Friday, June 3, 2016

When does 18 = 26? When buying cheap cables.


I recently bought some cheap molex to PCI-e power adapters from a seller on AliExpress.  Although there are deals for quality goods on AliExpress, I was a bit suspicious when I ordered these given just how cheap they were.  PCI-e power connectors are supposed to be rated for 75W of power carried over 2 conductors at 12V, which means 3.1A per conductor.  In order to avoid a large voltage drop the wires used are usually 18AWG, although 20AWG wires (with 1.6x the resistance) would be reasonably safe.

When the package arrived, I inspected the adapter cables, which were labeled 18AWG.  Despite the label, they didn't feel like 18AWG wires, which have a conductor diameter of 1mm.  I decided to do a destructive test on one of the adapters by cutting and stripping one of the wires.  The conductor measured only 0.4mm in diameter, which is actually 26AWG.  The first photo above shows a real 18AWG wire taken from an old ATX PSU next to the fake 18AWG wire from the adapter cables.

When I opened a dispute through AliExpress, things got more amusing.  I provided the photo, as well as an explanation that real 18AWG wire should be 1mm in diameter.  The seller claimed "we never heard of this before", and after exchanging a couple more messages said, "you can't say it is fake just because it is thin".  At that point I realized I was dealing with one of those "you can't fix stupid" situations.

So what would happen if I actually tried to use the adapter cables on a video card that pulls 75W on the PCI-e power connector?  Well you can find posts on overclocking sites about cables that melted and burst into flames.  If you have a cheap PSU without short-circuit protection, when the insulation melts and the wires short, your power supply could be destroyed.  And if that happend I'm sure the AliExpress seller is not going to replace your power supply.  How much hotter the cables would get compared to genuine 18AWG cables is a function of the resistance.  Each gauge has 1.26 times more resistance than the previous, so 20AWG has 1.26^2 = 1.59 times the resistance of 18AWG.  The 26AWG wire used in these cheap adapter cables would have 1.26^8 or just over 6 times the resistance of 18AWG wire, and would have a temperature increase 6 times greater than 18AWG for a given level of current.

It could make for a fun future project; create a resistive load of 75W, take an old ATX PSU, hook up the adapter cables, and see what happens.  People do seem to like pictures and videos of things bursting into flames posted on the internet...


No comments:

Post a Comment