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published: Monday 5 December 2016
modified: Monday 16 April 2018
author: Hales
markup: textile

Diodes part 2: Homemade aluminium diodes

If you find any of this text dry then just skip down to the pictures

Previously I’ve covered making copper diodes at home. Unfortunately this type of diode is almost as unwieldy and fiddly as using a cat-and-whisker detector. It’s a far cry from something that would allow me to easily make diode matrices on a PCB.

Traditionally we see semiconductors as being very expensive and difficult to make. Who could blame us? They all come from expensive cleanroom fabs that use multi-million dollar equipment. Heating a pizza in one of those chip ovens is industrial sabotage. How could you ever hope to get close to that level of control and precision at home?

A special thankyou to everyone who commented on the Hackaday article covering my last post about this. A lot of you suggested great ideas to investigate and I have been going down a few of those paths because of you.


Copper diodes look like a dead end

Everything I’ve read so far suggest that copper diodes were commercially made by heating copper up to around 800 to 1000 celcius. This would build up a couple of oxide layers and you would chemically strip the undesired outer layer.

Although I can easily get copper to that temperature, getting it to that stage whilst it’s on some sort of board or PCB is very difficult. I don’t have many materials at hand that can survive 1000 degrees, let alone ones that would be suitable as a base material and also easy for other people to get their hands on.

Whilst researching copper diodes I can across this interesting document:

By Thomas Mark Cuff, Engineering Department of Temple University
“Luck sometimes visits a fool, but never sits down with him.”

That quote bodes ominously.

Anyway, this doc covers some history of the copper diode, including some discussion about how copper sourced from Chile seems to work better. But in its historical overview it mentions ‘wet type rectifiers’:

At its simplest, a wet type rectifier consisted of, for example an electrolytic cell composed of an aluminum plate and a lead plate dipped into a solution of water and borax. When the aluminum electrode was positive (anode), oxygen was evolved at its surface producing a thickening of the alumina (Al2O3) passivation layer, which due to its insulating nature would prevent any current from flowing; the passivation layer rapidly dissolved when the aluminum electrode was negative (cathode), thus allowing the current to pass unimpeded through the cell. It was this growth and dissolution of the insulating alumina layer which gave rise to rectification in the wet type rectifiers.

And so the diversion begins.

Electrolytic capacitors

The paragraph continues:

Though wet type rectifiers are long gone, their direct descendants are still with us: aluminum electrolytic capacitors, which depend for their proper functioning on the electrolytically ‘formed’ insulating layer of alumina on their plates. These capacitors are, of course, polarity sensitive, if put in the circuit backwards the alumina film will dissolve resulting in the shorting out and possible destruction of the capacitor, due to overheating as a direct result of increased current flow.

Indeed, this makes sense.

If you use an electrolytic capacitor forwards: it slowly starts to block your current flow as it charges up. A tiny electrolytic capacitor blocks it almost right away.

If you use an electrolytic capacitor backwards: it looks like a short.

If you want to use one both ways: you wire two in series, facing opposite directions. ‘Non polarised electrolytic capacitors’ do this.

Baking powder versus Baking soda

Some sources suggest that you can use baking soda or baking powder instead of borax as the electrolyte.

I have a hunch that ‘baking soda’ and ‘baking powder’ are sometimes used interchangeably and sometimes used to mean different things. Later I’ll show some investigations into using both of them.

When I use the terms on this page:

Making and testing

Paper-separated construction

To make an electrolytic capacitor (read: rectifier) you need:

Ideally you don’t want to use aluminium on the other side as then your device will develop diode characteristic in both directions. From what I’ve read you can use most metals. I suspect that actual aluminium capacitor manufacturers use different metals or at least different aluminium alloys on each side, but I may be incorrect.

In my case I used:

Aluminium: A tiny strip of aluminium foil cut with scissors.

Electrolyte: Water and baking powder mixed together. I soaked a small piece of paper towel in this solution and use it to both hold the electrolyte and to seperate the two metals.

Other conductor: copper pads on a copper PCB.

