HP TouchPad Charging

I managed to get a cheap HP TouchPad recently.  One of the things I’m playing with is using Android and a Bluetooth OBD2 reader to monitor my car’s computer.  However, if I want to use it in the car for any length of time, I figured I’d want an automotive charger for it.  Similarly, I wanted a home AC charger as a spare and with a different design that fit better in the desired location.

Some people suggest simply using the stock AC adapter and buying an automotive inverter.  I already have an inverter in my car for other things, but there are a couple things that make this a less-than-ideal solution.  First off, the AC produced by cheap inverters isn’t the same as “real” AC in your house.  While it probably won’t hurt a charging circuit, electronics in general don’t like the power from these inverters.  A good inverter will probably cost more than your tablet did.  Second, you’re taking DC from your car, converting it to AC with the inverter, then using the AC adapter to convert it back to DC to charge the TouchPad.  There’s loss involved in each step, so this is a very inefficient method.  Additionally, even a small inverter adds quite a bit of bulk over a simple USB adapter that plugs into a lighter socket.

First, some USB basics.  On a standard PC’s USB port, the outside pair of wires (red and black) supply up to 500mA at 5V, to power USB devices.  The inside pair of wires (green and white) are the + and – data lines, which actually transfer data from storage and input devices.  According to the USB specs, all devices are supposed to handshake and let the PC know that they’re using power.  However, most cheaper non-data devices simply leave out the data section and just assume the USB port will provide 500mA @ 5V.  To allow devices to differentiate between a USB port and a charger that has a non-data USB-shaped port, it was standardized to short the two data lines together in the charger.  If the device detected the short, it would know that it was hooked to a charger rather than a regular USB port.

However, newer devices (advanced smartphones, tablets, etc.) use more power and have larger batteries, meaning they can benefit from chargers that supply more than 500mA.  Their AC chargers often supply ~2A, but they maintain the USB form factor so the same cable can be used with a PC.  To tell these high-amperage chargers apart from cheaper/earlier chargers that mimic USB’s 500mA limit, vendors started using voltage divider circuits on the data lines.  Depending on the exact value of the resistors used, the voltage on the two data lines will vary, allowing devices to distinguish between different chargers.  This means your tablet can pull 2A from its AC charger and limit itself to trickle-charging from your PC’s 500mA USB port (or your old cheap AC-USB charge adapter).  I don’t believe there’s a standard for this, but it generally works out fairly well.

The problem for TouchPad owners is that even though the TouchPad’s charging requirements pretty much match the iPad, the two devices are built to look for different voltages on the data lines.  From everything I’ve found online (my own research plus reports of others’ research), there are essentially no third-party chargers for the TouchPad.  A number of companies have rebranded their iPad accessories as TouchPad accessories.  Technically, iPad chargers will charge a TouchPad, but only at the default 500mA rate.  If you leave the TouchPad’s screen off, it will charge up very slowly.  If you actually have the screen on and are using it, the trickle will not be enough to charge or even keep the battery level steady; it will drain the battery, albeit a little more slowly than without the charger.  A number of people have reported that Brand X charger will charge their TouchPad without any modifications, but every report I’ve seen has indicated that this was at the 500mA rate, not the full 2A rate.  So yes, those chargers will work, but not in the proper sense that we’re looking for.

I chopped up a spare USB cable and went to work with my multimeter on the stock and third-party chargers.  This thread has some good info and diagrams for creating these voltage-divider charging circuits/adapters (though aimed at other Android devices, so don’t use their resistor values).  After mucking around for a while, I discovered a post by wilywyrm where he sacrificed his stock TouchPad charger to see its details.  HP uses a 240Kohm pull-up resistor between the 5V and D- lines, and a 300Kohm pull-down resistor between D+ and ground.  RICKV did quite a bit of experimenting with the resistors in different chargers, including the Griffin P2275 that I happened to choose.  RICKV removed the stock SMD resistors and soldered through-hole resistors to the pins of the USB port.  You can see the 240Kohm (red-yellow-yellow) pull-up on the left and the 300Kohm (orange-black-yellow) pull-down on the right, with solder bridging the two data lines in the middle.  I chose to get SMD resistors (300K, 240K) for the Griffin P2275 and RND AC charger that I ordered.  I didn’t take pictures of my own, but you can see where the stock resistors were in RICKV’s P2275 pic.  I simply removed the old ones and soldered in my replacements.  On the RND, only the top port (the one furthest from the wall when plugged in) is designed for higher amperage.  When you unscrew it and get the circuit board out, there are six resistors in the area of the USB port.  The two you want are actually separated a bit from the other four, nearest the edge of the PCB.  Once again, I removed the old ones and soldered in my own.

I put both chargers back together, and webOS happily charged at the full rate, showing the lightning bolt on the charging icon and not complaining that I was using an underpowered charger.  Success!

A few things to keep in mind…

  • When you modify a charger like this, you’re converting it from whatever device it was intended to charge and making it work on the TouchPad instead.  It may still work on other devices after the modification, but you should assume that it won’t.  If you want to use the charger for other devices, you may wish to make a charge adapter instead.  From the thread linked before, this post has a good pic of what the adapter should look like and this post has a good diagram of how it should be wired up (use the one on the right).  With this adapter, you’re using your own resistors to provide the desired voltages on the data lines (and completely ignore whatever is already present on the charger’s data lines) to make your TouchPad think it’s connected to the proper charger.  This will allow it to pull the full amperage from any charger; you should be able to use it with an iPad or Android tablet charger, regardless of what the charger was made for.  However, this also means that the adapter will cause your TouchPad to try to pull that amperage from a USB port or a weak charger as well.  If the charger isn’t rated to handle the amperage the device is trying to draw from it, you could damage the charger (which could in turn damage your TouchPad), possibly even starting a fire.  Don’t risk your new TouchPad or even your house because you didn’t want to spend the $10 on a charger that can actually handle 2A.
  • If your charger has multiple ports, make sure you know how the ports are wired up.  My RND only supports the higher amperage on one port, so that’s the one I changed the resistors on.  On my P2275, I only changed the resistors on one port; the other side was harder to get at and this way I can still use the second port for my iDevices.  I used a red permanent marker to color the white plastic tongue in my modified ports for easy identification.
  • The actual resistor mods aren’t extremely difficult.  However, unless you’re using the exact model (and possibly revision) someone else has already modded, you’ll need some electrical knowledge and a multimeter to figure out exactly what you need to change.  Likewise, SMDs are usually soldered with a hot air gun, not a standard soldering iron.  Depending on the charger and resistors you’re working with, you may be able to do it with a standard iron or make your own DIY hot air iron.  Also, SMD resistors are small.  When I was getting my new ones out of the packaging, I had to be careful not to lose them.  When I got my chargers apart, the stock SMD resistors were even smaller.  The size difference also complicated the task of installing the new resistors.  This mod isn’t impossible, but it’s more complicated than when you soldered a resistor to an LED in Electronics 101.

Hopefully this consolidated information will help some others out.  I did do a fair bit of experimenting on my own, but the posts that I linked to were a great help.  I couldn’t have done this so quickly and easily without their research and postings.  Thanks to everyone who posted something helpful and spent their own money destroying chargers in the pursuit of knowledge.

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