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TRB142 and TRB145 are only usable for USB tethering, meaning they have access to 5V power from USB. This 5V power line goes to (at least) the Quectel module. For other components on the PCB, 3.8V power is only present if external 9..24V power supply is connected.  

This dual powering arrangement seems unnecessary to me, and I would like to use USB power alone. I tried modify the schematics with a 5V input/3.8V output LDO, but this did not give the wanted result. Seems that I need to use a DC/DC converter to get it working on USB supply alone. Easy enough, as the external power supply current is less than 100 mA.

But the need for an external power supply for an USB-connected router seems like a serious design flop to me. What was the reason behind such a solution?

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By standard definition, with a USB which provides up to 2.5W, the router would not be able to powered by such a solution, as it was a higher maximum power consumption than that. You can check the power consumption base on the device load on our wiki.

The USB port on these devices is used as a network interface.

The external power supply that comes with TRB14 series device is a 9V 1A AC to DC power supply which comes with an industrial 4pin power socket, which also has 2 configurable inputs and outputs.



According to my measurement, the supply current @9v is below 50 mA in average. Occasional peaks up to 135 mA during connection setup.

As there are many USB modem or router dongles that can work with [email protected] (acting as WiFi AP at the same time!), it must not be impossible to achieve just router functionality under 2.5W.

And it would be also easier to use the 4pin socket for IO without the need to supply the router via the same socket...
Have tried my DIY fix, using 1.1 EUR DC/DC converter (isolated 5V in, 5V out). Works well with 10V resulting supply (5V from USB+5V from the converter).