Low Budget Bench Supply: Digital Version

This one costs slightly more than the analog tattoo power supply:

Tattoo Digital Power Supply - front panel

Tattoo Digital Power Supply – front panel

The gold lion really spiffs up the Electronics Workbench!

Somewhat to my surprise, the circuit uses a switching power supply based on a Reactor Micro RM6302 controller that can produce about an amp at voltages up to about 14 V:

Tattoo Digital Power Supply - internal view

Tattoo Digital Power Supply – internal view

CAUTION: Everything on the input side of the transformer runs at line potential. I have my doubts about isolation, particularly under fault conditions.

The trimpot on the PCB seems to adjust the output voltage, although it’s not clear what’s going on.

The three wire AC line cord has a standard IEC entry block on the rear panel, albeit with the ground terminal not connected to anything inside the plastic case. It arrived with the hot wire soldered to a tiny fuse on the PCB and the neutral wire (red!) to the back-panel switch. There being no practical way to put the fuse before the switch, I rewired the hot side to the switch before the fuse and the unswitched neutral to the PCB; that’s as good as it’ll get.

I also flipped the AC switch to put the ON position at the top. Sheesh.

The two 1/4 inch jacks on the front panel are wired in series, so it didn’t matter which one got the tattoo needler or the foot switch:

Tattoo Digital Power Supply - jacks in series

Tattoo Digital Power Supply – jacks in series

I rewired the sockets in parallel to eliminate the need for a shorting plug, although I cannot imagine any need for two outputs.

The knob seemed unusually sloppy, which turned out to be due to a broken threaded sleeve around the pot shaft that prevented the crudely made nut from seating tightly:

Tattoo Digital Power Supply - broken pot threads

Tattoo Digital Power Supply – broken pot threads

Given that the builders stuck everything else to the front panel with hot-melt glue, I followed suit:

Tattoo Digital Power Supply - glued pot threads

Tattoo Digital Power Supply – glued pot threads

Which actually held it in place reasonably well, despite the hideous appearance. The knob covers the blob, so It Doesn’t Matter.

The output range extends from about 1.2 V to just over 14 V at about an amp, but the knob seems erratic and the digital meter has only a casual relationship to the actual output voltage.

I think if you regard this one as a parts kit, reverse-engineer the schematic (which surely descends directly from the RM6302 datasheet), and rebuild the electronics, it might work better.

Bottom line: The analog version seems to be better as a low-budget power supply, not least because it has a metal case and an actual power transformer for galvanic isolation.


Leave a comment

Wouxun KG-UV3D: Another Failure

Once is happenstance, twice is coincidence:

Wouxun KG-UV3D - failure

Wouxun KG-UV3D – failure

Three times is enemy action, but we’re not there yet. I was willing to believe something I’d done had killed both of the radios, even though it seemed unlikely for them to last five years and fail almost simultaneously.

So I dismantled this one to see what’s inside. Pull off both knobs, remove the two screws at the bottom of the battery compartment, pry gently with a small screwdriver, and the whole PCB pulls out:

Wouxun KG-UV3D - disassembly

Wouxun KG-UV3D – disassembly

A bit more prying separates the big pieces:

Wouxun KG-UV3D - interior

Wouxun KG-UV3D – interior

Looking closely at the main PCB showed some problems I definitely didn’t cause:

Wouxun KG-UV3D - PCB overview

Wouxun KG-UV3D – PCB overview

Although it’s been riding around on my bike, the white blotches on the PCB came from inadequate flux removal after hand soldering.

A collection of images taken through the microscope reveals the problems:

This slideshow requires JavaScript.

I swabbed off the crud with denatured alcohol to no avail. The bottom side of the PCB has even more components and, I’m sure, even more crud, but I didn’t bother removing all the screws required to expose it, nor did I dismantle the other failed HT.

I doubt Wouxun’s QC improved over the last few years, which means the two replacement KG-UV3D radios I just bought are already on their last legs, despite my paying top dollar to the same reputable source that sold me the first pair.

We’ll be ready for new radios on new bikes by the time these fail.


Low Budget Bench Power Supply

The idea behind this gadget surfaced while I was looking for something else and, although the front panel makes my skin crawl, it’s just an adjustable DC power supply:

Tattoo power supply - original front panel

Tattoo power supply – original front panel

This is apparently the cheapnified version of the item appearing in the eBay listing:

Tattoo Power Supply - eBay listing photo

Tattoo Power Supply – eBay listing photo

Let’s say it has the potential to be a DC power supply, although we might quibble about the “Precision” part.

As delivered, it’s a deathtrap. Of course, it’s not UL listed and I didn’t expect it to be.

