Need Opinions/Advice on Niche Power Supply

[tkoham]

Hey everyone! New here, but I've lurked for a long time. I need some help with a project I'm planning on turning into a limited-run hand-built product for a niche market. It's a one-off tattoo machine supply I made a while back that several friends and coworkers have expressed interest in buying. I've outlined my design rationale and questions below. Here's the schematic I drew up from my prototype:



The aesthetics of the power supply are as important as its function, So I've opted to use some large case mounted BJTs and an analog panel meter.

Other than that, the design requirements are as follows:

• Linear
• Variable output with functional range between 3-18v (wider is fine)
• 4.5a Peak power, 2a continuous
• Light enough to mount with magnets

The reason I'm seeking advice is that I have little formal education on electronics and design. While I feel I have a good foundation from years of building simple electronics for my own use, there may be factors that I haven't considered in my design that could potentially cause problems for others. This would be the first thing I plan to sell, so I want to make absolutely sure it's safe and functional long-term.

My schematic may not be the neatest (again, self taught, sorry) so here are some clarifying notes in case I've left something ambiguous:

• the first jack at the output is for a remote switch that runs the load
• The Listed transistors are all NPN BJTs, the 2 in parallel are the ones that'll be case-mounted externally.
• The zener across the output is a 22v 5w which seems to be just above the top range of the prototype
• EDIT:yes there is a switch and fuse on the hot of the actual device, i forgot to add them when drawing the schematic

I've also got a few questions that no amount of googling seems to answer:

• Are there off-the-shelf AC supplies that aren't wall warts (have a cord on both the input and output) and if so where can I source them?
• I've found a panel meter that I'm probably going to use, but I don't want to fabricate a faceplate for it to make it look less cheap if I don't have to. Do any of you know where I can reliably source small, metal-front analog panel voltmeters that don't look cheap?
• is there a reason to use more sophisticated power filtering if your load is reversible and doesn't have any sensitive components?

Any Criticisms or notes welcome. Thanks in advance!

[nctnico]

I'd change the transformer for a AC-DC converter (open frame power supply or board mounted). These aren't very expensive and much lighter.
I have a feeling this PSU will be prone to overshooting at switch-on and switch-off. What kind of jacks do you intend to use? And where is the current limiting? And there is no sensing of the output voltage so the drop across the 2N3055s and series resistor isn't compensated. C9 doesn't make sense because it blocks DC to the meter. Is this  a circuit you found online somewhere? I suggest you look for a better circuit which someone has actually build and tested thouroughly.

[tkoham]

C9 doesn't make sense because it blocks DC to the meter.

Oops! yeah that's not supposed to be there, it's not on the actual prototype, I must've copied it over accidentally while laying stuff out. Updating schematic with switch and this accordingly

I'd change the transformer for a AC-DC converter (open frame power supply or board mounted).

I wasn't aware they made discrete linear modules for this, thanks!

Is this  a circuit you found online somewhere? I suggest you look for a better circuit which someone has actually build and tested thouroughly.

No, like I said, built something similar for personal use, Trying to improve it because several friends have expressed interest in purchasing one. It hasn't exploded or harmed any tattoo machine thus far, but you may be right about the overshoot. I'll look into that.

What kind of jacks do you intend to use?

Mono 1/4" TS Audio Jacks. Not my choice, it's standard in tattoo-land

And where is the current limiting?

I was under the impression that the LM723 was doing current limiting, when I lower the trimmer pot @RV2 the load reads less current draw. It might be coarse or flawed in some way I'm not aware of though.

Thanks for the input so far, you've been very helpful.

[mariush]

You could just get a single LM1084 linear regulator : https://www.digikey.com/en/products/detail/texas-instruments/LM1084IT-ADJ-NOPB/363557

Can do up to 29v in, and up to 5A out , as long as it's cooled enough, and only needs around 1v above output voltage to regulate properly.

You could use a transformer with two taps, and switch between taps using a mechanical relay when the output voltage is configured above some threshold ... ex when you want more than 12v output, change tap from 12v ac to 24v ac
 
You could limit current with a second LM1084, or you could use a  LM1085 which is cheaper, but limited to 3A (you say 2A but peak 4.5A)

[nctnico]

Isn't a tattoo machine basically a DC motor and what you want is speed control? If yes, then using PWM to regulate the motor speed is much simpler and more energy efficient. Still there has to be some current limit and thermal shutdown but a modern PWM controller chip has that built in. If you still want to go linear then the chips suggested by Mariush are a good choice but you have to do the math on the cooling to see if you can get maximum current at the lowest voltage with a real world heatsink.

