Teardown: Generic QC2.0 15W Charger (TCAU15U-050912)

Owing to the growing market-share of Qualcomm devices sporting Quick Charge 2.0 and 3.0 capabilities, the market for such devices has grown substantially. As part of reviewing some power banks with QC-capabilities, it would not be possible to assess their ability without having a QC charger myself.

Because of both being in a hurry and needing something cheap, back in May 2016, I purchased a generic QC2.0 charger for AU$10.22 just to try it out. At the time, none of my phones were QC capable, and it was really only to test the Anker PowerCore 10050+ power bank. While I have used it just a handful of times to test power banks, I had my doubts as to the quality of the unit, so I decided to give it a teardown.

The Unit

The unit originally came with two blade pins configured for US operation. With a quick modification enabled by a set of pliers, they now fit into an AU socket. Not recommended, but works in a pinch if you’re desperate. The outside of the unit has rounded corners and a tapered design, with a matte white finish and “generically” shows the Qualcomm Quick Charge 2.0 logo.

The opposite side has a single USB port, although this one did feel a little loose.

The bottom side has a list of specifications, with a model number of TCAU15U-050912. The only thing I could find was an “Itian Premium Design Quick Charge 2.0 15W Wall Charger”, with no clue as to who actually makes it. It claims an input of 0.8A which is quite high – maybe it’s got a poor power factor. It is capable of 5V, 9V and 12V DC as required by QC 2.0, and offers 15W maximum output, rather than the more customary 18W. The currents are not listed, but “assuming” 15W is 15W, that’s 3A at 5V (I doubt it), 1.67A at 9V (plausible) and 1.25A at 12V (slightly anemic). It claims an efficiency level mark of V, which is possible I suppose. The date code is Week 40 of 2015. It claims to be FCC approved (doubt it), double-insulated (it passed an insulation resistance test), and for use indoors. Interestingly, the pin-out makes no mention of D+ and D- which are used to negotiate QC2.0.

Given its already dubious credentials of low price and strange specs, I wouldn’t be leaving this one plugged in all the time, or trusting it with a precious phone. I can’t even be bothered to test it electrically for standby power or efficiency … partly because it does get a little warm in use, but also smells a bit like burning electronics. It hasn’t failed though, which is good …

Teardown

As the unit features no screw fasteners, opening the unit proved to be a semi-destructive act. This involved securing the power prong pins in a vice, and then slowly applying lateral pressure from each side until the end-plate removed from the body.

In my case, this resulted in a slight crack to the front facing plastic piece, but also the snapping of a fusible resistor leg as it was adhered to the front plate and mechanically separated upon opening. No matter, a dab of solder and it was repaired. Interestingly, the pins are moulded into the plastic and end as “stubs”. This goes into a matching tapered loop receptacle that protrudes upwards from the board that makes contact with the pins. I suppose this avoids having to solder wires, and allows for an easy “push and glue” fit of any region plug you might need. The loop receptacles do have some sign of corrosion, and the internals do smell a little bit of capacitor fluid.

The board is easily extracted from the board as it is guided by rails. The unit is a cost-optimized design on a single-sided PCB with silkscreening. The MOV used for surge protection is removed, and is the NTC for thermal feedback. However, the construction doesn’t appear all too shabby, with the exception of the cheap flimsy USB receptacle that has quite a bit of play and doesn’t seem quite “tight” enough with a weak outer shell.

Looking from the top, the responsible company is TDSEN, with a website at www.tdspowers.com that redirects to www.tdsen.com. This product is no longer within their catalogue, it seems. There are really not many components, but at least we see that the design incorporates opto-isolated feedback, a primary side inductor and proper Y-capacitor is used for interference suppression. Disasters are averted with a fusible resistor also acting as an in-rush limiter. There is a transistor for switching on the primary side, but there is no heatsink.

The main let-down so far is the downright random capacitors which come from Yihcon, y.u.g and CapXon. Of the three names, only the latter is known, and is not associated with positivity. However, that being said, the CapXon cap is an output capacitor that has been specified as a solid electrolytic, so it should have a better lifetime. A quick in-circuit measure of all the capacitors appeared to show that they still are in spec despite the smell and heat.

