USB Apple-style clone power supplies

Analysis of the Apple iPhone USB charger: and the fake charger, which sucks:

I bought 10 of the fake iPhone chargers on eBay. One exploded and another is intermittent. The others are still working.

So, 80% okay.

The one I have is different from both of those ones. It has two boards, but a similar circuit to the "fake" above. Self-oscillating TO-92 "MJE13003", regulation is just a zener series with an opto IR LED. No input fuse, no input CM choke, limited output filter, no OVP.

To trust your life or even a $600 iPhone or $300 iPod to such a circuit would be madness.

Best regards, Spehro Pefhany

Perfect for charging/feeding ANY USB battery/device requirement.

EMI from the charger to the touch screen? Supply noise, maybe, but EMI?

"Apple's power adapter is clearly a high-quality power supply designed to produce carefully filtered power. Apple has obviously gone to extra effort to reduce EMI interference, probably to keep the charger from interfering with the touchscreen."

There's CM filtering in the Apple supply. If the power supply has a lot of HF hash relative to the mains then that will appear on the capacitive touch screen inputs.

Best regards, Spehro Pefhany

Yep. Projected Capacitive Touch (PCT) screens can be sensitive to EMI sources, such as switchers. The big advantages of PCT over resistive sensors is that the front screen can be hardened glass (i.e. Gorilla Glass) and that you don't have to stomp on the display in order to get its attention. (Try your Android thing with 1 to 6 pieces of paper between the screen and your finger).

Vendors use various methods to reduce this sensitivity. (See Pg 4 for charger interference)

Reduction of Electromagnetic Interference in a Capacitive Touchscreen System

Thanks for posting this, useful!

Best regards, Spehro Pefhany

We furnish a nice Samsung supply with some of our USB-powered products. It's rated 700 mA, costs $4 with the cable. We tested some "1 amp" cheap (like, $1) ones, and they wouldn't start some of our boxes up. They saw the startup surge (into our switching regulators), shut down quickly, and tried again later. Bip.....Bip.....Bip.

When programming our induction power supplies, we sometimes have the dongle connected to a laptop while in operation. The trackpad becomes, less useful shall we say :) That the connection can be made to actually work with "only" a couple ferrite beads is probably testament to the power supply's design.

Tim

Well this all comes down to the question "How big is an inductor". There is the physical size, then there is the region that it effects.

I have two touch screen devices, neither Apple, and no issues when used with the usb power supply connected. Maybe this is an Apple problem more than a power supply problem.

I guess the next question is how clean is the 5V you get from a PC connection?

It's not unique to Apple. Because of the demand for scratch proof touch displays on everything from instruments to smartphones, the predominant technology is (PCT) Projected Capacitive Touch. The interference problem is common to all PCT displays.

Dunno and don't care because I don't run my Droids from a PC power supply. Much more interesting would be the effect of EMI on the 12V battery line via an automotive cancer-stick igniter powered cell phone charger.

Find a switching power supply. Give it a device to power (because most small switchers turn off when there is no load). Put one hand on the switcher (or on its ground terminal), and use your other hand to scroll or paint on your Android screen.

My Droid X seems unaffected, but a Droid A855 and iPhone 3G show a slight problem. Scrolling becomes slightly erratic and no longer smooth. Selection require repetitive finger pressing. Admittedly, it's barely noticeable and I had to try about 8 different wall warts before I found one that conducted enough EMI to be noticeable. I thought it would be much worse. Trying other switchers within reach, the worst is a Statpower 10A float battery charger. With one hand on the negative terminal, and the other on the phone, the touch screen is usable, but jerky. Again, the Droid X seems largely unaffected. Still, it illustrates the power supply interference problem.

I guess I don't have any crappy USB power supplies. I can't see any difference, and I'm trying stuff like wrapping the power cord around my finger or the phone, holding the mains/cig-socket end, etc. Phone is a HTC Desire Z.

Think about it. These phones have DC/DC inside and freakin' RF transmitters. That should be plenty of noise to cause problems.

Maybe the iphone, in the desire to be thin, skimps on filtering the DC input. Caps and inductors require space.

I thought for sure the cigarette lighter adapter would be noisy. It is some $5 Fry's special.

Put me in the dubious camp on this one.

Probably not so bad since it's probably not isolated, and the voltages are smaller.

Tim