RoadTest: AIM-TTI Bench Power Supply
Evaluation Type: Test Equipment
Did you receive all parts the manufacturer stated would be included in the package?: True
What other parts do you consider comparable to this product?: None. This is the first medium power bench supply I have worked with.
What were the biggest problems encountered?: Lack of a USB host port moves data logging of voltage and current outside of the instrument, whether into a connected PC or into external logging capable instruments This can complicate field use cases where a computer may not be available.
How this review is structured
This review of the Aim-TTi QPX750SP Programmable 750W DC power supply unit offers a discussion of my experiences using and testing this instrument. I wish to extend my thanks to Randall and his team for managing an awesome and successful Road Test program at element14. My gratitude and thanks also are extended to Aim-TTi for providing three instruments to be reviewed by e14 members. A good portion of my enthusiasm in applying for this Road Test grew from my desire to learn more about the Aim-TTi product line. I am formatting this review in a hub and spoke model. High level discussion of findings is provided here with links provided to detailed testing and application blogs.
My proposal to Road Test this supply was largely based on evaluating the supply in a specific use case. I will introduce that use case, summarize my findings, and provide a link to the complete write up of the use case experiments. For a very thorough and well written technical evaluation of the instrument I refer readers to Gough Lui's element14 Road Test of the QPX750SP.
Why would you want a QPX750SP on your bench?
The QPX750SP is a higher power version of the ubiquitous bench power supply found on every polytechnic lab bench, on most engineer's bench's, and on many hobbyist bench's. Electronic circuits need power to operate. In many cases power to operate electronic circuits under development, during test, or in troubleshooting and repair situations comes from a bench power supply. In my experience many circuits encountered in day-to-day electronics work can be powered from a supply that provides 0 to 30 VDC at up to a few amps. If you do work with op amps or other analog components it is nice to have a dual supply so that positive and negative rails can be set up. Even better is a three output supply that offers a beefy digital logic supply up to say 6 VDC at several amps and a pair of adjustable supplies that offer up to 30 V at a few amps to run analog circuits.
I have two benches in my workspace. A general purpose bench supply sits on each bench, waiting to energize whatever gadget I bring forth to experiment upon. A Keysight E36313A triple output programmable supply sits on one bench. The E36313A is a great bench supply that I purchased after reading dougw's Road Test review. It has two 25 VDC channels at 2 A each and a hefty 6 V channel at 10 A. Like most modern bench instruments it can be connected to a network for remote operation and data logging. There is also an in-instrument front panel mounted USB port to connect a memory stick to log measurement locally. It does not have a touch screen, but it does support 4 W connections, however, the sense connections are on the back of the instrument, so I tend to think twice about how seriously I need the accuracy of a 4 W connection when using the E36313A. My second bench is equipped with an older, but reliable B&K Precision 9130 triple output programmable supply. This supply does not have a touch screen, does not support 4 W connections, but it does allow for connection to a computer - via an RS232 port. These supplies, and others like them, have met my needs about 90% of the time. Rarely do I need higher voltages or higher current capabilities. When a use case arises that needs higher voltage or current I typically connect supplies in parallel (more current), or in series (more voltage). Both of the supplies I use support internal series and parallel configuration from the front panel, so no awkward jumper wires are required. However, even operating in series or parallel, my two supplies, and most standard bench supplies, are going to come up short in some specialized use cases. What if you need 20 A of current? What if you need 75 VDC? For use cases with demands outside the range of standard bench supplies it may be best to consider a heavier duty bench supply, like the QPX series from Aim-TTi.
I had a use case that fell within the output capabilities of the QPX750SP, but fell outside the capabilities of either of my bench supplies. My use case was experimental characterization of system behavior in a small off-grid solar photovoltaic (PV) power system. This particular solar PV system as presently configured works with voltages up to about 50 VDC and currents up to 30 A. It was mostly the QPX750SP current capability that I needed. I have posted a blog that describes all of the solar PV characterization experiments I performed with the QPX750SP. You can read it here. I have copied the PV system block diagram below to give you an idea of how the system is configured. This diagram will also allow me to explain how I used the QPX750SP.
Because this PV system is installed and operating in an out-building, I had to move the QPX750SP from my bench to the out-building to perform the tests. This "remote" use case revealed a functional limitation of the QPX750SP that made me frown for a moment, but it was not a show stopper. I used the QPX750SP to characterize two aspects of PV system behavior. First, I used it to simulate the capabilities, if not the actual behavior, of the photovoltaic solar modules. As you can imagine, and as I detail in the linked blog, PV module output is all over the map on any given day as it is strongly associated with solar radiance falling upon the panels. That in turn depends on the time of day, cloud cover, and seasonal changes in the path of the sun. There are too many variables to deal with here to run controlled experiments when depending on the Sun as a power source. Thus the need for a power supply that can simulate the power capabilities of two series connected 85 W PV modules in a controlled manner. The PV simulation tests did not stress the QPX750SP much since even under optimum conditions the panels only output 170 W of DC power and the QPX750SP can output up to 750 W within its PowerFlex+ envelope. With the QPX750SP substituting for the solar modules I was able to gather data to observe mode switching on the Renogy charge controller from MPPT to Boost to Float (Details of MPPT operation are provided in the blog). My second test application for the QPX750SP in this system was as a substitute for the 12 V AGM lead acid battery. In this application, the QPX750SP behaved as a controllable battery, allowing me to experimentally determine current flow into various loads at several levels of battery charge. These tests revealed a wiring omission that I describe in the blog post.
