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We're all familiar with the USB-A 2.4 GHz receivers from companies like Logitech that are absurdly small and barely protrude from the port. But when looking at available USB-C equivalents, they are quite a lot larger even after accounting for the fact the width of the port is reduced. Why is this, and can they actually get any smaller?

USB-A Example:

USB-A

USB-C Example:

Adam
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  • In addition to the answer below -- no, they can't get smaller. **Something** needs to stick out for the antenna! And also so that you can grab it to get it out again. – Dave Tweed Apr 14 '22 at 01:22
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    Have you actually figured out the volume of both these dongles? That USB-C looks no thicker than the USB-A, about the same length, and less wide. Yes it protrudes more but that doesn't mean it is overall larger. – Ben Voigt Apr 14 '22 at 22:57
  • Probably hard to make them smaller in volume for now, but I for one would prefer one that's wider but doesn't stick out as much. Less leverage to break your port when you inevitably hit it with something. Might run into trouble in laptops where there's other ports right next to the USB-C one, though. – Matti Virkkunen Apr 16 '22 at 12:55
  • USB-A dongle seems to be very excited in this picture. – Farid83 Apr 28 '22 at 06:16

2 Answers2

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Usb thumbnail sized adapters take advantage of the larger size plastic support of the usb A connector.

It goes from this:
enter image description here

To this:
enter image description here

They take the 2.25 mm of plastic support and replace it with a double sided pcb.

But Usb C is tiny. Smaller in width and height and depth. The female side connector floats in the shell, and is double sided. The male side connector has spring pins on both sides taking up 1mm each of the 2.6mm connector. It's almost paper thin shell and has no room for a normal fr4 pcb.

enter image description here

Frankly there is no space to replace the connector with a pcb and parts.

https://fccid.io/HQX-R07BT/Internal-Photos/Internal-Photos-2286089 https://www.mouser.mx/blog/usb-type-c-explained?cm_mmc=PressRelease-PR-_-Molex-_-USB_Type_C_Solutions-_-2016-04-13

Passerby
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    Great answer! Honestly, they'd probably be able to make the USB-A dongles even smaller, but at some point you wouldn't be able to pull it out of the socket without tweezers anymore. – Ilmari Karonen Apr 14 '22 at 17:25
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    @IlmariKaronen some are smaller than pictured, that one needs more space due to the antenna. Usb flash drives barely stick out. – Passerby Apr 14 '22 at 17:54
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    @IlmariKaronen We created a USB-A solution that was meant to be sealed in the slot (intentionally with glue) and it didn't stick out at all, so it's quite doable even with fairly low tech work. No custom silicon, just PCB and a tiny (ARM?) processor if I remember correctly. – DRF Apr 14 '22 at 22:24
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    @DRF Just curious, can you disclose some details about the scenario in which your solution was employed. It sounds like some security thing. – LorenzoDonati4Ukraine-OnStrike Apr 15 '22 at 23:10
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    @LorenzoDonatisupportUkraine In very general terms it was used as a secondary under control TPM. Mostly it held some keys which couldn't be accessed without a second element. Nothing all that special. Use was pretty much for full disk encryption. – DRF Apr 16 '22 at 10:51
  • @DRF. Interesting. Thanks! – LorenzoDonati4Ukraine-OnStrike Apr 16 '22 at 14:50
  • @DRF Why not install the component internally rather than glue it into a USB port? The latter seems more like a temporary hack than a permanent solution. – vikarjramun Apr 16 '22 at 21:43
  • @vikarjramun: I'd guess (without knowing the details, of course) that the answer is probably something like "because the USB port was there, and it was cheaper and easier to use it than to design a custom board with a custom connection for the TPM." Plus using USB presumably meant that the TPM could be used with *any* board that had a USB-A socket on it, not just ones with a specific internal connector. A hack, yes, but a pretty clever one IMO. And it's not really that much more hacky than, say, using a USB wireless receiver instead of a built-in one. – Ilmari Karonen Apr 16 '22 at 22:16
  • @vikarjramun Ilmari got most of it. Cost was a big part, ease of installation another. We couldn't choose which boards it would be used with and we expected to have maybe small thousands of these solutions made and installed. Making a proper internal module would mean finding space for it in each model along with a way to actually connect it. – DRF Apr 17 '22 at 12:00
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USB-A receivers fit most of their electronics inside the connector shell. USB-C shell is so much smaller that there’s no way to do it yet. It’ll happen eventually as new tricks in packaging are developed.

Recall that you can fit two C connector shells on top of each other inside the A connector shell. There’s lots of room in the A shell even allowing for the contacts. You can shove a nice sized PCB there.

A USB-C connector is more like a pipe with contacts on the inside. A USB-A connector is like a doghouse with an attic open for storage.

Kuba hasn't forgotten Monica
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    “USB-C shell is so much smaller that there’s no way to do it yet. It’ll happen eventually as new tricks in packaging are developed.” I doubt it will ever happen. Even if you’d try to cram a silicon die more or less directly into the connector you have to put the wires somewhere and you need mechanical stability. USB-C connectors are already pretty much as small as mechanically possible. Maybe carbon nanotubes could solve this ;) – Michael Apr 14 '22 at 11:37
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    On the other hand, Yubikey Nano USB-C is about the same chickness and size as the USB-A Logitech receiver, so I guess it can be done if you try hard enough: https://www.yubico.com/ch/product/yubikey-5c-nano/ – Mavrik Apr 14 '22 at 18:50
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    @Mavrik when all you need is a microcontroller with built in resistors and a cap sense trace. Their usb a model is so small I need to use tweezers to pull it out lol. But the C is using a standard C connector unlike the pcb trace trick with the A model. https://www.yubico.com/ch/product/yubikey-5-nano/ so it's still an example of the A being more form fitting than the C. – Passerby Apr 14 '22 at 22:00