Instead, a decent amount of smell was emanating from the transformer itself, which is probably made by a company with initials HDY. It may be due to hot operation and outgassing of insulating plastics, however, while it looks decent from the outside, it’s almost impossible to determine the insulation separation without unwinding it and destroying it.

The underside of the board has a smattering of surface mount components. The bridge rectifier, a control IC, a 6-pin SMD marked 4N12 and a MOSFET seem to make up the majority of the major components. There is a nice isolating boundary on the PCB, complete with anti-tracking slot to ensure primary to secondary isolation for user safety.

I have no idea what the markings on the chip are supposed to say.

I’m not sure this chip is the N-channel MOSFET it claims to be – it may be working as an active synchronous rectifier, but I didn’t take the time to analyze it. The mounting is very strange.

Conclusion

It is a cheap charger, but it doesn’t look too nasty now that I’ve seen inside. While it seems to be very much a cost-optimized design with as few components as possible, the PCB appears to be laid out in a good manner with clear isolation between primary and secondary. Most of the components don’t look too bad, with the exception of the primary side capacitors. The controller is still a mystery, but the unit does work. The other downside is the flimsy USB output connector that seems to be high resistance with the cables I have.

I wouldn’t, however, recommend this unit for regular use to charge a precious phone – why risk an expensive phone to a AU$10 charger? Besides, its 15W rating seems to imply that it doesn’t deliver the expected 18W-24W that a proper QC2.0 charger would deliver, and it does smell bad when in use, suggesting that something is getting quite hot.

Posted in Electronics, Tablet, Telecommunications | Tagged , , , , , | Leave a comment

Moments to Disaster: Hard Drive Failure & SMART Data

While I was away on holiday, blissfully enjoying my time outdoors, I wasn’t aware that a disaster was brewing at home. I got a message from my brother that his machine had started making a clicking noise, and it was already too late. I was sure that a hard drive had failed.

Luckily for him, the machine I built for him had an SSD for a boot drive, so after a bit of waiting, the unit still booted and continued to work. However, whatever was stored in the hard drive was inaccessible.

Attempted Rescue

When I came back from holidays, I tried to rescue it. The drive in question was a re-certified Samsung HD502IJ drive from my uni salvage spree back in 2015. The particular unit was serial number S1W3J9ASB00270, received with 21886 hours on the clock and passed commissioning tests just fine.

Upon pulling the drive and sticking it in my recovery cradle, it was confirmed that the drive spun up just fine, but it was failing to “find” its location and read-out the firmware from the disk. I pulled its serial-number mate S1W3J9ASB00276 from my spare pool and swap out the PCBs. No change in behaviour. Click-click-click-click … and eventually, spin-down.

At this stage, I was pretty sure recovery would be quite farfetched. Professional data recovery would have been a potential option, but it would probably be quite expensive. I didn’t feel it would be worth it to send the drive away. However, I was determined not to give up, and rather than waste my time taking photos of things, I just got on with it and cracked open both drives and “ghetto” swapped the whole head-stack assembly from one drive to the other. It was a long-shot, especially without all the fancy tools a proper lab might have, but I thought it’s better than nothing.

After powering-up, the drive still exhibited no change in its behaviour. It still couldn’t read. Worst still, transplanting the heads back to the original drive resulted in the original drive failing to work in a very similar way. Now I have two dead drives and nothing to show for it. At least, they didn’t cost me anything in the first place, except for some of my brother’s data. I did have a back-up before I left for holidays, but apparently, there was quite a bit of additional data added to the drive since I left … unfortunately, I can’t do anything about that.

Declared Dead – Here’s the Autopsy

At this stage, I declared both drives dead, and proceeded to dismantle them into component parts for disposal (as I usually do).

The Samsung HD502IJ is a 7200rpm PMR drive with 334Gb per-platter technology, with fly-on-demand (FOD) control and rotational vibration sensor. It uses old-fashioned contact start-stop instead of the newer ramp-loading technology for the heads.

When I disassembled out the platters, of which there are two in a 500Gb drive, I noticed some unusual damage.

It’s not the first time I’ve taken apart a drive and noticed the platter surfaces seem to be “ground down” in concentric rings. Notice how the landing zone section and mid-platter both have significant wear on the three active surfaces, although the degree of wear varies. The unused surface had no head, and thus without contact, it suffered no wear.

A closer look at the broadest wear band seems to show it has a interesting striated pattern, almost like a washboarding of roads.