The QPX750SP easily provided the voltages and currents the system needed for both sets of simulation tests. It also performed well during a "what if" experiment where I used the QPX750SP to simulate 4 PV modules in a series-parallel arrangement. My only disappointment is with the lack of a built-in USB port that would allow local data logging. I used two external data logging multimeters in various configurations to record current flows out of the QPX750SP, out of the charge controller, and into the pure sine inverter.
Investigation of QPX750SP protection systems
One of the many reasons I pursued a career in electronics technology rather that in electrical technology is that I thought I would be less likely to get zapped working with lower voltage and currents typically used on the world of electronics technology. For the 40 years I spent professionally working in electronics that thought proved to be correct. Any zaps I experienced in my career were the result of excursions into the electrical world. Not every excursion resulted in a zap, of course, but any that did occur were the result of carelessness while working in electrical (mains powered) circuits. My healthy respect for electricity leads me to always consider what sort of protection systems are included in test equipment. The higher power, voltage, and current capabilities of the QPX750SP drew me to study it's protection systems. My findings on the behavior of over current and over voltage protection systems in the QPX750SP can be found in this blog post. I can report that no instruments, or technologists, were harmed during the making of this review.
Less serious use cases
My interests in electronics these days mostly involve microcontroller applications around the house and garden. Very much in the low power universe where the idea is to not produce a lot of heat. The solar PV system is one exception where voltage and currents are just beyond the low power universe. In an effort to find loads that would employ lots of current to useful ends, I transitioned from thinking about the world of low power electronics into the world of electrical appliances. Many electrical appliances draw relatively high currents for the purpose of generating useful heat. Examples include ovens, dryers, and toasters. These appliances typically operate from AC sources and many contain electronics that may not appreciate being driven with high voltage DC. After considering risks and rewards, I settled on an experiment to make toast with QPX750SP. The results of that experiment are detailed in this blog post. The image below shows heating caused by the QPX750SP pouring current into resistive elements in a toaster, doing its best to make a toasty snack.
Another load that can draw a lot of DC current are motors. I found an old battery operated Power Wheels Jeep in the garage and decided to run the motors on it using the QPX750SP.
Turns out this was not a very exciting test. I was able to determine the nominal no-load operating current of the rear wheel motors in forward and reverse (see current chart below - forward and reverse produced similar charts). In order to get some idea of loaded current draw I simply held my hand against the rim of the rotating wheel and applied increasing pressure until the wheel stalled. This was a once only test in forward and reverse because I prefer my fingers to remain unabraded. The chart shows that loaded current seems to be limited by the QPX750SP Iset value. In this case that was 10.0 A. The QPX750SP can supply up to 50 A at 6 V, which is only 300 W and a whole lot less than the 750 W spec printed on the front panel of the supply, but that aside, I didn't want to run 50 A of current through my 4 mm banana leads. The fuses on the batteries are rated at 30 A and I recall replacing them more than once when my kids were little and would drive the jeep full speed into a fence, stall the motors, and laugh with delight until the fuse blew and their toy stopped working. The best of times.
This toy has two switches by the steering wheel, one labelled LOW, the other labelled HIGH, referring to speed. In LOW the two 6 V batteries are connected in parallel, in HIGH they are connected in series. I discovered this arrangement when I switched to HIGH with the QPX750SP driving the connections in parallel. The motors stopped and the QPX750SP went into Constant Current limiting mode. As this was a field test, I had to cart along the FLIR CM65 logging clamp meter to record current flows. You may be able to see the clamp meter sitting on top of the QPX750SP in the photo above as it sends Bluetooth samples to my phone.
User Interface (UI) discussion
To me, so much of the utility of a piece of test equipment is wrapped up in its user interface. An instrument can have awesome technical specifications, but if it is difficult to use, then, in my experience, it often gets left on the shelf gathering dust while other, easier to understand tools, see more regular use. Through a lengthy professional career in industry and academia I have worked with many brands and types of test equipment. I have seen students struggle with awkward UIs in ways that impede learning or result in incorrect use of a tool, and I have seen the benefit of well designed UIs that enable user curiosity and learning.
Unfortunately the transition over the last couple of decades to increased use of computer interfaces, touch screens, and firmware based UIs, in my opinion, has increased the probability of encountering wonky, awkward, and downright weird or faulty user interfaces. I have encountered menus that take users to dead ends, instruments that can accidentally be configured to go into catatonic lock out conditions, issues with network communications that prevent instruments from initializing, display screens with so much clutter that reading values becomes impossible, and many cases where what I expect a button press or knob twist should do is not what the designer of the user interface thought it should do. In general I got along with the QPX750SP user interface once I accepted that I would have to accommodate the vision of the user interface designer. Here is an example of what I thought to be odd UI design choices.
