eBay Listings: Read Carefullly

What’s wrong with this picture? (clicky for more dots)

eBay - 40 pin IDC cable - header
eBay – 40 pin IDC cable – header

Not obvious?

Here’s the description, slightly reformatted for clarity:

New 5m IDC Standard 40 WAY 1.8” Multi-Color Flat Ribbon Cable Wire Connector


Type: IDC standard.

10 colors, 4 group, total 40 pcs cables per lot

5 meter per lot.

width: 4.7 cm / 1.8 inch

Package content: 5M Flat Color Ribbon Cable

If you divide the 1.8 inch cable width by its 40 conductors, you find the wires lie on a 45 mil pitch. If you were expecting this “IDC standard” cable to fit in standard insulation displacement cable connectors with a 50 mil pitch, you’d be sorely disappointed. You can get metric ribbon cable with a 1 mm = 39 mil pitch, but this ain’t that, either.

Here’s what an individual eBay wire (black jacket) looks like, compared to a wire from a standard ribbon cable (red jacket):

Ribbon cable - 26 AWG - eBay vs standard
Ribbon cable – 26 AWG – eBay vs standard

A closer look at the strands making up the wires:

Ribbon cable - 26 AWG - eBay vs standard - strands
Ribbon cable – 26 AWG – eBay vs standard – strands

As nearly as I can measure with my trusty caliper, the eBay ribbon cable has wire slightly smaller than 30 AWG, made up of seven 40 AWG strands, as opposed to standard 26 AWG wire made of seven 34 AWG strands. The good stuff might be 28 AWG / 7×36 AWG, but I was unwilling to break out the micrometer for more resolution.

I’d like to say I noticed that before buying the cable, but it came to light when I measured the total resistance of the whole cable: 80 Ω seemed rather high for 200 meters of 26 AWG wire. The wire tables say that’s about right for 31 AWG copper, though.

Changing the AWG number by three changes the conductor area by a factor of two, so you’re getting less than half the copper you expected. Bonus: it won’t fit any IDC connectors you have on the shelf, either.

Turns out a recent QEX article suggested building an LF loop antenna from a ribbon cable, so I was soldering all the conductors in series, rather than using connectors, and it should work reasonably well despite its higher DC resistance.


10 thoughts on “eBay Listings: Read Carefullly

  1. Ha, another big one is quantities, is it two? two pairs? or two kinds and sells per unit? Of course the main picture will show a whole bunch of them.

    1. Sleaziest one I’ve seen recently: a “10 set terminal adapter” for an Arduino Nano that turns out to be one board with 10 of the terminal blocks, selling for just slightly less than a “10 x terminal adapter” offering with ten boards, each with ten adapters.

      There’s a big steaming pile of caveat emptor out there!

  2. I gave up on eBay a while ago, and am trying Amazon for components. I have a $5.60 bag of 10 uxcell rocker switches waiting to go out to the shop for testing. Reviews were mixed, but the worst one said 50% passed. The big Gel-cell battery I got from them did fine.

    I like the QEX loop antenna idea. An old Scientific American project I’ve had on the way-back burner is a lightning locater. I don’t want to use the vacuum tube design, but I think I can dust off the FET theory enough to get by. The ribbon cable beats hand-winding loops (the article suggested Hula-Hoops as forms, I suspect from when they were a new thing).

    1. Amazon’s offerings are at least as bad as eBay, and Amazon’s corporate practices offend me. I figure if I’m going to buy cheap Chinese gear, I should do so directly, so I’ve bought some things from seeedstudio, aliexpress, and hobbyking. Same quality, lower prices. I did pick up an IC based lightning detector breakout board that seems fine, some nice VFD displays, and even a UV-based flame detector that turned out to be (as far as I can tell) a genuine Hamamatsu evaluation board and tube.

      1. So far, so good. The switches ore OK, with brands and certifications claimed on the cases. They also have holes to let the dust in, but the 120VAC application is pretty dusttight, and the other one is in the 100s of millivolts.

        I used to live near retail and distributor suppliers, along with a bunch of surplus shops, but don’t live there any more. Not sure which of those suppliers would be there, either. Radio Shack doesn’t carry what I needed, and Amazon Prime covered shipping, so I’m ahead of the game. (Prime is getting really popular around here–a lot of retail disappeared when the logging/lumber jobs died, and it’s really filling a need. “Hurting the little guy” doesn’t count when he went away before Amazon even started…)

    2. As soon as I saw the off-by-one loop wiring trick, it was so obvious I just had to do it.

      It was also obvious that I’d never think of that trick… [heavy sigh]

  3. To be fairhopelessly optimistic, I’m sure the 4.7cm is the actual measurement, which could be as much as 4.749, which would be 1.8696″, which would be 47.9 mil pitch after allowing for the fencepost error, which is within 5% of 50 mil nominal… have you check to see if they actually don’t fit the IDC connectors? :) It’s a damned clever antenna design though! If the cable did fit the IDC connectors, i’d just punch it down onto a mating pair off-by-one…

    1. After a reasonably diligent search, I cannot find my IDC connector stash, which means one of two things:

      • They’re in a box with a misleading label or
      • I inadvertently gave them away

      There’s a lot of that going on lately.

      My trusty plastic ruler reports a scant 1-7/8 inch: not even close to a crappy fit in a 2 inch connector.

      Besides, those skimpy 31 AWG conductors would probably snuggle into the IDC contact blades with just enough metal touching to make you think it’s all good…

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