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: Testing purity of Titanium Bracelet

January 25, 2014

Sumangal asks:

“I have a titanium bracelet with germanium stones embedded in it. Is it possible to test its genuinity and purity without damaging the bracelet or the metal?”

The term “possible” covers a lot of ground. What came to my mind first was a simple mass spectrometer; part of any electron microscope. You’ll need to know somebody who works for a university or industry that has one, and is or knows a microscopist in residence. But it is a quick and simple test, if the chamber is big enough for the bracelet. Back when I worked in a radiological dating lab, I knew the right people and did such testing on meteorite and geological samples. That was around 1980.

Laser spectrography also can do the job. There are hand held devices that can tell you the exact content of the metal and stones without visibly damaging them. But you still need to know someone with the tool. Here’s an article about how that works at

In brief, both mass-spec and laser-spec ionize surface atoms (chase away their electrons) and as the atoms relax and recover their charges, they give off a fingerprint of colors. Every element has it’s own distinct spectrum. So these gadgets count how many atoms of each element the beam (electron or laser) stimulates, letting one know exactly what proportions are there.

I’ve already listed simpler tests in my post: How to tell if a piece of metal is really titanium, but many of those would leave bigger marks on the piece.

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.

Q: Can I use a Gold Acid Test for Titanium?

August 9, 2011

Tim asked:

I have what I believe to be a piece of titanium that was found on a beach in Florida. I tested it using an acid test kit. The metal tested between 14 and 18k using a gold test kit. Should titanium test like gold, or has my acid possibly gone bad?

I’m pretty sure that the acid test bottles for gold use a variety of blends of acids, with Aqua Regia being the one that actually etches gold.

Titanium will be etched by these blends, as will platinum and a variety of other metals. I don’t know at what level each titanium alloy might test. I’d suspect that common Al6-V4 alloys would result in a lower karat rating than the commercially pure grades (#1-#4).

But the acid test will not tell you that it is titanium, as opposed to platinum or tantalum or niobium.

So I don’t recommend this as a reliable alternative to the suggestions I make in “How to tell if a piece of metal is really titanium.”

Q: What common materials can be used to etch titanium?

August 9, 2011

To niggle the semantics, it depends on what is common in ones world.

Any chemistry lab would have hydrofluoric acid, the fastest way to etch titanium. Its helper molecule sulfuric acid is available everywhere (battery acid or some drain cleaners). The combination of the two makes for a smoother etch, but you’ll have to ask a chemist, why?

I’ve found a blend of oxalic acid (HCO) and sodium bi-fluoride in a grocery store laundry section bottled as a rust remover. This etches the titanium, but can leave a carbon residue, that is easy to remove.

Supposedly, concentrated oxalic acid by itself could do the job. But I don’t see how from an entropy standpoint. Also, there is the risk of carbon monoxide fumes (oxalic acid is carbon-monoxide-acid).

ABF (ammonium-bi-fluoride) is common due to its high-volume use in the nuclear industry. I’ve used this by itself at high temperatures. It behaves like weak hydrofluoric acid; essentially buffered.

The key ingredient for etching titanium is loosely bonded fluoride ions. This means that anything that will eat titanium can kill you if it gets into your system. Some people are sensitive enough that a splash of HF on the skin can kill.

Some other suggestions and cautions are here:

But my usual recommendation is to order Multi-Etch, a balanced blend of sodium-bi-fluoride and ammonium-sulfate, shipped dry and ready to mix:  Visit

Q: Can I anodize titanium with my 12v Power Supply?

June 19, 2011

Christopher asked:

I want to do a large piece of titanium (28″ x 3″ x .5″), around how many amps would I need to push through it, or how long would it take?

I have a power supply that is 12V @ ~19A, could I use this to color my titanium if I just leave it on for a long period of time?

If I color my titanium and dont like it, can I do it again and will I get around the same results? I’m scared that I will try to color it blue and get a terrible result and be stuck with it.

Okay, three questions, but the most critical one is in the title. Titanium colors are voltage controlled. A twelve volt power supply (or battery charger) would work for electroplating or aluminum anodizing, but not for titanium. More precisely, you can get the fingerprint-prone bronzes and deep purple at or under 12 volts. But not any of the other colors.

Because the final color is voltage limited, the current is less critical, in theory.  In practice I find that to reach well saturated colors beyond about 50 volts you need a supply that can support an initial surge of at least 0.1 amps/ This can be done with lower rated supplies by charging a large capacitor in parallel with the electrodes. Your total piece is 184.5, so 19 amps should be enough.

Burrs or sharp edges can have a negative effect on your final color.

If you don’t like your color, you can subsequently anodize to higher voltages, but not lower. The best color results appear on a clean and freshly etched surface. If you overshoot a color, or get a hazy or gray result, the only recourse is to grind, polish, or etch the color off and start over.