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The new MacBook Pro have a bunch of USB-C ports and no ethernet port, so you have to use an adapter.

The issue is that as long as the adapter is connected to the MacBook, it gets very noticeably hot, even more than the power adapter.

This happens both when the ethernet cable is plugged into it or not.

Now, from reading other comments this seems to be normal behaviour, but I'd love to understand why! From an user's perspective, this does not seem right, and looks like it could cause the adapter to fail faster than it would without heating up.

I have a USB-C to Ethernet Adapter, model belkin F2CU040. It gets hot as soon as plug the 100 Mbps Ethernet connector. As you can see in the figure, we are more than 20°C above the ambient temperature.

enter image description here

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  • The adapter has active circuitry inside, which is active whether it is in use or not. FWIW, this has been the case for all Ethernet adapters I've ever owned.
    – IconDaemon
    May 26, 2017 at 17:09
  • Heat in electronics is merely the result of current being resisted as it passes through a medium. All electrical devices generate a certain amount of heat, even if you can't feel it. Gigabit ethernet adapters tend to use a lot of power relative to other peripherals, so a bit of radiant heat is to be expected.
    – William T Froggard
    May 26, 2017 at 17:11
  • and consumes a lot more power than WiFi. will not buy again.
    – AqD
    Feb 15, 2019 at 15:26
  • This adapter also does not go to sleep, it keeps the link while the host is sleeping which means the host is still delivering energy to it. I contacted Belkin and they told me this adapter is NOT compatible with the MBP2019. Seriously, they said that. It says so on their website, it is only compatible with the 2016. Despite being listed on the Apple website.
    – PeterT
    Nov 27, 2020 at 18:47

2 Answers 2

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It heats up because it's consuming/manipulating current.

Even though the adapter isn't in use, it's technically powered up.

Without going too deep into electronics engineering, it's important to remember that the USB port is supplying a minimum of 5V. Most USB-to-Ethernet controller chips operate at 3.3V (i.e. Microchip LAN9500A controller)

The factors that would create heat are basically twofold:

  • Reduction of the voltage to 3.3 volts
  • Powering the chip

Voltage Reduction

To reduce the voltage, manufacturers use one of two components - linear regulators or buck converters. Linear regulators, which are quite cheap compared to buck converters reduce power through the dissipation of heat. Buck converters on the other hand, are extremely efficient (meaning the run cooler) but more costly. In low voltage scenarios, it's not uncommon to find linear regulators since the tolerances are so generous.

Powering the chip

Looking at the specifications page, the chip referenced above uses 395mW (.4W) in "low power" or "Suspend0" mode. Keep in mind, the device manufacturer may not implement these low power features for cost savings.

When it's transmitting (sending data) it uses 692mW (.7W) of power.

In terms of heat transfer, 1W = 1 Joule per second. Where a Joule = "The heat required to raise the temperature of 1g of water by 0.24 °C"

TL;DR

Between the chip transmitting/receiving ethernet packets as well as converting ethernet to USB as well as the heat generated by reduction of the voltage it's supplied, it's not surprising the adapter can appear "hot" especially if the manufacturer uses less efficient components for cost savings.

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  • While this is technically impressive, since the user who posted the question did not specify what USB-C dongle or adaptor they are using this answer cannot be precise. You are providing an example of a particular kind of chip (how do you know that is what is used in any USB-C adaptor/dongle by the way?), but that is not necessarily the same chip in each USB adaptor/dongle. In any case, maybe I'm just grumpy because my last answer got deleted where I gave (far more general/less technical) descriptions of two specific dongles that get very hot to the touch. But the OP question is very general.
    – SeligkeitIstInGott
    Nov 2, 2017 at 16:14
  • Also, as I observed (in my first and second attempt at an answer) I have noticed that the two dongles I have used (the SIIG JU-H30712-S1 & SMK Link USB-C Multi-Port Hub) will significantly heat up just by plugging in the ethernet cable even when the computer is idle and there are no specific user-initiated network traffic transactions going on (though perhaps some minimal amount of other traffic generated by the OS might be doing something in the background, like checking for software updates or what not). But the point is that it's not necessarily dependent on the amount of user utilization.
    – SeligkeitIstInGott
    Nov 2, 2017 at 16:27
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    @SeligkeitIstInGott Without both ends using the Energy Efficient Ethernet standard, Ethernet requires a signal be transmitted continuously, even when no data is being carried. It's an inherent design limitation of all high-speed serial protocols.
    – user71659
    Nov 2, 2017 at 18:58
  • @user71659 That makes sense. Thanks for the response.
    – SeligkeitIstInGott
    Nov 2, 2017 at 20:43
  • It's this adapter: https://www.apple.com/shop/product/HJKF2ZM/A/belkin-usb-c-to-gigabit-ethernet-adapter. In this case it's probably the voltage reduction, since the heat dissipated seems to be the same whether the USB cable is plugged in or not.
    – brocoli
    Mar 23, 2018 at 22:03
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Circuits heat up when electricity goes through them. There must be some circuit created regardless of the status of the actual Ethernet port. It probably has something to do with converting an Ethernet signal to something readable via USB that is always on.

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