Remember the basic definition for AC: that the current alternates direction.

This can only happen if the waveform crosses the x-axis at zero. The waveform can do anything above "0" on the x-axis and the current will still be uni-directional, therefore DC.

Granted, it may not be a clean, steady DC voltage like the kind you get from a DC power supply or battery, but it is DC nevertheless.

As for the reference to a bridge: the output is a rippled DC voltage. That's why DC power supplies have other components in them ... to filter and smooth that ripple into a "more usable" "steady" DC voltage.

 

I consider pure DC as a signal with only frequency content at 0.

Just to stir the proverbial pot; don't forget that a square wave is the sum of the odd harmonics of the fundamental frequency. That fundamental frequency might be very low, such as a switch that's turned on and left on for a year - but still, the square wave is the sum of the odd harmonics of the fundamental frequency.

 

Hello Wookie.

Yes I didn't like to stir this particular pot, that even unvarying DC can be regarded as having a period of infinity and indeed some analyses are based on this.

 

My brother was playing around with xenon lamps (strobe circuits). He wanted a quick and dirty 180VDC, so he used a widowmaker plug (a power plug with aligator clips), a diode, and a high voltage cap, all loose in his lap. It worked allright, he accidently triggered the xenon tube before he was ready, and since this lamp had the full house power behind it it stayed on, a most abnormal condition.

I learned about this dubious experiment when the UFO arrived. Every crack, every keyhole, had streamers of light pouring out. Seriously, I haven't seen movies whose special effects were so good, and this particular house wasn't that tightly built. Of course Jim, my bro, was completely blinded by the light, gropping for the power plug and hoping he didn't find more than he bargined for (Don't go into the light Jim!).

By the time the family poured into the bedroom it was over of course. Since it was officially my Dad's job to give him hell I left, shaking my head. I had showed him xenon circuits using 9V to high voltage convertors, the tube would flash discharging a cap. He was trying to better that concept.