Oscilloscope meets PPBAdv meets PSU’s
This is data, there are very few conclusions here, because I do not understand enough about what I am measuring, but it has some interesting photos (well, four versions of basically the same photo) of scope traces.
I set up the following: A power supply (4 different ones), powering a Pegasus Powerbox Advance, which in turn was powering a 0.5A dew strip at about 50%, and I then measured three things: (1) what did the PWM output look like (not very interesting, as you expect a 50% PWM square wave), (2) what did the input power look like in terms of noise, and (3) what did the output power look like in terms of noise.
Here is an example: Yellow is input power, Blue is output, and Purple is PWM. Underneath is a FFT showing the noise frequencies, basically just a lot of “stuff” spread over a wide variety of frequencies, but pretty far down. Note the yellow and blue are at 200mv per division, so the range over which input and output varies is about 150mv (by eyeball).
The PWM output is about 13.8v in this case, and when it occurs the input, and output, drops in synch with it from (probably) current induced drop. The power supply is connected with a 14 AWG wire, all but the last 6″ where there is a purchased splitter, probably 20 AWG to barrel connectors. Most of the voltage drop is likely in that. The actual voltage drop is about 100mv, maybe 150mv.
This is a shift to a 200AH LiFePO4 battery fully charged, i.e. lots and lots of pure DC power available. To me it looks essentially identical. Maybe, possibly a tiny bit less drop but do not think so. (note to self: Need to learn to measure vertical values exactly with cursor).
This seems to support that all the voltage drop is coming from the wiring, and all the noise is coming from the PPBAdv, and not the power supply. Here’s a Pegasus 10A PSU:
This has a slightly larger voltage drop, at least 175mv, but still close; this may indicate it does not quite keep up, though that PSU has a long, relatively low gauge wire and barrel connector, so it could simply be the built in wiring. it is a 10A PSU, and this is drawing one or so Amp, so it really should not be from the PSU itself.
And final candidate is a cheap Comcast 12v power supply that came with a TV box, chosen as one with adequate spec (3A) but likely the lowest possible cost components.
Appears to be a good 200mv, so again slightly more drop, but it is both a less capable PSU and also probably has smaller wire to the output (no easy way to measure without destroying it).
None of these, to my eye, had significantly more noise (all the blue/yellow are at the same scale, shifting a bit as different DC voltages). Both the FFT and size of the fuzzy line look about the same. And since it showed on the battery, all that noise is from the PPBAdv itself (the PWM, if set to zero or 100%, does not affect that noise significantly).
I do not suggest that noise is a problem, just showing a photo. It does suggest to me a couple things: The output to the PPBAdv does not appear filtered (the input and output noise profiles look the same), and it does not appear regulated, at least not well regulated if at all. And even cheap DC power supplies look fairly “clean” at least at these frequencies.
I welcome any feedback especially on how I am using this scope. It is new, I have not used one since the early 1970’s (and I think they changed a bit). I do not really get comments through this blog (just malware trying to find a home), but most people arriving here know where to find me.