I also mixed up some conductive glue using powdered graphite and some clay. The clay was plain white stoneware, but anything should be fine. The graphite was from a bottle (you can get this for lubricating locks), but you may have success rubbing a pencil on a file instead.

Use as much graphite as possible and as little clay as possible. If you don’t have enough graphite in your mix then your joints will measure hundreds of kilo-ohms. I was able to get this down to the few Kohm region. My clay, even when wet, had very poor conductivity on its own (which was useful for the paste diodes I show later).

First up I prepared pads on a bit of copper-clad board to make the diodes on. They were seperated by cutting lines with a hacksaw.

I then starting laying the components onto this PCB:

Here you can see the little pieces of soaked paper towel and the aluminium strips. I found that sitting the aluminium strips onto the towel led to poor contact/adhesion. If you fold the towel over the aluminium then it holds together much better.

On the other side of the diode I simply needed to connect the aluminium foil to the pad. This is where the conductive glue came in:

The diode will work like this, but for it to last more than a short time you need to stop the electrolyte drying out. I originally tried covering everything in hot-glue:

Unfortunately this didn’t work well:

I need to find a better way of doing this.

The next step is to ‘treat’ the diode. By passing DC through the diode in the direction that it should be blocked you build up an aluminium oxide layer. Effectively you pre-charge the capacitor. This is useful for testing if the diode works — ideally the diode pre-treats itself once in-circuit.

I used 36V and a 1K resistor to do this step. You put the positive terminal of your power supply onto the aluminium side of your diode.

Finally I hooked the diodes up to a curve tracer. I used the circuit from my previous post. If you don’t understand these next diagrams then that page will help explain things.

Lo and behold:

5V per horizontal division, 5mA per vertical division. As you can see the turn on voltage was about 5V, which is much higher than the usual silicon 0.7-ish volts.

Unfortunately this behaviour rapidly degraded. Over time the reverse breakdown voltage (seen as a little turn on the right end of the graph above) slowly moved inwards. After a few minutes you would start seeing this:

The diode is still a tiny bit better forwards than backwards, but it’s by less than a factor of two, which is not good.

When the diodes were useless a day later (from drying out) I pulled the hot glue off. This picture reminds me of when people tear chips apart when removing heatsinks:

I also didn’t have any success using ‘baking soda’ instead of ‘baking powder’. The diode would not block current and the copper would leech into the electrolyte:

Electrolyte tests

What is this baking powder anyway?

In Australia the ingredients of all food products are listed on their packaging (with some exceptions, such as single-ingredient products). Importantly they are ordered depending on how much is used in the product.

In other words: this baking powder is mostly made out of rice flour. Time for some investigation!

Testing a variety of electrolytes

Process: try different metal combinations and current directions, see which electrolytes block current (ie act like a capacitor/diode). Again I was using 36V through a 1K resistor.

I didn’t have luck with a curve tracer in these tests: the capacitance was too large, giving me curves like this that you had to mentally average to get rid of the loops:

As such it appears these diodes are too slow to be greatly useful at 50hz operation. YMMV, especially with different diode sizes and loads.

I tested all of the chemicals shown in the photo above:

The results showed that baking powder, soda and borax all worked. Salt acted like an almost dead short (compared to the 1K resistor) and everything else was unexciting.

I tried using aluminium, copper and graphite as the second terminal across all of these electrolytes. Using graphite or copper as the second electrode both had similiar results. Bubbles always seemed to emanate from the negative electrode, so the electrolyte (and possible electrode) is being consumed during use, which may cause issues later.

But why was baking soda all of sudden working in this test when earlier it did not?

I tried to repeat this test by mixing a new batch of water and bicarb soda.

It failed.

I tried again with ‘de-ionised’ water instead of tap water. Same result.

Current would start to drop as if the setup were acting as a diode/capacitor, but it would give up after a short while and start rising again. If I left the setup unconnected for a few minutes then it would work again for a short while before failing.

It turns out that I have to mix soda bicarb and water, put the aluminium in and then leave it sitting for a day. After this it seems to work well. I did this accidentally in earlier tests where it worked, and I’ve confirmed it again today. I’m not sure what’s happening here: perhaps other interfering materials in the water are slowly precipitating out or otherwise escaping/changing. Perhaps the aluminium is dissolving into the electrolyte and saturating it.