When dealing with any kind of power electronics I want my circuits to have 4 safety features: a real fuse, current limiting, thermal shutdown and overvoltage / reverse voltage protection.

[tkoham]

You could just get a single LM1084 linear regulator

I know the discrete BJTs aren't really necessary, but like I mentioned in the OP, a lot of the interest I got from the market I'd be selling in was in the aesthetic quality of the device. Tattoo power supplies are as much a fashion statement as they are functional, and the two cans bolted to the top of the case add to the visual interest. I suppose I could buy dummy cases, but that seems disingenuous.

Looking at some of the examples, it does seem like the LM1084 easy to implement for current limit/protection though, thanks.

Isn't a tattoo machine basically a DC motor and what you want is speed control? If yes, then using PWM to regulate the motor speed is much simpler and more energy efficient. Still there has to be some current limit and thermal shutdown but a modern PWM controller chip has that built in. If you still want to go linear then the chips suggested by Mariush are a good choice but you have to do the math on the cooling to see if you can get maximum current at the lowest voltage with a real world heatsink.

When dealing with any kind of power electronics I want my circuits to have 4 safety features: a real fuse, current limiting, thermal shutdown and overvoltage / reverse voltage protection.

Well, yes and no. There are machines that use solenoids, electromagnets and springs in various arrangements, BLDC-based devices with integrated controllers, and a few other designs.  WRT the SMPS approach being more efficient, I'm painfully aware, but linear power supplies are favored for their performance characteristics, perceived or actual.

There's a real fuse on it, it looks like the lm1084 should be easy to implement for current limiting looking at some example circuits, that's probably what I'll go with. You only draw peak power on most tattoo machines when you're using fill needles, which need to be run higher to get clean punctures. Most cheap to mid-end supplies only claim 2A output, most rotary (Brushed DC) machines draw less.

Reverse voltage is something I'm still working on ATM, thanks to you both for the ideas. Glad to have your input.

[mariush]

You could also get some inspiration from how they control soldering station heating elements temperature and maybe use a triac and a zero crossing triac driver/optocoupler to make the output skip a few AC cycles, reducing the voltage on the output capacitor. cheap and little space used. 
Could use something as basic as a 555 adjusted dynamically based on set output voltage.

[tkoham]

You could also get some inspiration from how they control soldering station heating elements temperature and maybe use a triac and a zero crossing triac driver/optocoupler to make the output skip a few AC cycles, reducing the voltage on the output capacitor. cheap and little space used. 
Could use something as basic as a 555 adjusted dynamically based on set output voltage.

I'm not sure I understand how a triac would help with over-voltage or feedback, I thought they were mainly used as a cheap/dirty way to limit AC voltage for things like dimmers by switching off for a section of each cycle. could you please point me to some examples of what you're describing?

[james_s]

The only tattoo machine I've ever messed with was powered by a repurposed model railroad transformer, it was used to tattoo identifying marks on animals so aesthetics were not a consideration. I can't even relate to the aesthetics being a large concern in the design of a tool but if people care about that I'd probably just buy an off the shelf power supply and mount it inside a housing built by someone more artistic than I. 

[Doctorandus_P]

Often panel meters can be opened, and the scale replaced with something you designed yourself and printed on a piece of paper.
For the looks, you can bend some protection brackets from stainless steel rod and mount them partly over the panel meters.

LM723 has current sense and limiting capability. Adding at least a simple short circuit protection is always a good idea. Cables do get damaged and shorted sometimes.

[akimpowerscr]

To drive a dc motor, I would choose a very simple and very robust technology: a transformer with center tap and two thyristors with phase control to rectify the alternating voltage.

Given the robustness of the thyristors and of the transformer, only fuse protection is sufficient.

For such applications, the simpler and more robust is often the best.

The principle of phase control is still used for a lot of household appliances.

