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First time asking here, but

Is there any potential harm in using a charging cable with a higher current rating than a charger it is connected to? It's a tablet (Asus P027) and its charger is 5V/2A only.

I bought a 1-meter/3.3-foot USB-A to USB-C (3.1) cable rated for 3A, it has a 56K resistor according to the QA on the product's page (Belkin 1m USB-A to C cable should pull it on google) and this is all I know about it.

  • No, it's good. The opposite is bad - in extreme cases the cable can melt and catch fire (happened to me, thankfully I noticed the smoke) –  Mar 28 '19 at 14:39

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No, only the device should not draw more current than the adapter and the cable can handle.

So the device defines how much current will flow through the circuit (which consists of your adapter and cable).

If the device draws more current, the adapter or the cable may overheat (with a fire hazard).

Btw, it's better to be safe and not use exactly a 2A but at least more, so you are safe with a cable of 3A.

Remarks from others Also, this yields for most (cheap) adapters. See the comment of Jarrod Christman for an exception.

For USB, there is a negotiation protocol. See the comment of Yakk below.

Michel Keijzers
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  • @Crasic, what I’m curious about are: 1. Does that same smart cable feature set exist using a USB-A to USB-C cable? 2. Are the power negotiation features required for the use of USB-C or can negotiations be bypassed like in traditional dumb USB cables? –  Mar 28 '19 at 17:31
  • I don't think this is right. If the device tries to draw more than 2A, the adaptor should drop the voltage to keep the current down (until it reaches the point where the voltage is so low that the adaptor cuts out altogether). – Martin Bonner supports Monica Mar 28 '19 at 14:30
  • @MartinBonner I don't have the theoretical knowledge, but afaik an adapter just 'gives' the current the load needs, and if the load requests more, the adapter delivers more, even if it cannot handle it. Of course, it might be possible there is some kind of safety circuit built in the adapter, but this is not common (for cheap adapters at least). – Michel Keijzers Mar 28 '19 at 14:41
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    @MartinBonner, that depends entirely on the adapter design and intent. Some power supplies will attempt to keep voltage constant and allow more current even beyond their own spec and die. This coupled with intentionally underrating for a safety buffer (a 3A power supply may actually do 3.5A fine), it is best to assume the worst case. Thus your power supply and cables should be oversized to some extent for your use, so you're not constantly against their max rating. –  Mar 28 '19 at 14:43
  • @JarrodChristman Thanks for the addition; I referred to your comment in my answer. – Michel Keijzers Mar 28 '19 at 14:58
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    Are you aware of how USB-C devices "negotate" with USB-C cables to determine how much current they can draw? Your answer seems to completely ignore this issue, which is at the core of the OP's problem. I believe 56k resistor in the cable communicates to the attached USB device how much current it can safely draw; barring something I don't know (which is plenty) that cable might convince the USB device into thinking "ok, drawing 3A is ok from this", frying the charger. This isn't a "dumb cable"; for a dumb cable your answer seems correct (more A=more safe) –  Mar 28 '19 at 15:19
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    (note: I am not an expert at this. Possibly there is a way that the other end of the USB connection (the charger) communicates to the USB-C device the current it supports, and/or handles being asked for more power than it is rated for gracefully) –  Mar 28 '19 at 15:23
  • @Yakk Yes, I added a comment to look at your remark; however, it is not that the intelligence is within the cable, afaik it's inside the USB protocol. – Michel Keijzers Mar 28 '19 at 15:44
  • @yakk, the OP just vaguely mentioned a usb “charger,” which vary considerably in intelligence and negotiation. Even if you’re talking about using a computer USB port, as outlined by the answer linked below, that varies a lot in terms of intelligence and proper implementation as well. Since the range of devices using USB charging vary a lot in sophistication I believe a lot of vendors go outside the USB spec for compatibility sake. –  Mar 28 '19 at 16:52
  • https://electronics.stackexchange.com/questions/222260/usb-power-the-basics –  Mar 28 '19 at 16:52
  • On your topic of USB-C, I am not as familiar with it. The cables can be smart, as in they identify information regarding protocol and power control... but I would be interested in seeing how well chargers and devices work within the spec. Technically you can just use the cable as a cable and ignore any and all negotiations I think. I say it’s stikl best to be cautious unless you know the entire chain. –  Mar 28 '19 at 17:00
  • One last comment, as it’s turning into an extended conversation with myself, haha. The OP actually mentions the use of a USB-A to USB-C cable, which may mean it’s a dumb cable as those are meant for backwards compatibility and lack the majority of the signaling wires of USB-C. –  Mar 28 '19 at 17:14
  • @yakk USB-IF USB C (in fact USB3.1 spec ) cables are smart and do in fact have embedded configuration ic that communicates the cable capability, This is used to ensure higher voltage safely with power delivery (20V). –  Mar 28 '19 at 17:15
  • @JarrodChristman USB-A to USB-C contain that resistor, which is the "smart" part that communicates "this isn't a real USB-C cable" to the USB-C device. Lacking that resistor the USB-C will draw too much power and plastic will melt. (Ask me how I know...) –  Mar 28 '19 at 17:46