Hello, very new amateur here. Most of my experience is with guitar effects pedals, which I've generally had good success with or been able to troubleshoot myself. Recently, I built an AC/DC power supply for salt water etching aluminum pedal enclosures, which I got the "schematic" for from the knife forging community. I wired everything up and tested it out on a piece of scrap aluminum and it seemed to work perfectly for about 5 minutes, until I noticed the machine smoking. It did etch, but now the fuse blows immediately so I'm pretty sure the transformer is fried. I'm used to working with PCBs, mostly passive components and following a more detailed/formal schematic; so I'm not exactly sure where I went wrong or even how to go about troubleshooting this since the transformer is shot and much more expensive to replace than the little transistors and diodes I've killed before.

It's a pretty simple circuit and I'm fairly confident I had it wired correctly, since it did etch the metal, but I could be wrong. I can't see any obvious shorts in the circuit (no exposed wires/components touching). I did use a slightly different transformer than the original did, since the original was out of production, but as far as I can tell from the datasheets, the only difference is that mine was rated at 3A instead of 2A (both were 24v center tapped 0-117 primary 12-0-12 secondary). I'm thinking my problem may have been with the physical grounding of the transformer. I wired the center lug of the secondary side to the ground from the power cable and to a wire which I soldered to the base of the transformer, which is steel. It was much more difficult to get the solder to stick because of some sort of coating, so I ended up filing the steel a bit and eventually got it to stick. My thinking is that I didn't have a good connection and therefore wasn't properly grounded. I'm guessing I should have gotten the multimeter out and checked before just firing it up.

So, my questions are:
1) Would this have caused the transformer to overheat and fry? Or could something else have caused this?
2) If my soldering was to blame, should I have used a different solder (I used either 60/40 or 63/37 tin lead w/rosin core) or another connection entirely? Or just improve my soldering technique? (Never had any issues with PCBs, but the thicker steel chassis is a new material for me)

I am very much early amateur level and I'm probably jumping ahead in terms of the difficulty level of the projects I'm trying to tackle. I do have some books I'm working through and have been able to trouble shoot issues with guitar pedals by reading and looking over schematics, but I know I should probably do a little more homework before getting into these bigger projects. I've tried Google, but it seems like all of the resources I find are about much larger transformers related to building wiring and the knife making community where I found this project has limited electrical experience. Any input, advice, or even just recommendations for resources and teaching myself more about electronics would be greatly appreciated!

 

Your construction and wiring are likely OK.
You need etch power control for the application. Get an old-school two wire Triac light dimmer (a auto-transformer would be better but would cost more) after the fuse between the 110 input of the transformer to control the transformer AC input voltage and transformer output power/current. You need current metering on the secondary side so you don't exceed the output current limits of the transformer.

That transformer is going to get warm in normal operation. It needs cooling (ambient air or fan moving air with the transformer bolted to a good head conductor) external from the box

 

Welcome to AAC!

1) What would cause the transformer to overheat and the fuse to blow? A short circuit. More about this later.

2) The type of solder used has nothing to do with your problem. The only problem I can imagine with soldering technique is inadvertently creating a short with solder splashes somewhere.

Firstly, some comments about grounding.
There is no need to ground the metal housing of the transformer or the secondary winding of the transformer, except for the following reasons. The secondary winding is galvanically isolated from the primary winding and hence from the power line. Usually this is a good thing. However, metal housing and chassis are grounded to the power utility ground for safety. In the event of a short between LINE and GROUND, the circuit breaker should trip in order to prevent having live voltage on the chassis.

Back to (1). The transformer will overheat and the fuse will blow if the transformer is drawing too much current.
There are a number of ways this can happen.

1 There is a short in the primary winding.
2) There is a short between the primary winding and the metal frame.
3) There is a short in the secondary winding.
4) There is a short between the secondary winding and the metal frame.
5) There is a short between the primary and secondary windings.

Disconnect the transformer completely. Use your DMM ane check for shorts between windings and between windings and the frame.

Edit: Having read spook's response, I have to add:

6) Taking too much current from the secondary winding. If you try to etch with no current limit that is the same as creating a short at the secondary winding.

12V / 3A = 4Ω

You need to have a least a 4Ω 50W resistor in series with your output to limit the current to 3A.
Try using a 12V automotive headlamp as a dummy load, i.e. it goes in series with your etching probes.

 

Most of the time,
Electro-Chemical Reactions are highly dependent on the formulation of the Electrolyte used,
AND, at the same time, the amount of Current that is "allowed" to flow.

More care / documentation may be required in the formulation of your Electrolyte,
and some form of Current-Regulation may need to be put into place to better control the Process.

Sometimes the amount of Current allowed is critical to achieving the desired end-result.
.
.
.

