Trouble with Dimmer-Based Circuit outside of the U.S.

February 19, 2014

Tod from the U.K. said:

I have noticed that my dimmer anodizer won’t go below about 35v, even if I go above then come down. Would a different wattage bulb help do you think? Or is it down to the dimmer?

My discussion and solution:

I used to have trouble getting the dimmer to work below about 15 volts, coming from 110vac. So trouble below 35v at 220 vac seems about right. And most of the world has 220 instead of 110 vac.

One solution may be to modify the original circuit (discussed here) with a voltage divider: Put the two light bulbs across the capacitor with a tap for the output voltage in between to get at lower voltages more easily.
Voltage divider

The output voltage will then be Cap Voltage x R2/(R1 + R2).
But bulb wattage is counter intuitive here:

  • Power (watts) is proportional to the inverse of the resistance.
  • Also, the actual bulb resistance depends on the brightness of the bulbs. But we can pretty much ignore this because we are measuring only the output voltage.

So using P for the wattage (rated bulb power) the good-enough formula is

V= Vcap x P1/(P1 + P2)

Simply, if R1=R2, then P1 = P2 and it comes out half.

But to get even lower voltages, try 100 watt R1 and 200w R2 to get V = Vcap x 100/(100 + 200) = 1/3 x Vcap
Play with the ratios until you get the range and stability you want.

And note that the output resistor is still there between the cap and the electrodes; we just moved it to the other side of the original dimmer load bulb to put it in series.
Thus the smaller the R1 wattage is, the slower the anodizer will work.

And remember that this only works for filament or halogen bulbs, not CF nor LED bulbs.


Q: How do I choose a resistor when building an anodizer?

September 17, 2012

Daniel asks:

I just built an anodizer as you described, but used an old Variac I got from eBay. I used a light bulb as a resistor across the capacitor, and wanted to replace it with a resistor. You specify a 100 ohm 200 watt resistor, but since you can use different light bulbs, I imagine I could use a resistor with different ohm rating as well. What would be the difference if I used a different ohm rating and what range would be acceptable?

Well, Daniel,

If you have a variac (instead of an unstable and load-determined dimmer) then the only function of the resistor/bulb in parallel with the capacitor is to drain it, to reduce the voltage.
It is only needed when you turn the voltage down. My schematic shows the resistor with a series switch (use a momentary, normally open).
The resistance does not matter much, as it is dependent on the power produced and the time it takes are really the only things of interest.
Given up to 120v, you need to be able to handle the power of whatever the resistance is: (P=V2/R)
So a 120 ohm resistor needs to have a 120 watt rating at 120 volts.
But a 240 ohm resistor needs only 60 watts at 120v. Or 30 watts at 60 volts.
Also, as a momentary device you can use a lower power rating than if it were on continuously. The voltage (thus power) drops quickly, and the time to drain grows with the resistance (V2=V1 x e-t/RC) .

So my design suggests some middle-of-the-road options for resistors that are easy to find at electronics salvage places. And light bulbs are great because their resistance increases as the voltage does, so they drain faster than a fixed resistor. But they break.


Q: Can I make my DC anodizer an AC source?

May 9, 2012

Dwayne asks:

I have a home made power supply that a friend built for me a long time ago that I use for anodizing titanium liners and bolsters on pocket knives I build, I don’t know much about electricity but I’m trying to learn, I just finished putting another unit together by looking at the one my friend made me. It has a variac, a full wave bridge rectifier, a light, a fuse link, a on/off switch, a momentary switch, an ammeter and a volt meter. Is it possible for me to add a switch somewhere to change the electricity coming out of my leads from DC to AC and Back. I’m wanting to acid etch my logo in the blades with the same unit if possible, DC etches the Stainless Steel deep and the AC blackens it. If possible, can you explain how to add a switch and what type of switch I need in layman’s terms, as a lot of the schematics look Greek to me. Thank you for your time.

The short answer is: This is a non-trivial change. One problem is that you need a different type of voltage and current meters for AC than for DC. So either you need high end meters that can sense the difference, or a duplicate set of meters and thus a separate set of output leads.

The simple solution would be to simply add another pair of leads from the variac output (before the rectifier) through another fuse and pair of meters to a separate pair of external jacks or leads that are clearly marked AC. And always make sure that only one set of leads actually leads anywhere or connects to anything. Use a separate 3pst switch (and signal light) to turn on the AC leads (presumably like the switch and light that connects the DC leads).

Basically, you can share the power cord and variac, but everything else would have to be be a separate circuit.


Anode vs. Cathode Terminology

January 25, 2011

A reader named Scott suggested that I may have made an error:

“On your Anodizing page,  point #3 & #4 are backwards. The anode is negative and the cathode is positive. The work goes on the negative side (the anode) and we are ‘anode-izing’

“Just the first few words of each line are backwards.”

His contention is that the “Anode” should be the negative side. I guess that he is familiar with batteries or sacrificial anodes, where the polarity is opposite that of the electrolytic process that I use.

