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Today, the charger for the modem for my cable-internet connection went dead. With shops closed due to coronavirus, I thought I would cut the connector from it, connect the two cables inside it with a cut USB-A to microUSB cable (leaving the USB-A in place) and use a phone charger. And so I did. Alas, it was apparently not enough to power on the modem - the diods would light up but I would not get a connection. The sticker on the modem says it needs 12 V at 1 A. I also have a router (that I actually use to cover my flat with wifi signal) whose sticker says 12 V at 2 A and that also did not work. I have another older and slower router labeled 9 V at 0.6 A that could work with this cable but it is slow and could only hold connection to three devices at the time (and we have at least nine in the house). I used a charger that says it can output 5 V at 2 A, 7 V at 1.67 A, 5 V at 2 A and 12V at 1.25 A (presumably using a usb-C cable).

Now, would using a different cable change things? I could cut a USB-C to USB-A cable. Pre-USB-C cables are all the same, are they not?

Would using a different charger change things? If I understand it correctly, that needs a "smart" device on the other end to support higher power, right?

In the end, I got a second hand charger (and ventured out) rated at 1.2A at 5V. Interestingly enough, it can power up the router (12V at 2A) and not the modem (12V at 1A) - I guess that is because the router only uses that power when USB devices are attached to it (it has two USB ports). Is that hunch correct?

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    \$\begingroup\$ To use the AFC(Adaptive fast charge) settings (everything but 5V) on that charger, you need to find out how an AFC compatible device indicates desired voltage to the charger. \$\endgroup\$
    – K H
    Mar 25, 2020 at 20:06

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A USB charger provides 5V, usually 100mA (To get more power, it has to negotiate with the phone). This is not enough for your router, by an order of magnitude.

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  • \$\begingroup\$ I see. But it was enough to power up the slow old router. Is it really 100mA and not more? \$\endgroup\$
    – sup
    Mar 25, 2020 at 23:15

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