The width of the wear band is different, and the edges can be irregular. In the case of the lightest wear, it was a thin “scratch” which was impossible to get good focus on.

The Drive’s “Medical Records”

As I wasn’t home to watch over the drive as it failed, I relied on my installation of CrystalDiskInfo to record the drive’s information. As with most of my systems with actively spinning drives, CDI runs in the background collecting health status data every 10 minutes and writes it to a set of CSV files (one per SMART attribute) stored in the smart sub-folder inside the installation directory.

There had been a number of arguments that SMART is absolutely hopeless in predicting drive failure. However, there is a possibility that the viewpoint is biased to those who have suffered unexpected failures “out-of-the-blue”. Was this one of the latter?

Upon examining the SMART data, I plotted only the variables which changed with relation to time up to the event, with the exception of temperature. CDI only records “raw” data for some attributes, and SMART normalized value for others, so I can only use what is left.

Based on the power on hours count, it ran for about 474 hours (although the last week of data does not appear to be recorded for this variable, and is something I’ve seen happen in the past). The time from commissioning into the “new” system for my brother to failure was 146.3 days. I left for my holidays where the “flat” section in the curve was around 27th June, as my brother was also on holidays at the time and the system sat unused.

The “vital signs” show that on the 7th of September, the drive did show a sign of potential impending failure with sky-rocketing pending sector count. In general, any pending sector counts cause me to investigate the drive, as it’s a sign the drive knows of sectors on the surface it cannot read. Two weeks later, it finally has its first reallocation event of one sector. The drive lived almost three weeks or about 85 power-on-hours between the first sign of failure and total failure, and this means that SMART did give some warning. Whether acting on the first warning would have been sufficient to rescue most of the data, or whether failure was already assured at that stage, we cannot determine for sure.

SMART normalized values for Raw and Soft Read-Error Rate both degraded slightly, but not sufficiently to trip any alarms. It does, however, correlate with increasing difficulties experienced by the drive.

Interestingly the Spin-Up Time, a parameter which is normally an indication of mechanical problems, showed increased variance about four days after the first sign of failure. The variance trends towards lower SMART normalized values, indicating decreasing health. This suggests that possibly the roughness had built up in the landing zone area so much as to oppose the motor “ramping up to speed”, delaying “take-off” and also increasing wear due to prolonged contact period with the surface.

If only someone had noticed the warnings as CDI would have popped up alerts, and taken appropriate action, as there appears to have been a window where at least partial recovery may have been possible.

However, it seems that SMART on its own may not have sounded any alarms without any “third party” monitoring software installed, as the thresholds are set quite high, and the amount of pending sectors and spin-up variations may not have tripped them. In fact, looking at the SMART parameter listing from another drive, it seems that of the attributes that showed change, 01 Raw Read-Error Rate would have to fall below 51 to trigger an error, and it only fell to 97. Likewise, attribute 03 Spin-Up Time has a threshold of 11, and the worst value recorded was about 33, and thus no warning would have been issued. Attributes for Current Pending Sector Count, Reallocated Event Count and Reallocated Sectors Count are all set to a threshold of 0, indicating they cannot trigger a SMART alarm despite being one of the most indicative attributes of impending failure.

As a result, there seems to be some merit to the viewpoint that SMART is pretty useless, however, if you interpret the SMART data more closely (rather than blindly following manufacturer-set thresholds), the drive does provide some prior warning of its unhealthy status.

Cause of Death?

Given all of the data, to find the cause of failure is almost downright impossible. I’m no hard drive expert, but I have a few hypotheses.

Modern hard drives have operational head-flying clearances in the single to double-digits of nanometers range. That’s a gap smaller than the wavelength of light. This impressive feat is achieved by a mixture of technologies including a head-heater for active fly height control. The heater changes the position of the head “on demand”, so as to bring it close to the disk when needed for reads and writes. With such small clearances, contact is occasionally inevitable due to embedded surface defects from manufacturing and particles in the chamber.

When a head makes contact with a defect or contamination on the disk, this would be traditionally known as a head-crash and can cause damage to the disk surface and the head. However, there is a more technical term for this – thermal asperity. This is because the head heats up as a result of hitting a defect, and this has a secondary effect of shifting the baseline of the analog voltage produced by the GMR heads, making data recovery difficult. It appears that it’s probably impossible to make perfect platters, so having some thermal asperities is something we need to cope with in real life.