Setting an output voltage
This is one of the most common actions a user takes when using a power supply. Aim-TTi provide serval pathways to support users as they endeavor to set output voltage. Users can set output voltage from the front panel numeric keypad, the touch screen, the rotary knob, or various computer interfaces. Consider the image below as I take you along on my journey of learning how to change the output voltage on the QPX750SP.
The output voltage has previously been set to 10.000 V. Right now, the output is off. Suppose I want to change the output voltage from 10 V to 12 V. Because I know the QPX750SP has a touch screen (nice!) and I want to change Vset, my instinct is to touch the Vset field. When I do so, the instrument beeps and the display changes as shown below.
Okay, so the Vset field is now outlined in yellow and the "V" front pane button is lit up. But no on screen keypad appears, so I wonder how I am to enter my new Vset value. The illuminated V button draws my attention to the adjacent rotary knob and develops an expectation that the rotary knob is currently linked to the Vset function. I turn the rotary knob one step to the right, expecting the Vset value to increase. Nope. The focus moves from Vset to Iset on the display. "V" goes out and "I" is illuminated.
So, there must be another step required to make the rotary knob change the Vset value. I rotate the knob one step to the left to regain focus on Vset, then depress the rotary knob thinking that might enable it as an input device for changing Vset. The instrument beeps, "V" stays illuminated, and the yellow outline around Vset disappears, but the Vset value is now colored yellow with the least significant digit is now highlighted. Does this mean I can now use the rotary knob to change Vset in the least significant digit position?
Well, no. A twist of the rotary knob moves the focus to the next digit. I suppose I should have guessed that behavior would occur. I position the focus over the units digit, then depress the rotary knob to enable value entry mode. Two clicks to the right on the rotary knob and I finally have my new desired Vset voltage.
To recap, changing the Vset value using the touch screen and rotary knob in this use case took 8 steps (once erroneous steps were removed from the sequence).
Obviously, this is not the recommended process to change Vset. Turns out once you touch the Vset field, the front panel numeric keypad is activated, though there is no visual indication this is the case. You can also enable the numeric keypad buttons by pressing the "V" button by the rotary knob. As soon as the user depresses a numeric keypad button the Vset field clears and the pressed value appears as a most significant digit. Finish entering the desired value and then press "OK" to set the new value. That is simple and quick. You can also press the Vset field twice to bring up a numeric touch screen keypad.
I would have designed the UI so that one touch of the Vset field on the screen would bring up the touch keypad, but that's just my preference. My point is that my intuition about how I expected the user interface to work was not in alignment with how the UI designer made the UI work. To align my expectations with the designers expectations required me to go through one, or both, of the following exercises:
Until the day that instrument manufacturers start providing their products with 100% user configurable UIs, we will all be obliged to spend some time doing exercises 1, or 2, or both in order to develop competence using a new piece of test equipment.
Just one more observation. Why does the numeric touch keypad have a "+/-" button? This is a single polarity supply. You can not set it up to produce a negative voltage. I suppose that explains why the button is grayed out, but still, why? My guess is that some other models that are dual polarity need this button, so for sake of efficient code reuse, the same touch keypad code was used in the QPX750SP and the +/-" functionality was disabled. But still, why?
Sometimes there are issues with the layout, labelling, and sizing of buttons and switches on instrument front panels. My only concern with the QPX750SP front panel is in the use of wire based push sockets for the 4-wire sense inputs. I'm curious to know why banana sockets were not chosen. These little push terminals are difficult to use (they are small and too close together) and they require the use of bare, or better yet, tinned wires. Bare, or tinned wires are not typically found in most test lead sets, so I had to make a pair. They are also prone to severing at the pinch point. I am not a fan of this type of connection.
On the other had I am a big fan of real power switches like the one Aim-TTi provided on the QPX750SP.
I prefer a real hardware based power switch for power control because soft switches depend on firmware to work and firmware can be oddly implemented. The front panel power button on the Keysight E36313A has to be pushed twice to turn the supply off if the supply has gone to sleep with the output active but the display blanked. The first push wakes up the instrument, the second push shuts it down. My expectation as a user is that when I press the power button the instrument will immediately toggle to the opposite power state. My most important expectation when I turn a power supply off is that the output is deenergized immediately. Rapid shutdown is required in cases where excessive current is flowing in a circuit and risk of overheating is present. The power switch on the QPX750SP for the most part operates as I expect. When switched off the output is deenergized, but voltage does not disappear immediately. It falls in what appears to be an exponential decay. The fan also runs for a few seconds before turning off.
I like the Aim-TTi QPX750SP. It is a very capable instrument in the higher power category of bench power supplies. My first experience with the Aim-TTi brand has been positive overall. I would have preferred to have a USB memory stick port on the front panel and banana sockets for the sense terminals and maybe a few tweaks to the user interface, but in general the Aim-TTi brand has left a positive impression. My explorations with the QPX750SP allow me to recommend it as a good choice for those needing a supply in this range of power capability.