Power Supply Tests

Here are tests of some power supplies I’ve built, along with results from other commercial supplies for comparison.

Ripple Rejection Test

In this test, I put a 100 Ω dummy load on the output of the power supply. Since all of the power supples in this test are set for 24 V (with one minor exception, noted below), this puts the same load on each, 240 mA. I ran the loaded power supply output to my measurement preamp, set to amplify only AC noise by 100×. I measured this noise with a Fluke 189 DMM to get the mVRMS number, and with a Protek 6502 analog oscilloscope to get the noise characterization and oscillogram. Most of the oscillograms are taken with the same scope settings so they’re easy to visually compare, though it sacrifices resolution in the lower-noise shots. See the characterization text for a short description of what one sees at higher magnification.

You may be wondering why I used a low-end analog scope instead of one of the more expensive Tek DSOs I have access to. It’s because dollar-for-dollar, analog scopes have less noise than digitals. In previous versions of this test, I observed the noise with my M-Audio Delta 44 sound card which is quieter than even high-end DSOs, but you sacrifice bandwidth, important for some of the test results below. Also, feedback made it clear that the spectrograms captured from the sound card were difficult to interpret correctly.

	mVRMS	Characterization
YJPS v0.4	0.007	very fuzzy broadband noise, no fundamentals
STEPS v1.1	0.058	some 120Hz ripple visible, but pretty fuzzy
TREAD v1.2	0.060	essentially identical to STEPS, on this test
Velleman K1823	0.10	unstable waveform, giving large semi-random fluctuations beyond basic noise
B+K Precision 1710	0.17	HF noise nearly smothers 120Hz ripple
Creek OBH-2	0.25	primarily 120Hz ripple, very little HF noise atop that
Elpac WM080	1.8	strong HF content atop 120Hz fundamental
Elpac MW1224	7.8	little else but 67kHz sawtooth switching noise
Radio Shack 273-1668^***	32	standard 120Hz sawtooth ripple waveform

For the PCBs above that have been replaced by later versions, performance should remain the same. I have not changed any of these in a way that would impact their performance. The only reason I have test data for older versions up here is that I do these tests as part of the development process, so I have the data around already.

The Radio Shack supply was set on the 13.5 V setting, but being an unregulated supply it actually put out 15 V under the test load. You would expect 60% higher ripple with a 24 V unregulated supply, all else being equal.

Load-Rejection Test

I removed this test because I was unable to get useful results when testing with headphone amps, since their dynamic current varies so little. Either all supplies tested equally good, or equally bad, or it ended up just re-stating the ripple rejection results.

I believe I need to design a dedicated test rig to get useful data. If I ever get around to that, I’ll put the results up here. If you know of such a circuit that’s already designed, let me know.