As a result, the platter is coated with a diamond-like coating and a lubricant layer to minimise the damage of a head-to-disk contact. The sliders and head components are also made of resilient materials. Unfortunately, even with this, it is possible to damage the head slider through such events, resulting in scoring of the slider surface with signal changes and potential fly-height control difficulties as the aerodynamics of the slider are affected. The shape of the defect has an important bearing on how damaging it is, with the smallest defects being “mowed down” by the head without damage and rounded defects resulting in a deviation in fly height but with no damage. The defects that sit in the middle are potentially dangerous.

As a result, I think the cause of failure was probably some platter defect in a specific location. Perhaps this was already extant at the time of platter manufacturing, or caused through mechanical shock while in operation or with heads not parked in the landing zone. If not, another cause could potentially be a very unusual data access pattern (unlikely due to the low duty cycle of “home” storage use) resulting in the head resting over a track for prolonged periods causing localized lubricant loss and reduced fly-heights.

This may have very slightly damaged the slider every time the location was used, but not enough to render the drive critically damaged. Over time, possibly due to where the data was stored on the drive and being accessed, in our use, the drive probably did rest its heads over the defective area and accumulated damage to its sliders. This, then reciprocally damaged the platter surface.

By the time the damage was done to the platters, even microscopically, it seems quite likely that the damage would have rapidly accelerated as the defects affect the flying performance of the head slider assembly which would become more likely to make contact with the surface, further damaging both head and platter. The stiffness of the suspension may have something to do with the “washboarding” patterns seen, as there could have been some “resonance” occurring.

The fact that damage was coincident on the three recording surfaces to varying degrees suggests the possibility of coincident damage (external shock for example), embedded defects in similar locations on all surfaces (unlikely) or potential for cross-coupled damage. I suspect that this could happen, say if any “resonance” is caused, the vibrations could be sent up the whole HGA and thus cause the other heads in the stack to fly in an oscillatory manner, also damaging their surfaces due to increased probability of contact.

However, as I’m no expert on the tribology of hard drive mechanics, I’ll leave all of the above as just a hypothesis. However, I’d have to say that Youyi Fu’s PhD thesis (2016) on Tribological Study of Contact Interfaces in Hard Disk Drives done at UC San Diego was quite an interesting read, especially for someone who is not an expert in the area.

Conclusion

Unfortunately, data was lost in this circumstance, but it seems extremely unlikely that any affordable service would have been able to restore it after-the-fact. The damage to the platters was extensive, and the cause is not conclusively determined. However, it seems that SMART did provide some evidence of the drive’s unhealthy state prior to its total failure (about 3 weeks or 85 power-on hours based on the recorded data), although whether this is enough warning to make a successful or mostly-successful recovery is not known. If assessing SMART based on manufacturer-provided thresholds, this drive does not appear to have tripped any thresholds, thus would not have triggered warnings from the BIOS or “inbuilt” features in most operating systems. Only more detailed third-party software with a keen eye on the SMART raw values themselves would have uncovered the developing issues.

Posted in Computing, Obituary | Tagged , , , , , , | 1 Comment

USB Connector Resistance: Another reason for slow phone/tablet charging

It was back in 2014 when I first published about USB cable resistance as one reason why your phone or tablet might be charging slowly. At the time, quality cables with good gauge wires were hard to find, and many “cheap” 28AWG cables were bought by unsuspecting consumers without knowing the downsides. Since then, it has become one of the most-viewed articles on this site, and the market has responded positively by advertising cable specifications more frequently.

However, this is only one part of the puzzle, and it is something I had realized almost right after I posted that article. Unfortunately, I couldn’t find the motivation to explain the technicalities at the time. Since then, a number of comments and reviews later, I feel that I’m better prepared to explain another issue which has existed since the dawn of USB but has gotten worse as we attempt to push more current through our cables.

Is that USB connector designed correctly?

USB at its introduction specified the ability to transfer up to 500mA at 5V over the USB A and B connectors. Throughout the iterations, the number of connector types have increased, and so has the current demanded by end-devices. USB “legacy” connectors still remain with us today and still remain relevant to many chargers which feature a USB-A socket.

