ESTD 1999

DC Electronic Load

Posted: 19th March 2019

Recently I have had a need to perform measurements that were time-consuming to set up, or otherwise tedious to perform with the equipment I had. Things like measuring the I-V curve of a solar panel, or the capacity of a battery, all of which involve drawing a specific current from a device and measuring the resulting voltage, either over time, or over a range of currents. That's exactly what an electronic load does - a relatively uncommon bit of kit. As usual, I came up with a list of features I wanted that don't seem to exist in commercial products, so I made my own.

The features I wanted that I couldn't find include:

And other, more common things I wanted:

Hardware

Having already used one for a previous project, a Raspberry Pi was the obvious choice to build it around. They come with WiFi built in, and have plenty of peripherals available, specifically a touchscreen, DACs, ADCs, and GPIO to control it all. All that digital stuff comes later though. The first thing to do was to create the analogue circuitry that lies at the heart. It's fairly simple, consisting of a single large transistor controlled by an amplifier with feedback connected for either constant current or constant voltage operation. Constant power and constant resistance modes are also possible, but I didn't implement those as I have no use for them.

Schematic of the electronic load.
This is the actual electronic load. Every other part of this project is just to support it.

The above schematic looks somewhat more complicated at first than it really is, because it's two circuits squished together. The first one is the constant current part, consisting of the op-amp U1A, MOSFET Q1, the sense resistor R5, and various other resistors. The second one is the constant voltage part, consisting of U1A, Q1, and more resistors. Which one of those two is operating is determined by the mode pins J3, which are connected to the op-amps' shutdown pins. Note that Q1 is shared, to save having to buy two large transistors. This is possible because when one of the op-amps is in shutdown mode, its output impedance is high, allowing the other op-amp full control. Additionally, the base stopper resistors R3 and R11 are sized larger than they would otherwise need to be, so that if something goes wrong and both op-amps are turned on at the same time, they won't short each other.

I selected the TLV2373 op-amp for this, as it has the required shutdown pins. It also has rail-to-rail inputs and decently low offset voltage, so it can take feedback directly from the sense resistor that is connected to ground. On testing, it turned out that even a very small DC offset was too much, so I added the potentiometer RV1 to allow the offset to be adjusted. The constant-voltage side doesn't need offset adjustment because it doesn't need to measure voltages of only a few mV above ground.

The main transistor is an IRFP7530, which is a superb example of modern semiconductor technology, having an RDSon of only 2mΩ, which is so small that it would be swamped by the resistance of the wiring. It's a bit higher than that with the 5V gate voltage it's being driven by here, but still very small. This allows the electronic load to draw its maximum current down to very low voltages.

The current sense resistor is also important. It needs to be as small as possible to dissipate as little power as possible, as the more it heats up, the more its temperature dependence will affect the measurements. It also needs to be small so that it doesn't drop too much voltage, which would limit the minimum voltage possible at high currents. On the other hand, the smaller it is, the worse the signal-to-noise ratio. I decided to go with a very small 10mΩ.

PCB layout of the electronic load.
Electronic load PCB layout.

I just about managed to fit it all neatly onto a single-sided PCB. It would have been easier if I could have obtained the through-hole version of the TLV2373, but after waiting for more than a month for them to become available, I gave up and got a surface-mount one. I tried to get the feedback connection to the sense resistors away from the high-current traces, Kelvin-style. It's not quite perfect, but that doesn't matter too much, as any linear errors can be calibrated out in software.

Next is the interface, which holds the DAC that converts the digital output from the Pi into a voltage that controls the current/voltage of the electronic load, and the ADC that lets the Pi read the actual voltage/current, along with a pair of amplifiers to change the sensed voltage/current to the right range for the ADC.

Schematic of the interface.
Interface schematic.

The schematic is very simple because it's little more than a container for the DAC and ADC which come on their own PCBs and plug in to the interface board. Note that the amplifiers for measuring current/voltage are both differential. Even though theoretically one end is ground, and so single-ended amplifiers are sufficient, in practice the resistance of the PCB tracks and wiring is high enough that the low side of the load will be slightly above ground.

I chose the TLV272 for the sense amplifiers as they are rail-to-rail op-amps that I already had. I didn't add any DC offset adjustment for these, but I probably should have, to improve accuracy. The ADC is an ADS1115, available already on a PCB, which is helpful as they would be hard to solder by hand. It's 16-bit, which gives plenty of resolution. Likewise, the MCP4725 DAC is ready-made on a PCB. It's only 12-bit, but that's still enough to give about 2mA resolution over the full 10A range.

PCB layout of the interface.
Interface PCB layout.

Not shown are a couple of capacitors across R4 and R2. I added those after noticing a large spike of interference picked up by the current sensor at exactly 1 minute intervals, coming from the WiFi. It is of course not possible to shield the WiFi and still have it work, nor is it practical to shield the analogue circuitry sufficiently, so I mitigated it by reducing the amplifier's bandwidth with capacitors. I should probably have done that from the start, since the bandwidth is far higher than needed for the modest sample rate of the ADC, which means excess noise.

The last analogue part is the power supply. It seemed sensible to run all the analogue circuitry from 5V, to keep all the voltages within what the DAC and ADC can handle. Since this supply voltage also serves as the reference voltage for the DAC, I used a linear regulator to provide it. The Raspberry Pi and its touchscreen also need 5V, but at much higher current, and they don't care so much about it being clean, so I used a couple of tiny switching regulators for those. All three regulators are powered by 12V from a "wall-wart".

Schematic of the interface.
PSU schematic.

