and provide distinct values:
Power factor and cos(Phi) are two different things
Consumption profile capture
As the Yocto-Watt-V2-C can sample voltage and current measures with a resolution of around μs, we thought it would be useful to make the detailed recording available to the user through our API, to enable visualization or harmonic analysis for example. In fact, it is the Yocto-Watt-V2-C web interface that allowed us to obtain the consumption profile illustrated above, enabling the user to instantly understand the type of load they're dealing with.
And since the captured samples are available, we realized that it would be very useful to be able to ask the module to memorize the capture corresponding to a particular event we are looking for, such as a consumption peak, for example. So we added an interface that lets you select an automatic memorization criterion, depending on the parameter whose maximum or minimum you're looking for:
Automatic capture with extremum search
Once the criterion has been selected, simply wait a few seconds, and the Yocto-Watt-V2-C can provide the recording of measures around the chosen event.
Of course, this function can be used on both DC and mains-powered devices. It is thus easy to measure peak consumption when a device is switched on, for example.
Comparison with connected energy meters
When the Yocto-Watt came out twelve years ago, there was virtually no other affordable wattmeter with an API that could take power measures on the mains and read them from a computer. With the advent of connected energy meters, the situation is very different today: there are a multitude of small, so-called smart energy meters, capable of taking power measures and transmitting them via Wi-Fi. So, is Yocto-Watt-V2-C still relevant for mains measurements? To find out, we ran a few tests...
We have tested a number of connected meters...
Here are our general observations:
- All these sensors are, of course, limited to mains operation: no DC measure.
- They only work on the basis of a wireless link (2.4 GHz), either directly with a cell phone, or via Wi-Fi, but sometimes with an obligation to go through a cloud server.
- In terms of power measure accuracy, it's quite correct, except that most of these meters simply don't see loads below 3 W. None of them can measure a load of less than 1 W.
- None of these meters indicate the power factor, nor cos Φ of course.
- The refresh rate of the measures on the corresponding applications is so low as to make it impossible to take into account variations in consumption, when it is not constant.
- Only one stands out: the Shelly 1PM Mini Gen4 WiFi module, which offers a genuine documented API and a Web interface with an acceptable refresh rate. But don't expect the 10Hz of Yocto-Watt-V2-C...
In our opinion, the Shelly sensor is clearly superior to the others in all respects, including its own power consumption, which is the lowest - half that of the greediest sensor, which consumes over 1.6W. But these meters are clearly not intended for the same use as the Yocto-Watt-V2-C. They are suitable for long-term, low-detail monitoring of consumption at home, but are not intended to quickly provide precise information on a particular electrical consumer.
Conclusion
The Yocto-Watt-V2-C is certainly 15% more expensive than its predecessor, due to its more advanced electronic components. But with its tenfold increase in sampling speed and its ability to record consumption profiles, the Yocto-Watt-V2-C delivers measures comparable to those of professional power analyzers, at a much more affordable price.