Unfortunately, the design of USB focused on making things cost-effective, and as a result, these legacy connectors tend to be less-accurately specified from a dimensional standpoint. This means that connector sizes can vary significantly between manufacturers, but “it should still work”. How well this works in practice is a different story.

I’m sure that many readers may have experienced a headphone socket which crackles and might need you to turn the plug a little or pull it out slightly before a good connection is made.

Others may have even experienced a USB charging cable which refused to charge until a little lateral pressure is applied to the connector, and then it worked. Or maybe, you needed to pull it out just a tiny bit.

These are all manifestations of contact resistance which may arise due to connector incompatibility.

The design of the USB connector is such that this problem should be minimised in properly-designed connectors.

I will focus on the USB-A connector, although the principles hold true for most types of connectors. The first feature to look at is the shape of each of the pins in the connector. Properly formed connectors have a tapered design at the front, which leads to a “raised ridge” in the centre. This whole pin is gold-plated to avoid oxidation and maintain good contact.

When mated with its appropriate mate which contains gold-plated contact springs, the raised ridges concentrate the pressure to “break through” any surface dirt. The plastic tongues on the undersides provide a stable “backing” and increase the pressure between the spring surface and the contact while enforcing connector alignment. The outer shell keeps the connectors from pulling apart, provides a continuous shielding path and maintains the alignment of the pins.

This all sounds pretty simple, but in reality, I’ve seen a lot of things go wrong:

  • Some connectors I’ve seen have absolutely no gold plating whatsoever, which makes them liable to oxidation and potentially depositing dirt onto perfectly-good connectors. Others have false “gold-coloured” plating which wears off after only very few uses.
  • Others have tongues which are too “thin” in the vertical dimension, and thus the plug doesn’t make good contact unless it’s tilted downward or upward.
  • A cost-saving measure seems to be the use of “thinner” and more “flexible” shell materials for the plug, which are liable to bending and mis-shaping after the application of a mild amount of mechanical stress. This jeopardizes the tightness of the connection and can lead to premature contact wear due to intermittent contact.
  • Some are “cost-reduced” in manufacturing resulting in “flat” pin profiles that do not achieve as reliable contact as the raised-ridge profile which it should have. This is common on “PCB” style plugs.
  • There are some completely unconventional plugs (e.g. a “reversible A” or a “shell-less A”) which violate the standard and thus rely on other parts of the socket to compensate in order to make the connection work.

Another point is that connectors are not designed to be used forever. Most connectors have data-sheet lifetime specifications of about 10,000 cycles to a maximum contact resistance of 30mohms. This sounds plenty, right? For 10 cycles a day, it’s almost three years to failure.

The truth is that this data-sheet stated lifetime is for ideal circumstances. This means not exceeding any current ratings, using their appropriate mating partner connector, under perfectly clean circumstances with no added mechanical stress. Once you add reality into the mix, these ratings can go right out the door.

For starters, any trapped particles can act as an abrasive compound wearing away the precious gold plating and serve to impact resistance. Any contamination from atmospheric deposition can result in corrosion where the plating is broken. Arcing from “loose” connections due to poor “partner” sockets can destroy the coating. Damaged sections increase contact resistance. Once you add the possibility of the outer shell being dimensionally affected by bending due to stress, a good connection is hard to ensure.

Hopefully, now, you can begin to appreciate that a USB connector is not as simple as it appears. But you might not be convinced that this is a problem.

A look at the cable resistance shows that for a 1m 24AWG cable, the cable contributes about 168mOhm of resistance. The connectors are supposed to be up to 30mOhm each at the end of life, or 60mOhm total. This isn’t entirely negligible, especially if we improve the cable. A better cable can be hamstrung by poor connectors!

“QC2.0/3.0 and higher voltages eliminates the problem!” Really?

One of the biggest push-backs I’ve had in regards to the original cable resistance article is a misconception that higher charging voltages solves the problem. The problem is that there are two problems in reality and neither of them are completely solved by higher voltages.

The first problem is under “regular” 5V charging, some devices may show slow charging or no charging at all when connected to long cables. When they upgrade to a QC2.0 charger, they see the device can now take a charge, and think that the problem is solved. Instead, it hasn’t – and it’s merely been masked.