[tkoham]

The only tattoo machine I've ever messed with was powered by a repurposed model railroad transformer, it was used to tattoo identifying marks on animals so aesthetics were not a consideration. I can't even relate to the aesthetics being a large concern in the design of a tool but if people care about that I'd probably just buy an off the shelf power supply and mount it inside a housing built by someone more artistic than I. 

It's an industry full of artists, so it's definitely a different set of priorities. I don't know what to tell you other than it's like Deco -- form follows function, but there's no reason that form can't be pleasing at the same time. One of the challenges with this design aesthetically is that the level of embellishment is limited by how easy the device is to sanitize, so simple things like BJT cans and needle meters can go a long way before you paint, seal and otherwise finish the device. I'm also doing custom AA5 Radio style potentiometer knobs out of casting resin. It's very much meant to have a hand-crafted feel. If I wasn't concerned about longevity I'd probably even put a 24v tungsten lamp in to light the meter.

Often panel meters can be opened, and the scale replaced with something you designed yourself and printed on a piece of paper.
For the looks, you can bend some protection brackets from stainless steel rod and mount them partly over the panel meters.

LM723 has current sense and limiting capability. Adding at least a simple short circuit protection is always a good idea. Cables do get damaged and shorted sometimes.

That's good to know! I was planning on fabricating a facade for the panel meter model I had chosen anyway, but being able to remove the indicator inserts entirely will be a huge help. You can do a lot with adobe illustrator on that front.

Thanks for confirming that, I was second guessing myself and looking for ways I could have implemented the current limiting circuit improperly according to the datasheet(s) -- I'm gonna run some protection schemes through their paces with components I have around and I'll post the revised design ASAP.

[calzap]

If you are going to design and manufacture a mains-powered device for sale in the U.S., consult a lawyer about isolating your personal wealth (present and future) from liability actions.  Friends quickly turn into enemies when someone or their property has been harmed.  Then there are third parties who may eventually acquire the equipment.  And be sure to put a California cancer/birth defects warning on it even if you don’t intend to sell it there.

Mike in California

[akimpowerscr]

Here is a very simple example of a circuit with scr and phase control that would be suitable for your application.

Some values would have to be changed to operate with a 24Vac transformer or more.

You need a freewheeling diode in parallel with the load because it is an inductive load

[Alex Eisenhut]

I can't even relate to the aesthetics being a large concern in the design of a tool

Really? You've never noticed cars and houses all look different? 

[james_s]

I can't even relate to the aesthetics being a large concern in the design of a tool

Really? You've never noticed cars and houses all look different? 

I don't really consider houses to be tools exactly, but most look like one of a small handful of styles with minor variation and form over function is one of the things I hate about so many cars, the newer they are, the more crippled they seem to get by styling choices. I've been driving boxy old Volvos for almost as long as I've been driving, and whenever I drive someone else's car I'm struck by how cramped the interior feels and how terrible the visibility is even when when the exterior is a monstrous SUV that I can't see over the top of. There's a reason boxes are boxy, if you want to make something to hold objects, a rectangular form offers the maximum amount of internal volume for the external space it consumes. One of my pet peeves about car design is the proliferation of fake stuff that exists purely for style. Look around and you'll see most now have an enormous grill but if you look closely only a small portion of it is actually a grill, the rest is just cosmetic. On one of the new Jeep models the whole trademark 7 slot grill is entirely fake with the real air intake being a small slot down below. So many have completely fake air intakes in the lower front corners, I think this nonsense kind of started with Subaru in the mid 2000's when they put fake hood scoops on the RS. I didn't intend to hijack this thread though so I'll get off my soapbox.

[tkoham]

Thanks so much for all your suggestions, Here's the revised Schematic:


Changes are as follows:

• Added bleed resistors on bulk caps
• Added fuses/switches to reflect fuses on prototype
• Added Resistor to spec for panel meter range
• Added simple reverse voltage protection

Still To Do:

• Simplify by sourcing Linear open cage module or AC wall-wart
• Enhance Reverse protection with PFET/PNP overcurrent protection instead of relying on fusing
• Explore damping strategies for filter blocks
• Explore adjustable compensation/feedback for BJTs

For some reason the forum is saying the image above "Doesn't pass security standards" when I try to edit the OP so i guess the new design will have to live down here for now.