Advice sought on why 6.8A USB charger melted USB cable today

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Electronic components are more precise now than they were years ago. Thirty years ago most resistors were 10% or beter. Now they are often 5 %.
The electrolytic capacitors may be +100/-50, but most of the others are usually in the 10 % or less range. Even a cheep motor starter capacitor I have is labled as 6 %.
Back when the Simpson 260 was the 'standard' test meter it was difficult to measure many values very close. Now the 'free with purchse' Harbor Freight digital meter will go to about 1 or 2 % and good meters much less.
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tlvp wrote, on Thu, 03 Dec 2015 23:48:13 -0500:

That "bargain" 40W USB charger is being offered for $60 online. The more honest 35W USB charger is offered online for $15.
Which would you choose if you're buying watts?
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the same product line from the same company, with the only difference being 5w?
or are they actually two totally different products from different companies (one a more well known brand) and with different specs and different build quality??
in other words, your comparison of two unnamed products without any links is completely bogus.

why would anyone do that?
build quality and reliability should be a priority, not wattage.
if the charger fails, it could potentially fry the device, if not cause a fire.
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nospam wrote, on Fri, 04 Dec 2015 14:35:20 -0500:

Here is the EXACT SAME PART and BRAND and 40W price:
$30 (Amazon.com product link shortened)
$15 http://www.frys.com/product/8423918
Here is the competitor, again, with exact same everything but the wattage is correctly listed as 35 watts.
$15 (Amazon.com product link shortened)
$60 (Amazon.com product link shortened)
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nospam wrote, on Fri, 04 Dec 2015 14:35:20 -0500:

There is absolutely no way to tell the build quality. Absolutely impossible without an oscilloscope which you can't expect a consumer to have, and you can't even expect a consumer to tell the build quality.
So what you suggest, while utopian, is impossible for a consumer.
They are all UL approved.
They are all of a "brand" name. (What brand name would you choose anyway since Apple doesn't make them?)
They are all sold by brand name stores (Amazon, Frys, etc.) http://www.frys.com/product/8423918 (Amazon.com product link shortened) (Amazon.com product link shortened)
You suggest something that is patently impossible for a consumer to do.
This is all the consumer has for quality: Guaranteed Safety:UL certified design(No:E310745). Unibody design,size:8.9×7.3×3.3cm,Industrial-grade materials and premium circuitry ensure lifetime reliability.
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yes there is. stick with reputable companies, not noname crap.
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.. and most consumers (let's make up some numbers here ...), say >95% won't know/care. Remember, this is why windows "sells" so well - people don't know/care. :) The few complaints they get from people that do know and care, will be ignored. They don't care, because they are selling heaps of these to people that don't care. :)

.. then if the things break you take/send it back to the shop you bought it from for a refund/replacement. You did do this, right ? :)
(snip)
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On Fri, 04 Dec 2015 14:35:20 -0500, nospam wrote:

Don't be too quick to bogle at that discrepancy -- I too have seen exactly the same item being offered by a variety of Amazon Marketplace vendors at prices ranging from the ridiculous through the sublime to the astronomical. Not all that often ... but recurrently, all the same.
Books; automated cat feeders; BlackBerry PlayBook leatherette covers; ... . (For the cat feeder, the best price turned out to be the single unit price direct from the factory itself. One amazon vendor wanted double that :-) .)
Cheers, -- tlvp
--
Avant de repondre, jeter la poubelle, SVP.

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sure, but that's not the same thing. a seller can ask for whatever they want and buyers can choose to pay or not pay.
he didn't say it was the same product. often, people compare products that they think are similar, but it turns out to be different products with different specs, so it's no surprise that the price is different.
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nospam wrote, on Sat, 05 Dec 2015 07:16:49 -0500:

All the links I provided at various wattages and prices were the same product exactly (other than the name on the outside).
Most consumers aren't detail oriented, so, if they see only the $60 40 Watt part and the $15 35Watt part, they're likely to overspend to get the extra Watts which don't exist.
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On Fri, 4 Dec 2015 18:48:56 -0000 (UTC), Danny D. wrote:

Not the more expensive of the two, that's for sure. But not the cheaper one, either, *unless* I had reason to believe that at least one of the vendor, the manufacturer, or the marketplace in which the offering was made could be relied upon to guarantee my reasonable satisfaction.
Case in point: I just had delivered via USPS a CaseLogic 3-port USB charging device, described as having Output 5V, 4.1A, 20 W.
Translation: the nominal output potential is 5V (probably +/- 10%); the USB port markings show two ports marked 1A and one marked 2.1A. I wouldn't want to push those to the limit, either, btw. Anyway, those figures add up to -- are consistent with -- 4.1A total output); and the 20W rating probably should be read as "don't try to push your luck, 'cuz 5V at 4.1A will result in 20.5W of power, and this device isn't good for that much -- keep the output well under 20W, and you'll be OK".
IOW, individually, each USB port *may* be good for sourcing up to the current printed alongside it (1A, 1A, 2.1A), but certainly don't expect all three to deliver full current at the same time :-) .
(BTW, that was under $10 postpaid, tax included, from a vendor I trust. CaseLogic I see lots of vendors offering, over a very broad price-spectrum, so neither brand nor price was in any way a selling point, pro or con.)
HTH. Cheers, -- tlvp
--
Avant de repondre, jeter la poubelle, SVP.

