My NCE Power Cab wall wart part #is DP-1381350dU 13.5V DC 1.35A.
My Radio Shack Micronta multimeter model # is 22-171 No "A". I also tried my analog multimeter. Both behaved the same way as Steve's meters.
I then used my oscilloscope to check things out. When I mention "under load" it is just the Power Cab unit itself - no locomotives on the DCC-powered track.
This shows the DC voltage. Zero volts is at the bottom of the screen. At that range the voltage trace is pretty smooth, with tiny amount of noise visible.
This is just the ripple and noise that rides on top of the 13.5V DC. The scale is 20mV/division.
The main ripple is about 10mV (0.01V AC) with the very brief spikes dipping about 40mV below and peaking 45mV above the 13.5V.
Ripple with no load applied. I took this snapshot as it gives a better view of a sawtooth ripple which is expected from a switching power supply. The amplitude is still about 10mV. That is quite acceptable for this type of a power supply.
Since I had it on my scope I was curious what the frequency was. It looks to be roughly 367Hz. It is lower than I expected, but still normal for that power supply.
As it has been mentioned earlier, using a plain multimeter in AC Volt range does not work for determining how smooth the DC output of a switching power supply is.