Anyway, it’s something to try if you have similar issues.

Paste diodes

Manufacturing the paper-seperated diodes was fiddly and error prone. It required tweezers and the tiniest shake of the hands would ruin the diode.

Ideally I want something that can be ‘dabbed’ or ‘printed’ on to make diodes. Ie not require fiddly assembly.

Something like this:

The white paste is a mixture of clay and baking powder. The grey is a mixture of clay and aluminium filings. I rubbed a piece of aluminium against a file to get these. The red is just electrical tape to make the gap wider (the copper pads are seperated beneath it, as per earlier).

The performance curves were similar to earlier and did not seem to degrade as easily, but the overall resistance was much higher. I’m going to investigate further with these to see if I can get them to work better.

Also: these diodes still need to stay wet to work. Putting a drop of water back on a dry one does seem to work, but you have to be careful not to put too much (and bridge the copper pads).

Ending notes

I’m not quite sure what to make of things. I’ve gone much further than I expected. Although I’ve not exactly discovered anything new, I’ve had some success with making up my own manufacture methods. Soon I hope to make a diode matrix for 7-segment display driving. That would be fun to achieve.

Obligatory “why don’t you just buy diodes, they’re so cheap?”. Because that’s not fun. And you don’t learn anything new or discover anything new.

Have some fun, and post if you have some thoughts, ideas or if you give things a go. Unlike last time: no login is required to post.

Paul - Friday 29 September 2017

Crazy, pointless, and very cool.

I always wonder what we would do and how far we could go in terms of rebuilding what we have if technology were suddenly pushed back a millennium. Caveman semiconductors.

Hales - (site author) - website - Friday 29 September 2017

Hey hey hey, I'm not done yet! *puts down club*

Make sure to read the third part, where I get closer to making something useful with these:


I have not had a cent of time to do work on this site or my various experiments over the past few months. I've picked up some casual work as well as a full-time uni load.

Eventually I'll get back to doing more crazy things :D

Hamona - Sunday 8 March 2020

Great job. I will try it. Thanks

Magster - Friday 5 June 2020

Sheez! you lit up my brain when you said you can a symmetric electrolytic by facing them back to back! I never thought of that! (faceplam!)

Once I worked out 5cmx5cm will only give you 20 picofarad capacitance at a millimetre spacing of the electroplates, I was stuck at a brickwall that gaining capacitance is alot harder than I thought.
Then I looked up this guy Robert Murray Smith on the internet and he was showing unbelievable results for 2cm^2 using some carbon foam stuff - getting some kind of volumetric area for some horrible amount of multiplier for a dielectric permittivity constant, quite unbelievable results for me considering me reading the capacitance budget equations!
So I got the crazy idea to get some kind of nanoetching done with electrolysis, and I ended up with a funny acting "slow reflex" diode, very similar to what you seem to be getting here.
It seems like its charging up, but it doesnt seem to be drawing any amps from my power supply as it deactivates on one side and just passing through the other.
It may be completely useless, but maybe this is what making your own components from scratch is like for alot of us. (Im not the only one.) And if I turn the volts up and use it, it seems to "blow" it, and it just starts passing current both ways, as if it were some kind of tiny of capacitance which has had its charge speed slowed down like its thru a resistor.

My next experiment is going to be trying copper sulphate, and see what copper travel from the negative to the positive is going to be like, and im going to mix it in with PVA, and after the electrolysis im going to let it the glue dry out to a dielectric before I test it. (its on the way to my door, in the next couple of weeks hopefully.)

One more thing to say, if you managed to make all the passive components from scratch, then you can pretty much make computers/tvs/multimedia devices without buying a single component and it makes alot of sense, you could start up your own GIT-CO company and put out products, and stop china from taking over. (but Im pretty sure America's helped the China's takeover of the planet, in an ironic way.)