From a simplistic view, to convey the same power at a higher voltage requires less current, therefore the resistance of the cable causes less voltage and loss. This is true. However, the other half of the story is that Quick Charge is developed to help speed charging of the phone, and instead, it tends to use currents in the 1.2-2A region to best optimize the charging where possible. This is, coincidentally, similar to the currents drawn by regular 5V charging and hence results in a similar amount of voltage drop.

However, people then seem to think that because it runs at a higher voltage, a 0.25V drop isn’t going to be a problem because it’s a smaller percentage of the voltage. This is NOT true, and the reason is simple.

Any voltage dropped over the whole cable assembly’s resistance is produced as heat. This is dependent on current and resistance only, so P=I^2*R. As 5V “regular” charging and Quick Charging nominally operate in similar current regimes, the voltage drop and power lost in the cable is pretty similar.

The two dark green lines indicate the maximum resistance of a single connector and two connectors at end of life. The light-green lines are an estimate of a 50cm 24AWG cable with two connectors, and 1m 24AWG cable with two connectors at end of life. Note that at 1.5A for the cable, expected voltage drop is 0.21 to 0.255V, which is right on borderline acceptable for regular 5V charging. The power dissipated in the cable and connectors is 0.315-0.383W. It’s not an unsubstantial amount – it’s about the same as holding your hand in front of a small battery-operated 3 x COB LED torch.

If you are to expect that 12V charging, being about twice as high of a voltage as 5V, can tolerate twice as much voltage drop, this would increase the power loss to about 0.63-0.766W. This is not a wise move, because the heat has to go somewhere, and now the cable gets twice as much energy dissipated per unit length. Where the cable is well made, the power should be somewhat “evenly” distributed across the cable, but if there is a fault with a contact, the lion’s share of the resistance can concentrate at that point. In the worst case, blindly pulling current despite a falling voltage compounded with a localized resistance in the connector can result in melted connectors and damage to devices. A higher charging voltage does not fix the issue of power being dissipated in and along the cable and connectors!

Practical Effects – A “slow” quick-charge?

If this graph looks a little familiar, it’s because it’s basically the graph from my Mi Power Bank Pro review, just with two traces added to the graph.

In order to test charging current profiles, I grab an appropriate wall-charger and throw a modified charger doctor current shunt in line to measure the current with a Keysight U1461A multimeter connected to a PC.

When first testing the “regular” 2A charger, there was no problem. The unit clocked in a 5 hour and 45 minute charge time. But then I tried to test the QC2.0 charging ability and got the grey curve, clocking in a charge slower than with the non-quick charge adapter.

Thinking this was a compatibility issue, I decided to use a different QC2.0 source, namely the old Mi Power Bank Pro. As a 10,000mAh power bank can’t fully charge a 10,000mAh power bank due to losses in conversion, I took advantage of the simultaneous charging ability. I connected a regular 2A charger to the first power bank to keep it “topping up” while it output QC2.0 to the power bank under test. Because of the close proximity of the output port to the charge port, I fitted a 20cm 24AWG USB extension cable. Interestingly, the charge was still slower than just doing it from a regular 2A charger.

The fault was not with the power bank, as I reverted to the original QC2.0 wall-charger (to be shown in a future posting), but with a small modification.

I noticed that the shunt was not making quite a solid contact with the USB port on the QC2.0 charger only, so I decided to “crimp” the USB socket to flatten it slightly. This increases the contact pressure, which should improve contact resistance. Guess what? It worked and produced the ~3 hour 40 minute curve.

This is a practical illustration of the following points:

  • Having quick-charge does not necessarily overcome USB cable and contact resistance as this is a dynamic function of the “mating” of the two connectors. Each plug and unplug can produce slightly different results.
  • USB connectors are not made equal – what worked with my Xiaomi 2A charger did not work properly with my QC2.0 wall adapter.
  • While a known good source of the old Mi Power Bank Pro was used, the addition of an extension lead added additional contact and cable resistance, nullifying any benefits. As a result, it’s hard to know if you’re improving the situation or not when juggling around cables and chargers.
  • While it might be easy to fault the charger doctor’s connector as being “cheap and out-of spec”, chances are, most products have connectors which vary in dimensioning enough to create problems at high currents. The device sensed the resistance and backed-off its current draw appropriately to prevent any damage. However, compared to a proper contact with the 2A charger, quick charging proved to be (ironically) slower prior to this “hack”.