Rather than just calling him “wrong”, I thought that I would explain it here, in case it comes up again:


The anode is the side of an ion exchange that supplies positive ions.

In the case of an electromotive source (like a galvanic cell, “battery”) you would be correct. The immersed source of positive ions into the solution (anode) produces the negative voltage by pumping electrons around the circuit to balance the positive ions lost to the solution. So in a battery, the anode is the negative side.

But in an electrolytic cell, like an anodizing or plating bath, the anode is where the positive external voltage pumps positive ions into the solution. So the anode is the positive side.

For my purposes, I need to bond oxygen to titanium. Oxygen is a negative ion (2-), pulled toward the positive electrode by the external power source.  The anode simply absorbs electrons from the solution and oxygen is split from the water to keep the accounts balanced. Titanium loves oxygen, so sucks it up as long as there is current. Hydrogen (+) bubbles off at the cathode (negative electrode).

Here’s the Wikipedia article on Anodes, if you want to corroborate what I’m saying and follow to even more authoritative sources.


Why add a capacitor to the Anodizer?

August 6, 2008

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.


How can I build an anodizer for UK Current?

March 22, 2008

Another reader question:

I am thinking of making an anodizer based on your anodizer digram. I live in the UK but was thinking that the volts for your electricity is different from the UK’s 240 volts?
If so do you know any diagrams that can help me with this, what I need to change?
My first thought is: In the UK, you can use the same Variac circuit. I wouldn’t trust the dimmer circuit because of the instability at lower voltages.
The U.S. uses 110 vac (150 volt peak), so we use about 3/4 of the range of the variable transformer (less if it is wired to provide over-voltage).
In the U.K, you would just just use 3/8 or even half of the available range.

If you really want to use the inferior dimmer-switch design, you can probably find a simple step-down transformer to cut your voltage in half upstream of the rest of the circuit.

Then came a follow-up:

Thank you for your reply; it was a big help. I am going to be doing the variable transformer one. I have been looking for stuff, but wow its hard to find anything that is needed.

I didn’t like the idea of using light bulbs, so I wanted to get Power resistor 200w 100 ohm but no one sells them, any idea of other Power resistors that I could use?

Try eBay.

I found several by a simple search for 200 watt resistors on eBay. You could vary the search for whatever power and resistance values you want, or use a search to find a good seller, and then ask them if they have what you need.

There are also a wide variety of variable transformers on eBay. But these heavy items cost more to ship, especially internationally.

Before the internet, I always shopped an electronics salvage store in my county. Many cities have at least one of those. Some junk yards and metal salvage yards also have a room full of gizmos that seem too nice to melt down. Call around.

Note: If you are not comfortable rewiring a lamp or replacing an electrical outlet, then you are probably not qualified to build your own anodizer. Buy a read-made regulated 0-150 vdc power supply.


Uneven results from dimmer switch anodizer

March 18, 2008

Here’s another question I frequently get:

I followed your instructions on building an anodizer and I would like to say that you have made a great job illustrating it. My anodizer is the dimmer and light bulb type it delivers a maximum of 160 Volts.I prepared a solution of TSP in distilled water placed the cathode (aluminum foil) and the Ti at the anode ran the circuit. The voltage keeps rising slowly and I get shades instead of definite colors (mostly violet, golden and pale blue). I can’t hold the voltage at a definite value. What should I do to get smooth colors? I tried adjusting the voltage first then immersing the piece but the voltage after immersing is lower than what I’ve just set it to. Please help me out here and thanks in advance.
David S.

First of all, the dimmer based voltage control is going to be a bit temperamental and unstable. But I used one myself for years before replacing the dimmer with a Variac.

Aluminum should work for a cathode, but should be lightly sanded to remove the invisible insulating oxide layer that spontaneously forms. I usually use titanium, but have been told by many that stainless steel works well.

When you have a large capacitor smoothing a the choppy dimmer voltage, the top end will be a bit mushy. The lower voltages are the worst for this effect. The tan, violet and blues are at the low end of the voltage scale.

Another issue in getting smooth colors is getting the voltage everywhere simultaneously. You should have the piece to be anodized immersed in the solution before completing the circuit to the leads. That is, you need a switch to turn the leads on and off, while the anodizer is running at the voltage you want.

Cleaning and chemically etching the metal before anodizing also helps assure a uniform color, and is generally considered necessary for getting the higher voltage colors.

The voltage measured on the leads or capacitor will drop when you start anodizing, and should rise back to your preset voltage in a minute or so. The time depends on how big a piece you are anodizing, how big your cathode is, and on the efficiency of your electrolyte.

Another possible problem might be the material of your attachment to the anode piece. Only titanium or niobium should touch the electrolyte at the positive side. Never use copper wire or regular (galvanized or tinned) alligator clips to immerse your piece. The current will just go though that, and little will be applied to your piece.