Why add a capacitor to the Anodizer?

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.

RMS vs Peak Voltage 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.

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3 Responses to Why add a capacitor to the Anodizer?

  1. Tony says:

    Interesting. Resistance increases as the part anodizes and builds the oxide layer?
    I didn’t consider that but it makes perfect sense.

    Conversely, when you mention the difference between peak values and RMS values (given the same supply power) does this mean unfiltered supplies can potentially anodize to higher levels?

    I ask because I have large parts to anodize (polished titanium mufflers) and it may be difficult to achieve the associated current densities.

    Thank you,
    Tony

  2. MrTitanium says:

    It’s not the resistance that changes, but rather the voltage drop at the anode. Specifically across the oxide layer.
    I=V/R, but V = (Vsource – Vanode). The resistance doesn’t change, just the effective voltage.

    It’s not that a rippled voltage lets you get higher colors (in the short term).
    It’s that the rippled voltage displays a deceptive voltage reading.
    If you switch the capacitor in and out at a fixed rippled input voltage, there is about a 15% jump between the Peak and RMS values. But the peak (what titanium cares about) doesn’t change significantly.

    A 10 amp circuit should be enough for a muffler. You might want to use a big light bulb as a current limiter (maybe a pair of 500w halogen quartz tubes in parallel, or a space heater). Or else, watch the current and turn the voltage up to keep it at the limit until you reach your desired voltage.

  3. Jonathan M says:

    AC is NOT more dangerous than DC when you are at the same voltages! Edison was just trying to smear Tesla to win a big contract!

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