How listening helps detect short circuit issues

Article By : Brian Zabel

Simple observation is sometimes what it takes to solve a problem, at least, that is the case in this featured phone battery charger issue.

Addiction to mobile phone usage at the centre of a family conflict has not been an uncommon problem as of late. Per a Common Sense survey in 2016, 70% of parents and teens argue on phone usage with 77% of them admitting the arguments are due to distraction by mobile device usage during family time. I may not be having the same issue with my stepdaughter but I certainly would not have a similar argument when she approached me one school night.

It was spring and I was at home, winding down for the evening, when I got a troubleshooting request from my frantic stepdaughter. It seemed that her mobile phone was almost dead and would not charge at all. Those with young teens in their lives will appreciate that her social life would purportedly suffer irreparable damage the next day unless I provided a fast and immediate solution. To calm the “client” without making any promises, I started with, “Well, let me take a look.” This was followed by more than one “Hmmm” to assure all that much thinking (not to be confused with inaction) was taking place.

I started with the big picture, obvious stuff: The phone’s charger was a custom wall adapter, not a universal USB type, so a look through my stash of random USB cables/chargers would be no help at all. Of course we had no spares of this brand on hand, and one would be expensive at a store if it was even available for this older model. Not that I¹d go out at that time of night; sleep trumps texting at my age. I’d even loaned out my multimeter, so advanced diagnostics would be limited to say the least. It seemed like a whole lot of unknowns and few real options even if I found something; not a good start for the home team. But I wouldn’t be a good sport without at least trying, especially in the eyes of a teenager, so I rolled up my sleeves and got to work.

At first look, I noted that the cable connection to phone's charging port was obviously loose and wobbly—a casualty of a teenager's rough-and-tumble life. Moving the cable end around would sometimes let the phone’s charge light blink on; an excellent sign, since it meant the unit could charge. I knew all about finicky connections and have had some luck with careful wiggling, rubber-banding, and generally finagling a temporary rig to put just the right pressure in the right place to maintain electrical contact, which would last indefinitely as long as nothing looked at it wrong. But, surprisingly, there was no position of phone nor connector that worked consistently. The phone would still sometimes work, but it didn’t seem to be in response to my connector wiggling.

The scenario looked grim; I started to wonder if the surface-mount connector inside the phone had finally twisted loose, taking some traces with it. Such a failure could mean a painstaking repair or an expensive replacement. Plus, I had no great soldering tools available, and it was already past bedtime. Talk about pressure at a customer site!

Then I noticed a faint whine coming from the wall outlet, pulsing on and off about once a second. “Yeah, it does that sometimes,” she said when I asked. Aha! That’s the behaviour of a smart switching supply trying to source a short circuit. The supply shuts down to prevent over-current damage, but then dutifully attempts to bring power back a moment later, producing audible switching-frequency noise as it momentarily drives maximum current. I had sudden hope in that the phone was not likely to be internally shorted; it was still working after all and sometimes charged.

I removed the plug from the phone and still heard the same pulsing whiney noise. So the cord itself had a short, and not the phone. A close inspection along the entire length of the cable revealed a small hole in the insulation where something our dog might had pierced it. The hole, being so tiny and on black surface, was practically invisible among the scratches and creases that scarred the well-used cord. I listened carefully as I flexed the wire near the puncture site. A few firm twists and some bending silenced the whine, removing the short circuit without breaking any connections. The supply was ready to go.

I carefully plugged the cable back into the phone's input; sweet silence reigned. The input connector, though wobbly, proved to be electrically solid; that was a red herring all along, and a testament to the abuse those mechanical connectors can take. It looked like we were back up and running, at least well enough to charge through the night and endear me to my stepdaughter—a reward of great worth indeed! With the disaster averted, I got some good sleep after all. And the next day I spliced the cable properly, permanently patching the pulsing power.

It pays to listen, even when tired and under pressure, and especially when your ears are about the only diagnostic tool you have.
Originally published on EDN.

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