Tablet 110/240v Chargers, Charging Cables

[airdale]

A few months ago, I posted some results of experiments with 12/24vdc chargers and cables. The experiments were stimulated by a situation where my Nexus 7, while connected to a car charger with the GPS running, still lost battery charge and eventually died. I found a car charger that eliminated this problem. I also pontificated a bit on the importance of good USB charging cables.

Since then I have purchased a USB in-line ammeter and have been playing with wall chargers and playing some more with cables:

First result is that I have successfully reinvented the wheel. After testing a number of "2 amp" 110/240v chargers with Android and Windows tablets, including several chargers whose marking were outright lies, I found a Hewlett Packard charger (P/N 157-10157-00) that is really excellent. I was going to post the details but then I found this: http://www.righto.com/2012/10/a-dozen-usb-chargers-in-lab-apple-is.html from a couple of years ago. This guy reached the same conclusion I did and his research was much more thorough than mine. $10 each: Source: http://www.ebay.com/itm/310786548990?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649 or search eBay for "kmashi" and look for the round black ones. (Note: I did not test this charger with any iThings.)

Throw away the cables that come with the HP chargers. They are junk. In testing cables using a good charger and a tablet computer that would take 1.90 amps I found that the majority of USB cables in my inventory could not deliver the 1.9 amps, sometimes delivering only about one quarter of that. Without an ammeter, it is hard to spot good cables but I did find a reliable source: http://www.monoprice.com/Category?c_id=103&cp_id=10303&cs_id=1030307 and select the cables that are marked "28/24AWG." (This means that the current-carrying wires are wire gauge #24, heavier than most cables.) The good news is that monoprice is probably the best source of cables on the internet anyway and their good cables only run a little over a buck apiece. I have tested the 1.5 foot and the 3 foot cables and found that they reliably deliver the 1.9 amps that my tablet will drink.

 

[denverpilot]

Monoprice kicks ass again. No surprise there.

 

[pj500]

I like those MP cables but the ferret core is annoying for portability.

I bought a USB in-line ammeter off amazon a while back. Great toy for testing wall wart power flow.

 

[airdale]

... the ferrite core is annoying for portability.

Yup. I take them off.

Here's how: First squeeze the ferrite bulge in a vise a couple of times. The ferrite will crack easily. You'll hear it. Then cut the outside coating off, letting the broken pieces of ferrite drop out. I've had my best luck using a pair of diagonal wire cutters Finally, carefully cut your way down to the cable itself so you are left with just the thin plastic that is left-over coating from the inside diameter of the ferrite choke. Tear that off and you're done. With a little practice two or three minutes is all this takes.

Too bad they don't offer the good cables without the chokes.

 

[Ghery]

This EMC engineer cringes when you talk of removing the ferrites. I know why they are there and you may hear the results of removing them in your radio. YMMV, but that's the chance you take.

 

[airdale]

... I know why they are there ...

Actually (MSEE here), I think they are there to increase the perceived value of the cable and hence allow a little higher pricing. There is the long shot reason that they might have originally been tooled for an OEM who thought he might be a little marginal with the FCC, but I doubt it.

Regardless, I have never heard any symptoms of EMI from choked or unchoked USB cables even though, as you point out, it is always a possibility.

 

[denverpilot]

Actually (MSEE here), I think they are there to increase the perceived value of the cable and hence allow a little higher pricing. There is the long shot reason that they might have originally been tooled for an OEM who thought he might be a little marginal with the FCC, but I doubt it.

Regardless, I have never heard any symptoms of EMI from choked or unchoked USB cables even though, as you point out, it is always a possibility.

Actually there's been plenty. Let me fire up a legal and clean kilowatt of HF in your driveway. That Part 15 device on the end of the cable is going to be mighty unhappy.

I spent a few hours hunting down the noise problem on multiple receivers at a radio facility one day hat all tied back to the piece of junk cheap Ethernet switch someone installed in their equipment cabinet. The Fire Department engineer couldn't find any contact info for the idiots so he authorized yanking their power.

Their tech showed up an hour later when all their non-Public Safety gear dropped offline and we showed them the garbage their unshielded crap was throwing off on a spectrum analyzer after they got there.

Amazing how quick they drove up the mountain. And they learned how to shield and ground their gear, that day. LOL.