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$15 ... I could get 2, making 70 W ... and still have $30 left. :)
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And that's why % are such a bad idea. :) :)
Agreed about resistors. The *really* expensive ones are +/- 2% (or less). :)
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On 11/30/2015 10:00 AM, Danny D. wrote:

The device can be powered by 110V at 60Hz (US market) *or* 220V at 50Hz (european). Internally, the device can be thought of as "autosensing" the input and adjusting, accordingly. (in practice, there's no real "sensing" involved; it will also work at 190V/53Hz as well as a multitude of other combinations! Possibly even DC!)
In those conditions, it will not draw more than 800mA.

The output will be 5V (nominally... it may actually be 5.1V, etc.) The TOTAL current available is 6.8A. Think of current as water flowing through a pipe -- at a particular RATE (amps). This says that the "pipe" can supply water (current) at a rate of 6.8A.

The power supply/charger will consume a maximum of 40W. As it will only DELIVER 34W (6.8A x 5V) to your "loads", this suggests the device is ~75% efficient (34/40). The extra "power" drawn from the AC mains is lost -- as heat.
Each connector in "supercharge mode" can deliver 2.4A at the stated 5V.
This is what a "regular" USB connector delivers

That would depend on what's inside the tablet. The short answer is: there was a potential (voltage) drop across the cable (i.e., from the charger end to the tablet end) *and* current flowing through the cable. The drop at that current rate expresses a power (heat) that is dissipated *in* the cable.
Hook a *stiff* car battery charger to a *good* battery (in techno-speak, both devices want to have low apparent impedances... not easily "pulled" from their desired state). Charger puts out X volts. Battery is *at* Y volts. X > Y causes current to flow TOWARDS the battery; Y > X causes current to flow FROM the battery.
But, neither the battery nor the charger are "moving" -- so, the difference (X-Y) is appearing across the length of the wire connecting them. I.e., the *wire* is handling the mismatch -- by getting hot! :>

See above. You consume foodstuffs every day. If you don't gain weight, AND, don't sh*t, you are 100% efficient in your utilization of those calories (hand-waving as not everything you "take in" CAN be "burned").
The 40W is what you are consuming; what the electric company is billing you for [sic]. The 34W is what you are actually able to *use* (in your tablet, etc.) from that. An "ideal" device would consume exactly as much as your tablet requires -- no "insertion/conversion" losses.
You'll note that the device gets warmer as your load increases -- cuz the losses tend to increase (in absolute terms -- power/heat) as your demand increases.

A USB device can "negotiate" it's power requirements. The socket (charger) can only deliver what it can deliver. A conforming device will not ask for more than can be delivered.
E.g., I can connect an iPod to a low power USB connector on a *keyboard* (notoriously low power because THEY use some of the LIMITED power available to them on *their* USB connection to the computer!) and access the contents of the iPod as well as transfer files to/from.
But, I can't *charge* the iPod over that connection -- because there isn't enough AVAILABLE power to do so (at least, not quickly)
USB ports (on computers) have sensors in them that will sense if a connected device has drawn more power than it should. In that case, the USB port will shutdown -- like tripping a circuit breaker -- until the device is removed. At that point, the circuit will (should) reset so another PROPERLY BEHAVING device can be connected -- without having to replace any real fuses (which was a problem with old PS/2 and XT keyboards) or having to "reboot".
Not all USB sockets are well behaved -- as loads *or* supplies! Caveat emptor.

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Back to "Danny D." again eh?
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I often ignore posts from Google. Use a real news client instead.
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Jolly Roger wrote, on Mon, 30 Nov 2015 20:12:47 +0000:

I've always been Danny D.
My Google+ page is https://plus.google.com/112608173732813956628
My Flickr page is https://www.flickr.com/photos/98287134@N02/with/9329893219/
I have been posting mostly to alt.home.repair for years, and they all know me as a reliable and detailed poster.
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Then you have a PIA, nymshifting doppelgänger who uses a particular posting style. Hmmm...
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Savageduck
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Aah! You also noticed that.
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Regards,

Savageduck
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Danny D. posted for all of us...

Call the CPSC and put them on *RED ALERT* for a recall. The problem is that it is not an Apple product so therefore you are bleeped. Sound the klaxton.
--
Tekkie

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On Monday, November 30, 2015 at 3:54:06 PM UTC-5, Tekkie® wrote:

B00T3FQBHO

/6000193376994

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n.

My guess is that part of the answer to this is that this is a cheap, Chinese no-name product. It probably has all the typical confusing specs, specs lost in translation, etc. Why it burned up the cable, who knows. What kind of support is there for this? I'm betting not much.
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