The Joulescope, now a Kickstarter project, lets you measure voltage and current consumed by many an embedded or IoT system, then calculate power and energy consumption. It's a DMM and oscillograph.
With so many engineers making small, low cost embedded and IoT devices, power consumption is high on the list of design requirements. While engineers have been measuring voltage and current with meters and oscilloscopes, the Joulescope Precision DC Energy Analyzer by startup JetPerch combines both into a single USB-connected device. After you see how it works, you can get a look at what’s inside.
The Joulescope (Figure 1) is a small USB-connected device that measures voltage and current, from which it calculates power and totalizes energy use. It connects to a Windows 10, Mac OSX, or Linux computer. (JetPerch recently told me that someone is running the Joulescope on Windows 7, though the documentation specifies Windows 10.) Software is available for download and is open source.
The standard removable, open source front panel holds four banana jacks, two each for input and output. The upper two banana jacks (input) connect to the power supply while the lower two (output) connect to the load. To get a feel for how the software works, I started with a simple load: a 1 Ω, 5 W resistor load driven with 2.2 VDC. That kept the dissipated power just below 5 W.
The software opens with a DMM screen that lets you see the most recent readings for voltage and current. Power and accumulated energy are calculated and displayed below (Figure 2). That’s good if you just want to be sure that your DUT is working. The DMM screen also provides min, max, peak-to-peak, and standard deviation data.
Note the voltage reading of 2.01 V. IR drops in my connecting cables reduced the load voltage from the 2.2 V at the power supply. I used identical leads from the power supply to Joulescope and from Joulescope to load. You must account for loses in cables and wires.
Using the View menu, you can select what you want to see on the screen. Figure 3 shows the oscilloscope display. It’s really an oscillograph, adding the latest measurements from voltage and current to the far right of the screen.
At this point, you’re wondering why each plot shows three traces. The middle trace represents the average of several measurements with the maximum above and minimum below. That’s because each pixel column on the screen contains data from numerous measurements. Think of it as a bin. If you use the mouse wheel to zoom in, the three datasets converge until you see a single measurement per pixel column. That takes some getting used to. There’s no way to turn off the min and max traces. I’d like to have that option in a future revision. The video below shows the three traces converging as the time scale changes.
Because of my resistive load, the measurements were relatively stable. When using the auto-scale function, the software adjusts the scale in real time to provide the maximum range of the measurements. You can set the voltage and current ranges manually. In Fig. 3, you can see how the software adjusts the height between the min and max readings to fill the available area. All current measurements, for example, are between 2.00 A and 2.03 A.
Fig. 3 also shows a vertical marker. That lets you see the statistics at any point on the screen just by moving your mouse. By clicking the control button (upper left), you can stop acquiring data and look back. You also have the option of saving acquired data to disk. The Joulescope doesn’t have an acquisition memory. Add data streams to the host computer. By clicking and dragging the mouse, you can scroll through the plots. The mouse wheel controls the time scale.
I discovered what seems like a bug. It occurred while the oscillograph was running and I turned off the power supply. As expected, the plotting of new data stopped, but it did not restart when power was reapplied. In some cases, I had to close the app and open it again to get anything to work. In other cases, I had to disconnect the Joulescope from the PC.
Using a resistive load is good for getting comfortable, but you can measure voltage and current in a stable load with a DMM or oscilloscope. To see how the Joulescope responds to an active device, I connected an external hard drive as the load. To do that, I removed the front panel and replaced it with the USB front panel (Figure 4). You can accomplish the same thing with a USB breakout board. Removing the front panel exposed the internal board, which I’ll cover on the next page.
The external drive connects to the output connector while the input connects to the host PC. Future versions of the board might replace the input USB Type-A connector with micro USB. Eventually, a USB front panel will need USB Type-C connectors for both the input and output. Figure 5 shows the external drive connected to the Joulescope.
In Figure 6, you can see the voltage and current resulting from writing to the HDD. Here’s where having the ability to remove the min and max traces would make the plot easier to read. At the end of the video above, you can see how to zoom in on a rising edge contained in this waveform.
The Joulescope is a unique instrument. That’s not to say that you can’t make power and energy measurements using other tools such as a DMM or an oscilloscope, but the Joulescope does it all in one place. The Joulescope isn’t available for sale just yet, given that JetPerch just launched a Kickstarter campaign. The list price has not yet been set, but I think $299 is reasonable. Yes, people will say they won’t pay more than $20, but please don’t.
Wish list for future revisions
On the next page, we look inside the Joulescope.
[Continue reading on EDN US: Inside the Joulescope]