They also got a chat with the Fire Captain about labeling their cabinet with appropriate contact information.

 

[Ghery]

Actually (MSEE here), I think they are there to increase the perceived value of the cable and hence allow a little higher pricing. There is the long shot reason that they might have originally been tooled for an OEM who thought he might be a little marginal with the FCC, but I doubt it.

Regardless, I have never heard any symptoms of EMI from choked or unchoked USB cables even though, as you point out, it is always a possibility.

Immediate Past President, IEEE EMC Society here (38 years experience in EMC). It wasn't perceived value. The product qualified with that cable wouldn't pass emissions testing without the ferrite bead. Nobody is going pay the cost of putting that thing on the cable unless they had lab results showing the necessity.

 

[airdale]

Ghery, you're right of course. In trying to be brief I was not clear.

It is my understanding that MonoPrice tools their own cables for QC reasons. Assuming that is the case and since this particular cable is sold stand-alone, I see no reason for their adding the choke other than marketing "perceived value." And, speaking for myself only, I have never had an issue with an unshielded USB cable. I agree with pj500 that the choke is annoying, so I cut it off.

If the cable was originally tooled for an OEM who needed the choke, that would also explain its presence. But I would still cut it off. Because I don't need it.

Let me fire up a legal and clean kilowatt of HF in your driveway.

Certainly that would be a good RFI source, as would be my jr. high/high school ham radio transmitters and the AM and FM broadcast transmitters that I installed and maintained back in college and graduate school days. But that is not my world any more. If I lived next door to someone's antenna farm, things would probably be different as well. I might well be adding chokes to all my USB receiving antennas!

I would be curious to know if anyone has actually experienced EMI issues with USB cables, flying in our little airplanes, in cars, or at home. As I said, I have not. (The only EMI issue I have ever had is occasionally a turned-on cell phone will break the squelch in one of the airplane radios with its characteristic buzzy sound.)

 

[denverpilot]

I've had problems with whole PCs, didn't bother to see if their USB devices were installed.

But those problems were experienced at locations where I was driving receivers right down to the theoretical noise floor limit possible, at normal outdoor ambient temperatures, on various bands.

I suppose if I had a way to cryogenically cool those antennas like the Deep Space Network folks do, it would have been a bigger problem.

I've also seen the measured man/made noise floor on VHF 20 miles from Metro Denver rise a number of dB in two decades. All of these things contribute collectively to that problem. It's one of the reasons most Public Safety radio systems here have gone to some form of diversity receive system, be that true diversity and voting, or muti-site trunking. There are obviously other benefits to those also, but noise is a non-purchase-reason contributing factor.

So for the average Joe? Probably not much effect. Heh. It is noticinke to anyone trying to accomplish something they could easily do 30 years ago, though.

 

[Ghery]

Ghery, you're right of course. In trying to be brief I was not clear.

It is my understanding that MonoPrice tools their own cables for QC reasons. Assuming that is the case and since this particular cable is sold stand-alone, I see no reason for their adding the choke other than marketing "perceived value." And, speaking for myself only, I have never had an issue with an unshielded USB cable. I agree with pj500 that the choke is annoying, so I cut it off.

If the cable was originally tooled for an OEM who needed the choke, that would also explain its presence. But I would still cut it off. Because I don't need it.

Certainly that would be a good RFI source, as would be my jr. high/high school ham radio transmitters and the AM and FM broadcast transmitters that I installed and maintained back in college and graduate school days. But that is not my world any more. If I lived next door to someone's antenna farm, things would probably be different as well. I might well be adding chokes to all my USB receiving antennas!

I would be curious to know if anyone has actually experienced EMI issues with USB cables, flying in our little airplanes, in cars, or at home. As I said, I have not. (The only EMI issue I have ever had is occasionally a turned-on cell phone will break the squelch in one of the airplane radios with its characteristic buzzy sound.)

With that clarification, I agree. If MonoPrice is selling the cables alone there is no requirement for FCC approval for the cables. They can make them any way they want (so long as they work when I buy them). The ferrites come into the picture as a mandatory item when the cable is sold as part of a system and the ferrites were necessary to show compliance with the emissions limits contained in the FCC Rules.

 

[JohnF]

For the use described in the original post there are no emission concerns as only the dc power lines are in use.