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I have a project where I'm using an android tablet to power a serial relay, the relay is 20 m from the tablet so I'm using a USB C > USB A hub and a self powered USB over CAT6 extender. When the tablet is charging through the input in the USB hub all is well. However when I turn the charger off (which needs to happen from time to time) there's a small <1 s delay between the supply power switching from charger to tablet battery and my relay turns off. Would it be possible to add a capacitor or super capacitor to the circuit after the hub to keep the voltage up on the extender and relay board during changeover?

I found this thread:

Car bluetooth power delay- USB capacitor?

With this diagram enter image description here

I have found that I can get a 1 F 5.5 V supercap for a reasonable price, would this circuit be enough, it seems too simple and I don't want to damage anything with over voltage or overcurrent.

Thanks in advance

Tim

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  • \$\begingroup\$ Actually with such a large capacitor value you will also need to worry about the charge time. When first connected the cap will draw a huge current, likely enough to brown out the USB circuit or even a small power adapter. \$\endgroup\$
    – Nedd
    Commented Feb 24, 2023 at 8:13
  • \$\begingroup\$ Good point, thankyou. So something to slow the charge current is needed or a lower value cap? I can build a circuit from a diagram but I don't know what would be needed. Basically I need a circuit diagram that would work, or a product that already exists for a small cost. \$\endgroup\$
    – Tim
    Commented Feb 24, 2023 at 8:40
  • \$\begingroup\$ Welcome! On top of what @Nedd said, its both limit on how much current you can draw and max allowed capacitance connected to Vbus by the USB specification. IIRC, it's in the 1000 uF range, but please check and don't take my word for it. Worst case, you will need a DC/DC or some other current limiting device to make it compliant. \$\endgroup\$
    – winny
    Commented Feb 24, 2023 at 9:15

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