I received the following question:
I have a variac and full wave rectifier but no cap.
What is the reason behind adding a capasitor to the anodizer? I know it will reduce electrical ripple but what will it mean to the anodize process or final results?
In principle, the smoother, ripple-reduced output allows more even anodizing starting at the initial surge. Whether this is truly useful, I don’t really know. My experience is almost exclusively with a smoothed DC supply. But I have a switch on my main anodizer to disconnect the capacitor for those occasions when I feel like it.
The voltage will read wrong with ripple. The anodized color depends on the peak voltage. But a rippled current shows on a meter as the rms voltage, that is somewhat lower. So the color is less predictable, and the time spent at that voltage is more critical to watch.
Also, once you reach your final voltage (or at least asymptotically close enough), the smooth DC current is stopped. But a rippling supply still produces a trickle of current as the piece you are anodizing acts as a capacitor. If you wait long enough, you can see the color continues to rise at a fixed ripply voltage.
This latter point is more important if you mask and do a succession of lower voltages for multiple colors. With ripple, the higher voltage colors will creep as you anodize the lower voltage areas.
Another note is that AC is more dangerous than DC. Edison (General Electric) made sure that the first electric chair used the AC current promoted by his rival Tesla (Westinghouse), to popularize that point. (source) But I doubt it makes much difference in any practical sense of anodizer safety.