Replacing An Overheating Tablet Power Supply

[Dave] has an ASUS tablet PC with a little problem. The device is charged via the docking connector’s USB cable when plugged into a special wall transformer. The problem is that the wall unit tends to overheat, and is shut down by a thermister inside to avoid permanent damage. The word on the Internet is to drop it in a zipper bag and chill it in the freezer for a bit. Although this works, it’s not the permanent solution that he was looking for. Instead, he hit the parts bin and built his own power supply replacement without buying anything.

The device is simply looking for 12V on the power pin (pin 1) of the USB cable. [Dave] dug through his mountain of unused AC adapters and found one that fit the voltage and current specs of the stock unit. He also grabbed a dusty old motherboard and plucked the USB ports off of the back. A bit of protoboard makes for a good base to connect the AC adapter wires to the ports, which was then covered with one big shrink tube. The result is seen above, and demonstrated in the clip after the break.


28 thoughts on “Replacing An Overheating Tablet Power Supply

  1. 12V on USB pin 1! Is that right? Wow, ASUS is really bastardizing the USB spec. I feel sorry for the person who some day down the road tries to charge their USB device with this adapter expecting it to output the typical 5V on the USB ports.

    1. Seriously! That was my first thought too: “Somebody’s going to blow up a phone with that thing one of these days.”

      Guess that’s what you get for buying Asus. I’ll keep it in mind!

    2. I came to say the exact same thing. Plenty of high powered tablets charge over 5 volts at 2 amps so it was stupid on their part to make an adapter that will fry any other device you connect!

      1. Do you have evidence that the power supply has any thing higher than 5 V. on the power pins, when a device other that what the power supply was designed to power is plugged into it? Without that evidence accusations it’s dangerous to other devices is inaccurate

    3. The stock Asus adapter won’t fry anything – it supplies 5v by default. The Asus charging cable shorts out a pair of USB3 pins (7 & ground, I believe) which prompts the adapter to switch to 12v. This hack, on the other hand, could easily fry something since it is an always-on 12v.

      The fact is, Dave has a defective supply. They had some trouble with early units overheating, but Asus has been good about replacements and it hasn’t been an issue with retail units lately.

      I’ve thought about hacking up a 2nd power supply to save the $30ish that Asus wants, but I would do it by getting a cheap charging cable and hacking the USB plug off to hardwire it into the 12v supply. Doing it with standard USB hardware and not including the protections that Asus does is just asking to kill a phone or other device.

    1. One man’s violation of a standard is another’s “thinking outside the box”.

      And neither of those guys have to deal with this problem…because they have your money and you’re stuck with a cheap, overheating power brick.


    2. I have the newer Transformer (Prime) and I just checked the power supply. It’s listed as 5V 2A or 15V 1.2A. I’m assuming they use some auto-switching based on the cable being plugged in, since it has a standard USB plug on it. To provide >5V out on the stock charger would be insane with a standard connector.

      Here’s a picture of the adapter from my Prime:

    3. Some of these wall supplies are REALLY noisy. I have gone through half a dozen to find one that will work for an Evaluation Module that I am creating. Some radiate so badly that it can be seen on a scope from a completely unrelated PCB that is several feet from the radiating supply and associated wiring. BAD!!!! This cost me more than an hour of work last year, trying to figure out what the hell was going on, only to find out that this crappy aftermarket laptop adapter on the other bench was screwing me over. Funny, but a big pain in the ass.

    4. Only a violation if the power supply has anything other than 5 V. available when devices other than the one it was designed to power, plugged into it. I haven’t read that it has 12 V. on the power contacts as a steady state

    1. According to somebody on the Internet, the stock charger provides 15V only when pin 7 of the USB connector is bridged to ground. (“USB connectors don’t have a pin 7!” — USB 3.0 connectors do.)

      So, yeah, nobody’s going to cook a phone with a stock Asus charger; it’s just the hack depicted here that’s dangerous.

      1. The Asus supplies are perfectly safe, since they only provide 15V when a USB 3 cable is used to connect a TF. (See the parent post).

        For USB 2 devices, they provide the standard 5V, albeit at a higher current (my phone charges in half an hour).

    2. Asus isn’t doing anything dangerous or stupid. Watch the video.

      What Asus provides is a NORMAL laptop-type power supply – terminates in 1.8 or 2.1mm male circular plug. An Asus adapter is used to convert that to a USB plug… which goes into the Asus. Everyone knows to assume USB plug means 5V, and everyone knows that round plugs mean assume NOTHING.

      The only non-standard thing about Asus USB is their port can handle either 5v (data) or 12v (charging).

      I respect the author’s right to do what he wants, but I think it’s bad engineering to make something that LOOKS like a 5V port but is 12V. It’s easy to say only you will use something, but you never know… friends will reach into your backpack for a charger if you step away for just 1 second. If he kept the adapter, something bad like this could not really happen.

      1. Actually the video does NOT show a coaxial power connector on the tablet end of the cable. However there was mention of a barrel connection to USB connection adapter cable cable to be able to use the universal wall warts.

  2. I’ve had similar problems with overheating “wall wart” supplies. Here are a few of my solutions:

    1. I drilled a large number of small holes in the plastic case to provide some airflow. Do it in a drill press and set the depth limiter so you barely go through the plastic, to avoid hitting anything inside.

    2. Open the supply, and see what is actually getting hot inside. They may have used an undersized diode, or too small a heatsink on a transistor, etc. I’ve replaced conventional diodes with Schottkys, or replaced the MOSFET with a part having a little lower on-resistance to get the heat down (and improve efficiency in the bargain).

    3. Put the supply in a larger and/or a metal case. It’s easiest if you add a short AC line cord, so the supply becomes a “lump” in the middle of the cord.

  3. The person that made the video documenting his hack, the comments to the video, or the comments to the Hackaday post haven’t stated there is a voltage higher than 5V. at the USB jack when it’s just simply connected to the line voltage, so there is no evidence that the power supply is not USB compliant. The fact that, it’s able to deliver 12V. when the device it was design to power commands it to,doesn’t take it out of compliance. Asus has had a history of manufacturing quality, so it’s interesting that many are so fast to pass, judgement with analyzing the facts or lack of them. Yea this hack may present a mortal danger to some devices that will be plug into it, but I thought hackers weren’t required to think very far ahead of their own immediate need. ;) In looking at one other you tube video on the topic, the root problem may lay in the design of the USB connector design. In that the the strength of the connection weakens, allowing the power contacts to generate heat, causing the power supply’s thermal protection to do it’s job.

  4. did the same a lot of time ago.

    it is the only way to charge a transformer that has a completly dead battery (the original charger waits for something in the central pin of the usb connector to supply 15Volts)

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