Earlier this summer, I bought a beefy power supply from Jameco (1952361 PWR SPLY,TBL,REG,9V@6A,F2). One warning: the power supply does not come with a power cord, which must be purchased separately. The online catalog makes this clear, but in my comparison shopping for power supplies I somehow missed that. The nice people at Jameco sent me a power cord when I pointed out that their competitors were including the power cords with the power supplies, even though I admitted the oversight was entirely my fault. With the cost of the power cord, the prices at Jameco are about the same as other reputable sources of power supplies (like Pololu).
now that I have some power resistors, I finally got around to testing it. Because I have 3 different size resistors, I can test the voltage drop across many different load resistances: ∞, R1, R2, R3, R1||R2, R1||R3, R2||R3, R1||R2||R3, R1+R2, R1+R3, R2+R3, R1+R2+R3, R1||(R2+R3), R2||(R1+R3), R3||(R2+R1), R1+(R2||R3), R2+(R1||R3), R3+(R1||R2). But many of those are very similar resistances and won’t tell me much. I was mainly interested in how the power supply behaved when near the top of its power range, so I only tested the first 8 resistances on that list (no load and the various parallel arrangements).
The current is computed by measuring the voltage and the resistance and dividing. Because the resistance values are so low (1.34Ω to 10.17Ω), the repeatability of the measurements is not great (about ±0.04Ω), possibly due to differences in the attachment of the clip leads. This variation in the resistance probably accounts for most of the deviation of the points from the fitted load line, though the ±0.005V variation in the voltage measurements probably also contributes.
I made the measurements at the barrel jack that I plugged the power supply into, as that is the relevant point in the circuit for designing around. The output impedance of the power supply is 47mΩ, which is probably due mainly to the resistance of the cable from the power supply to the barrel connector. The cable is 1m long, and 47mΩ for 2m of copper wire is 23.5Ω/km, which is about the resistance of 18-gauge copper wire (20.9Ω/km), which is consistent with the diameter of the cable and what I would expect a 6A supply to use.
The supply delivered 6.647A at 8.907V (59.2W), comfortably above the specification of 54W. I did not detect any increase in temperature of the power supply at this power output, even after a few minutes (during which time the resistors got quite warm). So it seems that the power supply is indeed what it claims to be.