 

Gotta check the VA rating of the transformer. Go over that and the transformer will generate magic smoke or simply melt one of the secondary windings open and go dead. Electrolysis is gonna pull a whole lot more amps than most circuits boards see unless it's for a whopping big amplifier. You have to know how much power is needed for the electrolysis and design for it.

 

Gotta check the VA rating of the transformer. Go over that and the transformer will generate magic smoke or simply melt one of the secondary windings open and go dead. Electrolysis is gonna pull a whole lot more amps than most circuits boards see unless it's for a whopping big amplifier. You have to know how much power is needed for the electrolysis and design for it.

+1

Pretty sad that nice transformer was sacrificed to the magic smoke god. There are transformer types better suited to this sort of duty than the types designed to power electronics.
https://www.foster-transformer.com/products/survivor-class-2-transformers/

 

reading the label on the transformer:
60hz 117V 0.4A max
CES 67-1243 (SS4-24C3A)
Class A (105 degree C) insulated
1.5A 0V CT 1.5A
Sec 12V 3A (36VA) total at CT

This transformer's 24 volt AC output is only rated at 1.5A. The 3A "rating"
is at 12 volts using both sides of the center tapped secondary.

Still there should be some current control and/or limit of the current in the circuit.

 

Strictly from a wiring perspective - don't ground the transformer. It's not necessary. Second, you don't need a bridge rectifier if you're going for 12 volts. If you want 24 volts - you showed 12-0-12 then you'll need a bridge. Your DPDT switch - you show it as being AC on one side and DC on the other side. That's an error. This is the way you'd want to draw a schematic. With 12-0-12 you want only two diodes unless you want a +12 - 0 - (-12V).

Here's an easier to follow schematic:

When current is moving (let's call it) clockwise the center tap is negative and the upper diode is positive. When the current reverses and moves counter clockwise the center tap is still negative but the bottom diode is positive. You can switch the DPDT switch and your probes can either be Pos & Neg or Neg & Pos.

Your diagram has a lot of errors. Would have to sit down and break it out just to show you the error. Rather than go that route I've banged out this drawing. Should make it a lot easier to understand. You CAN ground the transformer if you feel it's important, but the secondary is completely isolated from line voltage. Also, notice the two different ground symbols: The three barred triangle is Earth Ground whereas the empty triangle is Common Ground. The two have nothing in common other than the fact that you don't need to draw out all the return lines, which makes a schematic much harder to follow. Use of the Common Ground makes that a whole lot easier. There IS one more ground, it's called a Chassis Ground. Looks like a rake with three forks on an angle. Chassis Ground and Common Ground are related but do not mean the same thing. Chassis ground is the zero volt point anywhere on the chassis whereas common ground can be floating above or below chassis ground. Don't get confused with all the particulars just yet. Learn the basics and you'll be well on your way to understanding how things work and how to draw them out in a universal language. Well, mostly universal.

One last thing: You can put a capacitor between the point where the two diodes connect and common ground. I didn't draw it. But doing so would give you a smoothed out DC signal. There's a lot to learn there as well. The circuit above will block all negative going sine waves and present only the positive going sine waves.

 

Thank you all! This is all very helpful and informative! I’ll have to take some time to digest all of this and make sure I understand everything. It's definitely answered some questions, but also brought up some new ones, as well. In the meantime, I got the DMM out and the transformer is for sure toast, though I will mount it on my workbench as a reminder that electrical hubris will invoke the wrath of the magic smoke god! And I'll definitely be spending more time learning the basics and how to apply them before I jump into any other more advanced projects.

This also got me thinking about another reason I think my machine failed where others have had success using this same build without any type of current control/limiting. Most of the knife making community is using this for small maker’s marks/logos and use a Q-tip or some other small etching tool connected to the negative lead and dipped in the electrolyte solution and etching in small, quick bursts. On the other hand, I went for a larger, full graphic and used an electrolyte bath with a much larger piece of copper connected to my negative lead. Am I correct in thinking that the larger surface area of my etching area and the negative electrode increased the current in addition to putting that stress on the transformer for 5 straight minutes?

Thanks again, I really appreciate all the responses to this!

 

There is one GLARING ERROR in the connections, That was connecting the center tap of the transformer to the power source green lead. That was an error because if the etching tank is also in contact with the power ground, which is probable, it puts a hidden shorted circuit across half of the secondary. And it is a shorted circuit you will not see when checking the assembly for errors. It is also the sort of thing that will allow the system to function even as it is overheating towards destruction.
So while connecting the framework of the assembly to the supply ground wire is a good choice, connecting any part of the low voltage section to the safety ground can lead to damage and destruction.
In addition, showing a wiring diagram instead of a circuit schematic drawing makes analysis and evaluation much more difficult, and it certainly tends to hide errors.