How many lethal problems do you see?

Tattoo power supply - original AC wiring

Tattoo power supply – original AC wiring

For starters, it has a three-wire AC line cord with the green-and-yellow conductor chopped off flush with the outer insulation inside the heatshrink tubing just behind the transformer:

Tattoo power supply - ungrounded AC line

Tattoo power supply – ungrounded AC line

The blue wire is AC neutral, but it really shouldn’t be connected to the finger-reachable outer fuse terminal.

The brown wire is AC line, which goes directly to one power switch terminal. In the event of a hot wiring fault, an unfused conductor touching the case will test the GFI you should have on your bench wiring.

The AC line cord uses some mysterious copper-colored metallic substance that’s about as stiff as music wire:

Tattoo power supply - stiff AC wire

Tattoo power supply – stiff AC wire

The strands cannot be twisted together like ordinary copper wire, although they can be soldered. They may be copper-plated aluminum, because a magnet ignores them.

After soldering the strands together, they snap when bent:

Tattoo power supply - soldered broken AC wire

Tattoo power supply – soldered broken AC wire

Generous strain relief is not just a good idea, it’s mandatory.

After some Quality Shop Time, the ground wire now connects to the case through the transformer’s rear mounting screw, the neutral AC wire connects to the transformer, the hot AC wire goes to the tip of the line fuse, and the fuse cap terminal goes to the switch:

Tattoo power supply - AC line rewiring

Tattoo power supply – AC line rewiring

I relocated the white LED to the middle of the meter, where it looks a bit less weird:

Tattoo power supply - revised front panel

Tattoo power supply – revised front panel

I have no idea what “Porket indicate” might mean. Perhaps “Precision indicator”?

The right 1/4 inch jack, labeled “Foot”, normally goes to a foot switch you don’t need for a bench power supply, so I converted a length of drill rod into a dummy plug to short the jack contacts:

Tattoo power supply - dummy switch plug

Tattoo power supply – dummy switch plug

The tip comes from a bit of lathe and file work and the white cap comes from a bag of wire shelf hardware.

A genuine hologram sticker (!) on the back panel proclaims “1.5 – 15 VDC 2 A”, which seemed optimistic. Some fiddling with power resistors suggests tattoo liners (I learned a new word!) don’t draw much current:

  • 4 V @ 1 A
  • 8 V @ 800 mA
  • 10 V @ 600 mA

It can reach a bit over 18 V (pegging the meter) at lower current, so it’s Good Enough for small projects with un-fussy power requirements.

Caveat emptor, indeed.

, ,


MPCNC: Epoxy-filled Connector

When I wired up the MPCNC’s tool length probe, I planned to reinforce the wiring with a dab of epoxy. What I didn’t notice in my enthusiasm, alas, was the opening from the rear to the front in each pin slot:

Epoxied connector - rear

Epoxied connector – rear

Which let the epoxy flow completely through the connector:

Epoxied connector - front

Epoxied connector – front

So I cut the mess off and applied heatstink tubing on each wire, just like I should have in the first place.

Now you know the rest of the story …

I really dislike pin headers as cable connectors, but that’s what the Protoneer CNC board uses:

MPCNC - Protoneer Wiring - SSR

MPCNC – Protoneer Wiring – SSR

It’ll be Good Enough if I don’t do anything else particularly stupid.

, ,


Monthly Image: Snow-dusted Spider Silk

The recent bitter cold and gusty winds swirled a dry snowfall around our back patio, where it clung to the (otherwise invisible) spider silk strands on the cedar shakes:

Snow-dusted spider silk

Snow-dusted spider silk

It’d be Art if a human did it!

Leave a comment

Stainless Steel Water Bottle: FAIL

Although I repaired the spout a while ago, those water bottles were never satisfactory and saw very little use. A recent cabinet cleanout showed the “stainless steel” has passed beyond its best-used-by date:

Stainless steel water bottle - rust

Stainless steel water bottle – rust

With no regard for whether the patient would survive the operation, I peeled off its rubber foot and applied the Lesser Hammer:

Stainless steel water bottle - insulation

Stainless steel water bottle – insulation

The “insulation” seems to be a rigid urethane-like foam disk few millimeters thick on the bottom of the interior flask, with good old air around the sides.

The bottles never worked very well and now we know why.



Eyeglass Temple Re-re-repair

After three years, the temple screw on Mary’s oldest and most-battered “reading” glasses worked loose. A dab of low-strength Loctite should hold it in place forever more:

Reading glasses temple repair

Reading glasses temple repair

That brass stake pin certainly adds a Steampunk flair to the proceedings …

Leave a comment