One of the products being shown a CES is a dongle intended to let you charge a tablet from a laptop USB port that normally wouldn't work because it didn't provide enough power.

The CNET writeup implies that it's actually boosting the power output from the port beyond the normally available amount. This seems improbable to me because the ports should have current protection to protect themselves from shorts. Also, if it was possible to bypass the current limit externally, I'd've expected at least some tablet vendors to have included the ability to do so directly in their cables.

One possibility that occurs to me is that the dongle is just spoofing responses to the tablet when it queries the laptop about the maximum power it can deliver while only providing a trickle charge that the tablet would otherwise reject as too slow to bother with.

  • \$\begingroup\$ "the ports should have current protection to protect themselves from shorts" is correct, but that does not necessarily imply that this limit is at the 500 mA (or any other specific figure). The only explicit obligation of the host is to limit the current "to a safe value". Read Olin's answer if you need more words :) \$\endgroup\$ Jan 11, 2013 at 14:42
  • \$\begingroup\$ When I was in first year of Comp-Eng, I use to carry around a USB cable with some wires sticking out so that I could get 5 V whenever I needed. A few times I made silly mistakes which shorted out the circuit. The USB power just turned off and there was never any damage. I wouldn't recommend it though. \$\endgroup\$
    – JSideris
    Jan 11, 2013 at 14:56
  • \$\begingroup\$ @Kortuk This seems off-topic to me as a clear impact in consumer-electronics support. There is no design here, just discussion of what designers at the product manufacturer are constrained by, but since it is an end user rather than a product designer posting the question it is clearly off topic and must be closed. \$\endgroup\$ Jan 11, 2013 at 19:59

3 Answers 3


It might be possible in some cases, and it is probably safe to try.

USB only provides 500 mA maximum, and that is only after requesting and being granted that level of current. However, the spec does not say a port can't supply more, only the minimum it must supply in various circumstances. The extra logic to limit the port current only to what was negotiated isn't worth it on systems that have plenty of power availble. Consider that 500 mA at 5 V is only 2.5 W, which is small fraction of a 150-350 W power supply commonly found in desktop systems. On such systems, they usually just put a polyfuse in series with each USB power line and call it a day. If you don't draw more than 500 mA, all will be OK. Polyfuses don't have tight trip points, so there is usually considerable margin.

Laptops, on the other hand, have limited power and therefore do usually manage their USB ports carefully. There will always be some margin built in, but don't expect a lot more from a laptop USB port than what you're supposed to get. The margin will be just enough to guarantee the minimum is met under all conditions.

As for it being safe, probably most of the time. All these devices can't afford to blow up when something bad happens and power and ground are accidentally shorted at a port. I'd be surprised if you find a computer that is damaged by that. It's not that much more expensive to build in some basic protection, the probability of crap happening is high enough, and the cost of damage from it high enough that such protection is worthwhile.


There has been some comments that claim the USB spec requires a host to limit the current it sources to 5 unit loads (500 mA). That is not correct. It was also noted that some hosts blow physical fuses if devices draw too much current. That may be true of those devices, but then those aren't real USB ports.

From the USB 2.0 spec, section 7.2.1, first bullet, page 171, referring to hosts sourcing current:

... must supply at least five unit loads to each port.

Then in section "Over-current protection" on page 173:

... The preset value cannot exceed 5.0 A and must be sufficiently above the maximum allowable port current such that transient currents (e.g., during power up or dynamic attach or reconfiguration) do not trip the over-current protector.

And later in the same paragraph:

The over-current limiting mechanism must be resettable without user mechanical intervention. Polymeric PTCs and solid-state switches are examples of methods, which can be used for over-current limiting.

So clearly:

  1. A host is not only allowed to provide more than 500 mA, it actually must at least for long enough to ride over connection transients.