Like the interface, this PCB is little more than some sockets for other boards to plug in to. The two switch-mode regulators are cheap ones from China, while the linear one I made myself, with the same pinout for convenience (I haven't bothered to show the schematic for that, as it's too simple). There are some other components present though: A common-mode choke and capacitor at the input, and some capacitors across the outputs of the regulators. The filter at the input is necessary to keep out noise generated by some of the lower-quality wall-warts I might use to power this. The capacitors at the output do the same for noise generated by the switching regulators, although they are not too noisy by themselves.

Several PCBs mounted inside the case and wired together.
The touchscreen is mounted by using a Pimoroni frame as a clamp.

And there it all is inside the case. The two stacked PCBs at the top are the Raspberry Pi and the PCB for the touchscreen. The analogue stuff is all at the back of the case (at the bottom of the photo). On the left, the interface, then the load itself in the middle, and the power supply on the right. The main transistor is bolted to a heatsink on the back, which is big enough to handle around 100W before the transistor overheats.

The back of the case, mostly covered with a large heatsink.
That's the biggest heatsink that would fit on the back without covering the vents.
The sloped front of the case with the touchscreen on it.
The case is made by Hammond, chosen to match the bench PSU which is on the left.

Software

With the hardware assembled, I then started work on the software. As with all the other projects I've used a Pi for before, I started with WebIOPi, which has a webserver and drivers for a few I²C devices, like the DAC and ADC I'm using. Sadly it's no longer supported, and doesn't work on the newer Raspberry Pi 3, but there is a fork available, rpi3-webiopi which does work, so I used that one instead. I had to make a few modifications to it, such as adding support for more of the features of the ADS1115.

I used that to provide a simple HTTP API, exposing functionality such as turning the load on and off and setting the current/voltage. So that it can be set up and started from one device, then the results viewed on another device, it will run once started until it is told to stop or it reaches some preset limits, and it will keep all the measurements it made until asked for them (or they can be read frequently, to keep the chart updated in real time). To provide programmability, it can also accept a Python script instead of a single value for the current/voltage, which it will use to continuously adjust the load.

The client (which is just a web page) is where most of the interesting stuff happens. It has a UI for setting the voltage/ current etc. and a chart for displaying the results, which can show not only the current/voltage, but also some other values that can be calculated from those over time, such as electric charge in mAh for measuring battery capacity. Charting is done using Chartist, which is a very nicely written bit of software, if a little bare of features. The data can be exported as CSV too, so it can be further analysed elsewhere.

The completed electronic load showing a chart of energy drained from a supercapacitor.

Above you can see the completed thing having just taken a measurement of the energy contained in a supercapacitor. The capacitor was charged up to 2.5V, then the load set up to drain it at a constant 1A until empty. You can just about see that the chart is configured to show energy in Joules, and the final value is 1377. Since the energy stored in a capacitor is E = ½ * C * V², the capacitance must be 2 * 1377 / 2.5² = 441F, which is acceptable for one that is supposed to be 500F±20%. In fact I'm pleasantly surprised, as it was a very cheap one purchased from eBay, and the manufacturer doesn't advertise anything bigger than 400F, so I was half expecting it to be fake.

Also visible in the photo are the extra connections for remote voltage sensing. Without those, the measured voltage would be too low due to the voltage drop in the wires and connectors. There is a switch to toggle between remote sensing and local sensing (which measures the voltage from a point just inside the case). The switch is recessed so that I can't accidentally toggle it.

The completed electronic load showing a chart of current, voltage and power varying up and down according to how the load was programmed, and the on-screen keyboard, similar to those on mobile phones.
The standard fonts on the Raspberry Pi lack the tab symbol (⭾), hence the square on one of the keys on the left. I should fix that.

Although the Raspberry Pi has an offical touchscreen, which I used, it runs a desktop OS that lacks touchy features like an on-screen keyboard. It does have a USB port on it, which I brought out to the front panel, so a physical keyboard can be connected if required, but I really intended it to be controlled with the touchscreen alone, so I programmed an on-screen keyboard in JavaScript. It has both a small numeric keypad for entering numbers, and a full alphanumeric keyboard for text input. The above photo demonstrates the ability to use a script to control the load, in this case simply ramping the current up and down. It's hard to read the script in a photo of a small screen, so here's the same thing again as a screenshot from my PC:

The same as what was showing on the screen in the previous photo, but without the on-screen keyboard. The Python script can now be seen to be (t % 10) / 2 if ((t - (t % 10)) / 10) % 2 == 0 else 5 - ((t % 10) / 2)
The background colour matches the case panels when shown on the Pi's screen. It looks different in photos and on other displays.
The data points are in time order, so the chart can't draw lines between them when the x-axis is not time.

The final screenshot is a test to find the maximum power point of a solar panel. The load was set to constant voltage mode, running the script 20 - (t*t) / 10 so that it ramps down from 20V, spending more time (and thus taking more samples) at the initial higher voltages, which is the region of interest. The minimum voltage limit is set to 0.5V, which causes the load to stop once the voltage drops below that, otherwise it would run forever. The chart is set to use voltage as the x-axis and power as the y-axis, so it can easily be seen that the maximum power is achieved a little below 18V.

Source code is available both on GitHub where it's easier to browse, and as zip files here.

The modified WebIOPi that it runs on top of is also on GitHub, and here in a zip file.

Conclusion

Overall it works pretty well, doing everything I wanted it to do. There are a few shortcomings though. It has a slight problem with the temperature coefficient of the sense resistor. It's ok at currents up to an amp or so, but at higher currents the resistance slowly increases as it heats up, leading to the actual current decreasing. To completely fix that, I would need to add a temperature sensor. It's also hard to read the chart when it is displaying multiple quantities, since they all share the same y-axis, which is a limitation of Chartist. Since this is my device that I have full control over, I could work around that if I really wanted to, or add any features I might need in the future.