While pinching the connector does improve the contact pressure and contact resistance, it is not advisable to do. This is only a “temporary fix”, as by pinching the connector, you can cause permanent damage to the mated connector. This can happen by “grabbing” the tongue too tightly such that it fractures on applied stress or attempted disconnection, additional fatiguing of the contact springs through over-pressure and wear to the plastic tongue which can cause it to “thin” more rapidly and fail to apply proper pressure to other connectors. It will also cause the plug’s pin’s to be “flattened” more quickly, and the gold plating on both plug and socket to wear rapidly.

Other Issues with USB cables

While I have mentioned cable and contact resistance, there is another element of resistance that can come about from the connection of the wire to the plug itself. This point is usually soldered, and depending on the quality of the cable-strip and solder operation, the resistance can vary. It should be quite small by comparison, however, there has been some evidence to suggest that poor quality connections can occur in cheap cables due to over-stripping of conductors resulting in a few strands being fractured at the strip and not making connection. This produces a localized area of increased resistance, and could be responsible for cases where the connector-plug side shows melting.

Some other cables have been made with non-copper wires as well, which will likely have a higher resistance for the same wire gauge. Be careful!

There are “charge only” cables with only the two power conductors connected, but no data connections. Some may have them locally shorted to present as a dedicated charger to the end device. These cables are incompatible with Quick Charge technology which requires the use of the D+ and D- lines for signalling.

Another issue is with insulation wear-out. As USB cables get flexed quite frequently, areas near the plug may be flexed sufficiently for insulation between wires to break within the outer sheath. This is especially probable for specially “thin” or flat cables where regular PVC insulation is replaced by enamel coating (as with headphone cords). While not directly observable from the outside, such loss of insulation results in an internal short circuit within the cable, which can serve to heat the cable and cause it to burn. I suspect this may be what happened in this “viral” image of a burnt cable on a bed. Regular chargers are generally current-limited to source a total of 5 or 10W, whereas fast-chargers can source up to 18W. This is not insignificant, as a hand-held glue gun or battery-operated soldering iron needs just 7W to operate.

This isn’t as rare as it might seem either – I have been approached by a friend who claimed the Xiaomi power bank I recommended was poor because it charged the phone slowly and kept discharging on its own when connected to the cable with nothing attached. The truth was, the cable had a high-ish resistance internal short which was not enough to cause visible damage, but enough to waste energy. Had the cable worn further, a more drastic result may have eventuated, but the power bank was not at fault.

Conclusion

While the market was quick to respond to the wire gauge problem, it turns out connector resistance is equally as important, or even more important. Problems with connector resistance can arise due to improper connector design, improper dimensions in manufacturing and wear and tear to gold plating on contacts.

Simply upping the voltage may help the symptoms, but does not solve the underlying inefficiency of energy being lost as heat. Where quick-charging is used to improve charge times, the current flow magnitude is roughly the same and there is still a risk that poorly constructed cables with poor contact resistance can heat-up and melt. As a result, even if higher voltages are employed, devices are still sensitive to voltage drops in cabling and connectors to avoid the possibility of excess heat build-up which could lead to fire risk and property damage.

Traditional USB connectors were not specified with close tolerances for dimensions, which results in such problems in ensuring good contact between connectors of different makes. Cost reduction, by reducing materials used, and unconventional USB connector designs also can cause issues. Connectors wear out and become damaged over time. Newer connectors such as USB-C are much tighter on tolerances and should improve the situation, despite the smaller connectors and contact areas.

The solution to the issue is, unfortunately, not so simple for existing “legacy” USB connectors. Most of the time, consumers have absolutely no say about the connectors used in their devices, chargers, cables, etc. and take it for granted that it works well as it’s “USB” standard. Ensuring that products use “name brand” connectors from trusted connector companies (e.g. Amphenol, Molex, Foxconn, Pheonix Contact, etc) that are properly terminated can help. Reducing unnecessary connection/disconnection to avoid wear and avoiding physical strain on the connector is also advised. In a pinch, the situation might be helped by “crimping” the connector slightly, but that is likely to cause accelerated connector wear and is not advised.

Aside from the connector, the wire-to-connector joint is also a potentially vulnerable point, as well as the insulation within the cable assembly. Reports of “phone fires” and “connector melting” often fail to distinguish as to the true cause.

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