There are a lot of power quality analyzers on the market today. Most rely on current clamps, can measure hundreds or thousands of amps, and are more oriented for factory rather than consumer type products.
But power quality remains and important topic for consumer electronics, and with a bit of custom hardware (for safety) and software, the QA403 can perform a range of tasks related to power quality.
The board below is test hardware to allow safe measurement of AC voltages and currents by the analyzer. While the analyzer is isolated, to connect and measure AC voltages directly and safely you need thousands of volts of isolation. Most of this thinking by the safety folks contemplates things such as you are in your office making a measurement and a lightening strike happens to hit the power lines outside your office. Now, the board below isn’t ready for that since it doesn’t even have a case. But you can see the isolation barrier drawn in red. The pair of parts that straddle the isolation barrier are AMC1200 isolated opamps. These parts can withstand a 4.2 KV transient and survive. The part by itself straddling the isolation barrier is a Wurth 760390014 transformer, and it’s rated for a 3.1 kV transient. So, the pieces are there.
At the south end of the board are two IEC connectors. One connector comes from the wall, the other connector goes to the DUT. The white BNC connectors on the far side are for connecting to the QA40x analyzer. The left channel is voltage, the right channel is current.
The board above won’t be taken to production and so the precise details aren’t that important. What is potentially interesting to discuss is how we get line voltages and currents safely into the QA40x analyzer while preserving dynamic range.
The schematic for the board above is shown below:
The power from the wall enters at the lower left. Area 1 on the schematic is a capacitive drop off-line power supply. Here, the voltage is dropped by an X1 safety cap, and rectified by a zener, a schottky and big 470uF cap. From there it goes to a 5V LDO and an SN6505 push-pull driver that drives a transformer. Area 4 is where the transformer output is rectified, and then regulated. On the left side of the transformer we have a 5V supply derived from the line voltage. On the right side of the transformer we have a 5V supply that is derived from the line-side 5V supply. But it’s isolated. These are both used to drive the isolated opamps shown in area 5 (left side of opamp is non-isolated, right side supply of opamp is isolated).
Area 2 is the current sensing, done in a 0.01 ohm resistor. Areas 3 is voltage sensing. To the right of area 5 are difference amps as the opamp output is differential. Note the isolated opamps used have a fixed gain of 8, and a typical gain accuracy of 0.05%. Combined with 0.1% resistors, you are looking at measurements that are really tight right out of the box with little temp and/or ageing drift.
A proof-of-concept visualizer was written for the QA40x software. When charging a laptop, the visualizer output is shown as below:
The voltage waveform is about as expected: 121.5V, and a THD of about 0.7%. The dominant harmonic if the 5th (300 Hz). On the current waveform, we can see a pretty lousy sinusoid. This is characteristic of older style power supplies where nobody really cared about efficiency. These types of power supplies take their power when the sine is at the peak. As a result, you have a current waveform with poor THD: nearly 30%. We can also see the real and apparent power. Apparent power is what you get when you measure your supply voltage and current with a simple meter, and then multiply those values together. In this case, a DVM would report 121.5V and 0.356A, and that suggests the circuit is consuming 121.5*0.356 = 43.2W. But it’s not watts–it’s VA. The actual power that is doing the work of running and charging the laptop is 26.12W. That figure is determined by multiplying the V and I measurements together at every sample, and then taking the mean. And from that, you can determine your power factor (PF). That figure tells you how much power is being wasted. And in this case, with a PF of 0.604, about 40% of the power we purchase is wasted in this charger (when you pay for power, you pay for real power as consumers. But that price has the wasted power built-in. That is why electric companies are so obsessed with power factor).
And this is what Energy Star and all the other government and industry programs are all about–getting the efficiency up. And the way you do that is by making the current waveform look more like a sine that is in phase with the voltage waveform. This is commonly called “power factor correction” and it requires a ACDC converter that is quite a bit smarter than most “off line” converters in use today. And the more power you consume, the more it matters.
Someone that understands high-power electronics and efficiency is Tesla. Let’s take a look at a Model Y charging from a 115V wall plug. For the plot below, the current was set in the Tesla app to limit at 6A due to some issues in the test hardware that was built.
Here we can see the current waveform very much represents a sine wave. it’s not perfect, at 2.3% THD. But we can see the Tesla charger is taking current from the wall at nearly every point of the voltage sine. And as a result, the Tesla charger appears to be purely resistive. It’s not of course, but Tesla’s power factor correction is making the load look purely resistive through sophisticated algorithms. And when a load appears purely resistive its power factor hits 1.0 and its efficiency is thus 100%. The PF here is 0.988.
So, why use an audio analyzer for measuring power quality? There are a lot turnkey options out there today, and you might not need to. But most power quality analyzers rely on clamp-on current sensors. A front-end board as shown above can use shunt sensing, which has much lower offset errors, no drift and significantly better accuracy and noise performance versus clamp sensors. If you are designing consumer products with aggressive standby modes and large active loads, that will challenge the lower limits of what a clamp-on sensor can do. The QA40x also has an easy to use API that makes it trivial to capture waveforms and do whatever processing and logging you might need.
The QA40x also offer a large dynamic range–much larger than what you get from a scope. In the case of the Tesla charger, the peak power can hit 1.4 kW during charging, and just 40W or so when idle. In addition to the dynamic range win, the QA40x accuracy is quite a bit better than most scopes.