Joule Thief: Supplement: AC and DC Coupling, Input and Output, Scoposcopy
Here I use the little NE-2 Joule Thief to illustrate the function, effect and usage of the AC coupling / DC coupling settings on the oscilloscope. The “AC Coupling” setting is NOT for measuring AC, as some people seem to think! The terms refer not to the signal you are measuring, but how that signal is fed into the scope’s preamp. “DC coupled” means that there is just a straight wire connection for your signal. “AC coupled” means that this straight direct connection is broken and a capacitor… which passes fluctuating signals but not DC… is inserted in series instead. This has the effect of blocking the straight, non-fluctuating DC component of any signal, and moving the average of the fluctuating portion down to the zero baseline of the scope’s channel. This removes or hides the absolute voltage values!! Sometimes this is not important, but when making power measurements… it clearly is. I show how a false reading of negative voltage happens when AC coupling is set inappropriately, for a DC signal with superimposed “ripple” or fluctuating signal on top. Incidentally I also show how ungrounded probes can pick up signals from nearby apparatus even when they are not connected… or rather, especially when they are not connected. The 60 (or 50) Hz line current is always detectable in this manner, inside houses. One pitfall from trying to use AC coupling incorrectly is shown in the video.