The Aim-TTi QPX750SP:  Awesome power supply - but can it make toast?

The Aim-TTi QPX750SP is not a typical bench power supply.  A typical bench power supply may have two or three output channels capable of delivering 0 to about 30 VDC at up to say 3 A.  For example, my Keysight E36313A is a typical bench supply with three programmable channels.  It has no trouble delivering one channel of 6V at 10A, and two channels of up to 25 V at 2A.  That sort of power is fine for powering many common DC circuits including most small contemporary digital and analog systems.  Some applications are beyond the reach of typical bench supplies.  For example, a small off grid solar PV power system may see DC voltages around 50 to 90 VDC at double digit currents. Designing and characterizing a system like that requires a power supply with capabilities beyond those provided by standard bench instruments.

The Aim-TTi QPX70SP reaches into the next tier of voltage, power, and current capability.  It can comfortably deliver 750 W of power to your load anywhere from 80V to 0.01 VDC.  At 80 V, load current up to 9.375 A can be drawn (80V x 9.375A = 750 W).  At lower voltages, say 15 V, up to 50 A of current can be drawn.  In my Road Test I will be journaling my experience using the QPX750SP to characterize a small solar PV system, but in the meantime, I have been looking for things to pour heaps of current through. I happened upon two slightly off kilter test cases; one slightly specious, the other slightly capricious.

This blog details the slightly specious test case.

When I think of lots of current flowing through a circuit at high voltage I think of power, and when I think of power I think of heat.  Plenty of devices that we plug into the mains produce heat, like coffee makers, blow dryers, ovens, lights, and...toasters! I was interested to find a device that presented a simple resistive load that would get hot, but also would not have a lot of pesky electronics inside that might a) get wrecked by being driven with high voltage DC, or b) gum up the load impedance somehow.  I surmised a toaster might qualify.  A basic toaster is pretty much a resistive load that is designed to get really hot for a while. My North American AC mains operates at 120 VAC (RMS) and the unfortunate 4-slice toaster that I kidnapped from the kitchen is rated at 1500W, so would it not draw about 750W when making 2-slices? You may be starting to see why this is the specious test case, and you would be correct.  I don't actually know what is in my 4-slice Black and Decker toaster in terms of electronics.  It has buttons, so it might have electronics.  There is a button for bagels, but I surmised that only turns off the two resistive loads on the outsides of the slots.   It has a button for frozen items.  I surmised that just extends the toasting time.  It has a button for warming.  I surmised that just sticks more resistance in series to reduce the heating power.

I figured it would be wise to do a little exploratory testing, to see what I might learn about the toaster before hooking it up to the QPX750SP.  I took some resistance measurements.  Here is what I discovered:

So, yes, it looks like 2-slices presents about twice the resistive load of 4-slices.  This suggests that 2-slices will draw a maximum of 750W at 120 VAC (RMS).

So, because P=I²R and  I^{2=750W19.15Ohm} , 2-slices might draw about 6.26 A at 120 VAC.  Well, the QPX750SP is capable of delivering 9.375 A at 80 VDC, if the load demands it.  

First, I ran a 2-slice test using 120 VAC mains power.  The toast dial was set to 3.5 on the 5 point scale. I  took thermal imaging measurements of the toast slots during the test.  Here is what I found:

Notice that the Max temperature target in the left slot reads "OVER".  The Keysight U5855AU5855A IR camera I used for this measurement has a maximum measurable temperature of +350 degrees C.  So all we know is that the resistive elements appear to get hotter than +350 degrees C when running the toaster from120 VAC mains power.  The manual target, positioned between the two slots, reads +122.0 degrees C. This image was taken just before the 3 minute mark when the toaster popped. Bread comes out the toaster under these settings a medium golden brown at a temperature that easily melts butter.

Second, I ran some tests with the toaster powered by the QPX750SP.  I set the QPX750SP to output 80 VDC at up 9 A current.  The output leads were connected to the prongs of the AC power cord along with the remote sense leads.  I used Aim-TTi's Test Bridge software to control and data log the tests.  I wasn't sure 80 VDC would operate the toaster at all, but I am happy to report that the toaster latched the handle in the down position, the toast light came on, and the thermal IR camera detected heating. A thermal IR image taken just before the toast popped at 3 minutes is provided below.

A few things to note:  the toaster still works after these tests, so I didn't fry any electronics inside.  The temperatures achieved at 80 VDC are noticeably lower than those achieved at 120 VAC.  The resistive elements did not visibly glow red when powered with 80 VDC.  The toast emerged from the toaster crisp and capable of melting butter, but was not golden brown.  So, in answer to the question, can it make toast, I have to say, yes, sort of - not the best toast, but yes, it made toast.  And, I must declare, the Aim-TTi QXP750SP met this challenge with flying colours!  From the image below you can see it casually delivered 335.4W of power to the toaster as if it was taking a stroll in the park. It has reserve capacity at this voltage to deliver another 415 W!  The cooling fan in the QPX750SP sped up when the toaster was activated, but otherwise the supply seemed unimpressed with the challenge.

Data log strip charts of supply voltage and supply current during the toaster test are shown below.  These logs were generated by the Test Bridge control application.

Sample rate was set to 250 ms (the fastest rate permitted by Test Bridge).  Note that the toaster ran for about 3 minutes drawing a near steady current of about 4.2A.  Note too that the supply voltage only dropped from about 79.993 V under no-load to 79.9915 V under full load.  There is a very brief drop to about 79.977 V at the instant the toaster load is engaged.  The remote sense connection provided the feedback necessary to allow the QPX750SP to rapidly compensate for I2R losses in the main supply leads.

I am impressed by the performance of the Aim-TTi QPX750SP during this odd-ball test.  For a rough analogy, imagine holding your arm out to your side, parallel to the ground, ready to grasp whatever is passed to you. Now imagine you are asked to maintain that parallel stance as someone places a 10 L jug of water in your outstretched fingers. And, oh yes, please maintain your arm parallel to the ground for the next 3 minutes.  You may pant, but please do not dip the jug more than 1 cm toward the ground.

I am working on serious bench evaluation testing of the QPX750SP and getting ready to run characterization tests on the solar PV system, but I had to try something quick and simple to stress the supply at its higher end.  Thus the 2-slice toaster test.  One more high-ish current test at a lower voltage caught my eye during a clean up of the garage.  Next post I will use the QPX750SP to run the motors on this vintage item:

Until next time!