Patrick - Tuesday 16 March 2021


Hi there. I have been searching for a way to easily rectify the output current of my " welding contraption" made simply by using two microwave oven Transformers in series with 9 - 11 turns of around 8 gage wire as the secondary coils instead of the originals for high voltage. ( I also made the heavy wire using the focus coil out of a crt I found in the dump- everything must be made from scratch is the theme here. 16 strands of coil wire making a bundle is what I used for the secondary coils. I didn't even insulate the bundle! Just stuffed it into the transformers with tape around the inner walls and corners of the metal form and packed paper into any gaps left over. I ran low on wire when wrapping the secondaries, and consequently one transformer has 9 turns, and the other has 11. Since they both oscillate together from the same source, I decided to find out if they would emit showers of sparks when I plugged them in but they hardly even heat up after going through one stick, so there's no arguement there, and possibly there's even some benefit to having one putting out more current and one keeping the voltage a tiny bit higher. It's a little bit tricky getting the strike so more voltage for the start would be another addition to think about.

I was really surprised that it worked though with all the many variables and slap and dash craftsmanship! It will maintain a stable arc and will run a good bead on heavier steel plate, but it's too hot for light gage material so it becomes necessary to add metal with the other hand as you would with oxy/acetylene tourches.

I'm thinking that by rectifying the current it would have less spattering and better penetration, while retaining more of the steel from the rod. I bought e6013 (5/64") rods with the idea that it likely wouldn't have the current to do much anyway and those are the second smallest I found, so a thicker rod is in order also, but it would be nice to be able to make it DC and be able to adjust the current.

I knew that aluminum oxide acts as a diode when allowed to build up on a plate submerged in a borax or sodium carbonate solution ( concentration unknown and not baking soda either, carbonate and not bicarbonate), and subjected to alternating current. The only other bit of info I recall is that ham radio enthusiasts used a technique involving aluminum in solution to rectify current. A search online yields lots but none seem to really satisfy what I'm looking for exactly. (The link above is a paper from 1952 I found that I think may be of some interest to you since it describes what really goes on between the oxide layer and the aluminum that causes rectification. )

Anyway, I felt compelled to share with you the hillbilly welder and some thoughts on rectifying current without buying any heavy duty rectifier diodes. RadioShack is all but caput and I don't feel like waiting for the mail lol.

The next thing I am going to try is using a copper cathode and aluminum anode in strong electrolyte solution. Having the copper, a half inch diameter pipe on a plastic swivel arm, so I can adjust how much of it gets submerged in the electrolyte- this should act as a variac of sorts I hope. I'm not sure if the rectification should be before the transformers, in an attempt to provide them with pulsed DC, or after with all the heavy current boiling the electrolyte as wasted heat energy. Probably I'll end up with a steam pot with deadly consequences for any living thing that comes in contact with it. Call it the scaryac! But if it works to rectify the current and makes it possible to turn the heat down, then this little contraption will actually be a useful tool made from junk.

I'm interested in any thoughts you or your readers may have.

Hales - (site author) - Tuesday 16 March 2021

Thanks for the link Patrick. I think I came across this Hokkaido article a few years back, I'm not sure. Quick summary: attempts to build mathematical models of electrolytic rectifier behaviour, delves into theories of electric double-layer ionic flows and other such craziness to explain certain electrical & physical behaviours. The mathematical arguments go way too in depth for me to follow, but the theories on ionic flows are particularly interesting.

I suspect you will have troubles using these electrolytic rectifiers as a welder rectifier. From what I gather: you have wound your transformers to provide a low-voltage and high-current source (ie very low impedance). These electrolytic rectifiers drop around 5V min and could be considered a "moderate" impedance, depending on your point of view, which probably means a lot of the welder's power will be wasted in the rectifiers.

Buying some big ass rectifier packs is probably the "correct" solution. Not sure how much they cost. You might be able to buy big old thyristors and trigger them as a diode instead, those might be easier to get odds-and-sods of old big bolt-style units off eBay maybe?

> I didn't even insulate the bundle! Just stuffed it into the transformers with tape around the inner walls and corners of the metal form and packed paper into any gaps left over.