  2. The only stated upper limit on current a host can provide is 5 A!

  3. Fuses that blow and have to be replaced are not allowed.

  • \$\begingroup\$ The USB battery charging spec allows for up to 5A current draw over a USB port that implements it. \$\endgroup\$ Jan 11, 2013 at 15:51
  • \$\begingroup\$ @NickJohnson I'm aware of that; but cnet is claiming this dongle is able to provide that level of power from ports that don't implement it and from which tablets normally refuse to charge which has always stuck me as petty (an adaption of the "you can do this but you won't be happy with the results" message from putting a USB2 drive on a 1.x port while allowing trickle charging to occur would have been my choice). \$\endgroup\$ Jan 11, 2013 at 16:26
  • \$\begingroup\$ The 2.0 Spec does indeed say a device can only draw/a port can only supply 5 units loads (100ma x 5). It lists a maximum as well as the minimum. And look at the non-compliant devices like eeepcs and rasberry pis. Overcurrent draw on the usb ports result in blown fuses you have to desolder and replace. \$\endgroup\$
    – Passerby
    Jan 11, 2013 at 18:50
  • 3
    \$\begingroup\$ @Passerby - You've actually found a computer motherboard with traditional fuses on it? Really? Every motherboard I've dealt with either has polyfuses, or some sort of FET-based overcurrent cutout. \$\endgroup\$ Jan 11, 2013 at 20:28
  • \$\begingroup\$ @Passerby: The USB spec doesn't say the port has to limit the current to 5 units. It is the device's job to limit the current it demands. The port is within spec to shut down as soon as the device draws 501 mA, but it is allowed to let it draw more if it wants to. Congratulations if you found a device that actually blows a hard fuse on overcurrent, but most don't. I think that "probably" safe, with the accompanying explanation, is still correct. There are many many devices out there, including all PCs I've ever examined at that level, that won't be damaged at all. \$\endgroup\$ Jan 11, 2013 at 21:37

I can think of one cheap trick to do this:

USB charger schematic

The device thinks it is connected to a battery charger and draws much more current than 500 mA. Note the missing USB logo on the pictures of this adapter. Nobody said that there is a data connection possible with the adapter.

It works in most cases because the USB ports on a laptop have a common voltage source, so with 4 USB ports you might be able to draw up to 2A on one port without much risk.

Problems will arise once you have more USB devices - like Bus powered USB HDDs - and thus less current available. USB Hubs will probably not like this very much even when self-powered, so connecting this adapter to a Hub is a no-go.


The specific ChargeDr dongle you list claims to cut charging time (by 20% to 400%), and accepts "Input: USB 2.0 & USB 3.0 Port, 5V, 0.5A~2.1A Output: 5V, up to 2.1A". It claims to be FCC certified. The problem is that it is mostly marketing. The reason iPads connected to a computer USB port say they are not charging is not because they can't charge at 0.5a, but because they were designed not to. If the USB connection doesn't identify itself as a high power current source, like the iPad chargers or USB3 ports can, then it won't charge.

The problem is that this is a brand new device, claiming it can do what the law of conservation of energy says it can't. It's so new that no one has been able to open it up to see how it works. The FCC id for the device isn't available online, so no one can check the FCC id page for any documents.

And you are correct, it does "spoof" in a way. It's a man in the middle management chip. It assumingly enumerates with your computer one way, then enumerates with the charger in another. It acts like it's an off the wall power supply instead of a proper USB host.

From their website:

Will ChargeDr harm your electronic devices?

Absolutely not. Your devices have a power management chip regulating input power received to charge the internal battery. ChargeDr interacts with this chip to define the highest amount of power you device can safely handle and requests it from the computer or charger.

  • \$\begingroup\$ My guess is that it's even simpler than that; there are various "standards" that companies use that allow a device such as a tablet to identify when its connected to a wall charger. Apple etc have some resistors tied to the data lines in the charger that the tablet can detect, and based on that it will draw more than if it enumerated as a USB host. This device is cheating by adding those resistors in the dongle. World's most expensive resistors yet?... \$\endgroup\$
    – hex4def6
    Jan 12, 2013 at 1:19
  • \$\begingroup\$ @hex4def6 That's the thing, it needs (should) enumerate on both sides. At the very least it's a simple usb slave with toggleble downsteam pins. \$\endgroup\$
    – Passerby
    Jan 12, 2013 at 1:36

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