 

There is one GLARING ERROR in the connections, That was connecting the center tap of the transformer to the power source green lead. That was an error because if the etching tank is also in contact with the power ground, which is probable, it puts a hidden shorted circuit across half of the secondary. And it is a shorted circuit you will not see when checking the assembly for errors. It is also the sort of thing that will allow the system to function even as it is overheating towards destruction.
So while connecting the framework of the assembly to the supply ground wire is a good choice, connecting any part of the low voltage section to the safety ground can lead to damage and destruction.
In addition, showing a wiring diagram instead of a circuit schematic drawing makes analysis and evaluation much more difficult, and it certainly tends to hide errors.

Thanks, I'm guessing this is why the fuse didn't blow and the machine just kept chugging along to it's eventual death. And, yeah, the diagram is not good, but that plus a Youtube video (made by someone with probably the same or less knowledge than me) was my guide for this build. This experience has definitely taught me that I need to gain a better understanding of circuits and components before attempting other projects like this.

 

In the "ElectroChemical*" diagram the DPDT switch seems to be labeled AC & DC and the wiring in that
diagram seems to implement this by switching the output before or after the DPDT switch. In that diagram
the transformer is used as a 24 volt transformer, the ground to the center tap is misleading (and possibly
a problem as mentioned).

 

the larger surface area of my etching area and the negative electrode increased the current in addition to putting that stress on the transformer for 5 straight minutes?

Yes, electrochemical processes like plating and etching consider the current per unit area, more area takes more current.
The distance and conductance of the liquid also effects the current.

 

Thank you all! This is all very helpful and informative! I’ll have to take some time to digest all of this and make sure I understand everything. It's definitely answered some questions, but also brought up some new ones, as well. In the meantime, I got the DMM out and the transformer is for sure toast, though I will mount it on my workbench as a reminder that electrical hubris will invoke the wrath of the magic smoke god! And I'll definitely be spending more time learning the basics and how to apply them before I jump into any other more advanced projects.

This also got me thinking about another reason I think my machine failed where others have had success using this same build without any type of current control/limiting. Most of the knife making community is using this for small maker’s marks/logos and use a Q-tip or some other small etching tool connected to the negative lead and dipped in the electrolyte solution and etching in small, quick bursts. On the other hand, I went for a larger, full graphic and used an electrolyte bath with a much larger piece of copper connected to my negative lead. Am I correct in thinking that the larger surface area of my etching area and the negative electrode increased the current in addition to putting that stress on the transformer for 5 straight minutes?

Thanks again, I really appreciate all the responses to this!

.
Metal Surface-Area, ( in square-inches ),
and the physical distance between those Surfaces,
will play a huge roll in determining how much Current will be "DEMANDED",
at a given, fixed, Voltage.

A Current-Regulator will reduce the Voltage available to a lower value to
keep the Current at a preset-level.

You need a HUGE Transformer.
If you're on a super-tight budget,
a rewound MOT (Microwave Oven-Transformer), will get the job done,
But there are much more elegant options.
.
.
.

 

It sounds like the diagram I followed is worse than I thought, so I thought I'd clarify what the diagram is supposed to achieve. The goal is to use the DPDT to switch between AC (for darkening the etch) and DC (for the actual etching). My understanding was that was the purpose of the bridge rectifier (converting AC to DC), while the transformer simply steps the voltage down to 12 (or maybe it's 24 in this case, as some have pointed out). Sorry if the poor diagram caused any confusion/difficulty assessing the problem.

Additionally, if what I'm trying to do is on a much larger scale than I realized (size of transformer/current demanded), I might be better off going with the method of using a small electrode and etching small areas at a time, though some type of current regulation seems like a good idea regardless. Most of the enclosures I'll be etching are between about 12 to 18 square inches. I liked the idea of masking off my stencil and etching the whole thing at once, but my expectations might be overshooting my means. It would definitely take more time to do the etching using the Q-tip method, but this is purely a hobby and I don't have a huge budget for high end/large components. Basically, I don't want to half-ass it, but I'm not expecting a professional grade end result, either.

 

If you limited the current and did the whole thing at once the current per area would be low and I'd expect the action to
be slow. So perhaps just more time? Hours? Days?

I'd also watch out for non-uniform paths/distances in the liquid which would lead to non-uniform etching over the surface.

 

Ehe one critical item that is missing is the ammeter that would allow you to know the actual current. At least one plating handbook includes amps per square inch specifications for electroplating. That was in a plating handbook I borrowed and returned 30 years ago, I don't even recall the company that sold it.
AND for any future yootoob projects, you can post the circuit here and ask for comments on does it seem OK. That source includes a whole lot of poor ideas with faked demonstrations.