Lol insulation probably isn't required from a direct contact safety point of view (it will be low voltage output from what I gather), but it is useful for preventing shorts/arcs that lead to fires and meltdowns. Look up stories of technicians working around large low-voltage lead-acid battery banks, namely what happens when they accidentally drop a spanner across the bussbars. I believe a lot of low-voltage low-impedance places now either recommend plastic tools or short handles (mini-spanners) to avoid this problem.

Mind you, full shorts are basically what you're using to weld anyway, so my concerns may be misguided.

> The next thing I am going to try is using a copper cathode and aluminum anode in strong electrolyte solution.

If you put AC across this: the copper will dissolve and the solution will become conductive. AKA the electrolyte will become "poisoned" and it will no longer have diodic behaviours.

EDIT: did you just spell it 'aluminum' instead of 'aluminium'? Get off my blog!

> Probably I'll end up with a steam pot with deadly consequences for any living thing that comes in contact with it. Call it the scaryac!

Yep :D

Not really a variac, more like a variable resistor with intrinsic liquid cooling (TM). Just don't tell anyone that, scaryac is a better name.

> It will maintain a stable arc and will run a good bead on heavier steel plate, but it's too hot for light gage material so it becomes necessary to add metal with the other hand as you would with oxy/acetylene tourches.

Nice work. What wiring arrangement do you have for your primary & secondary sides of your transformers? With a bit of re-arrangement you may be able to get lower power outputs.

Eg I assume you have your primaries wired in parallel to each other (ie both getting full mains voltage) and your secondaries in series with each other. If you re-arranged to have both primaries in series then you would halve your output voltage. This could then be wired to a super-beefy DPDT mains-rated switch to change between the two wiring arrangements.

Whatever you do: don't wire your secondaries in parallel! The different winding coun AND the low impedances will together probably spell self-welding/immolating doom.

Whilst I'm here: it's worth mentioning part 4 of my blog posts which has my whole (shoddily written) thesis attached, complete with lots of pretty pictures and amusing reviews of other people's unreliable papers about electrolytic rectifiers. https://halestrom.net/darksleep/blog/043_diodes4/

Patrick - Thursday 18 March 2021

Yep, there's no way that a simple electrolyte will carry the necessary current. It sparks and will evolve banggas from the solution. It works well enough to play around with and show people that there's lots of interesting stuff to make and re purpose from out of the dump. In a pinch, someday maybe I'll be glad to know how easy it is to make a serviceable stick welder from a couple old transformers. I have a Lincoln mig welder already but I love to tinker around so the big pull for me was to make it entirely from things gleaned out of junk.

I still may try messing with the transformer cores just a little to change their coupling via an air gap. I recall that old stick welder that my dad had long ago had a lever that would move a plate or something mechanically changing the output. That could possibly be the solution for something like the MOT welder, I'll call it; complete with a c- clamp holding the two halves together. What's a little thin plate of plastic stuck in between those now?

Yes, you are correct in that the primaries are the originals and in parallel, and I ran the secondaries with one wire. It just stops twice to circle two fire hydrants checking its P- mail on the way to the other end lol. I'll definitely check out your blog part 4 on electrolytic rectifiers.

I can only imagine the scene at even a modest sized battery bank when someone drops a wrench across the bus bars! So yea, welding is essentially just controlling a direct short and it's safe so long as you are not a better path than the electrode is! Poor wrench...

Have fun! I need to think up a way to get rid of some gophers now- maybe two hho generators would produce enough banggas to fill the tunnels depending on the permeability of the soil...Bang!..hate to use up my propane and O2 on gophers.

Mathew Timu - Monday 26 April 2021

I took a d cell 1.5 battery that was old and useless that I found laying around the house and thought that I could use the carbon rod Inside of it to be used as a rectifier diode for a crystal set radio I also used steelo wool that you wash dishes with and connected them in place where the germanium diode would go the carbon rod on the cathode side and the steelo wool as a cat's whiskers on the anode side,they have to be just touching one another for it to work after a lot of probing your eventually get a hot spot where it's able to detect and rectify radio signals and turn them into audio sounds where your be able to here a radio station with music or talk back broadcasting network here's an idea use a crystal set radio to help you with figuring out how to make your own home made diodes just as I did

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