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Archive for January 17th, 2018

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 itemĀ appearing in the eBay listing:

Tattoo Power Supply - eBay listing photo

Tattoo Power Supply – eBay listing photo

Let’s say it has theĀ 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.

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