Designing a charger output protection circuit.

[nic55]

Hi Guys,

I've got a transformer here that I mainly want to use for charging USB devices in the car, it's a 12-24v to 5v 10a transformer that will be hardwired in. The input will be a direct fused feed from the car battery, triggered via a relay on the ACC signal.

The output side I'm not so sure about, I'm thinking I'll have 4-6 USB ports throughout the car and likely a jack or two to occasionally power other devices.

I'm not very knowledgeable on how I should isolate each jack, I'm thinking there should be a fuse and diode on each one and maybe a small capacitor to smooth out when a device is added/removed? What can I do to prevent against a faulty device/cable on one port causing damage to the others, would a fuse be enough to handle that?

Thanks for your suggestions.

[Skepticist]

Fuses are not going to protect anything other than the wiring itself in case of catastrophic short circuits. Your 'transformer' is a 'buck converter' and should provide its own protection against overload/overtemperature which is far faster and more effective than fuses (check the manual or the datasheet for the controllling IC). Devices you connect that use the USB supply voltage for charging their own internal lithium batteries will do their own regulation to protect the battery from doing a Chernobyl type meltdown so those devices only require that the supply voltage be within specs (5V).

Some do require specific conditions on the USB data lines in order to set the charge rate and that varies between brands.

[nic55]

Thanks for the info.

Apple devices look for resistance while most other devices are happy to just have the data pins shorted which is easy enough to cater for, I don't want to regulate current to each port was just wondering if I should be adding anything else to help protect the converter , wiring/other connected devices.

[Uncle Fester]

You could use a Polyswitch fuse on each each port, rated around 1-2A :
That way if one device/cable shorts out, the rest of the circuits stay alive and when you remove the offending device it goes back to normal.
You could add a red LED with 1K resistor parallel to the polyswitch to immediately display the fault.

[Skepticist]

As long as the switchmode controller IC has the usual protection functions like foldback current limiting, overtemp shutdown etc you shouldn't need any extra protection on the output, but a 10A fuse on the input might be prudent to avoid blowing the accessories fuse in the vehicle if the worst happens. The controller chip should be something like LM2678 or similar family that has excellent protection already built in for the output but check the data sheet online to be sure of the features provided.

[Uncle Fester]

Keep in mind the 1.1A poly switch I selected above, will trip over at around 1.8A. I would not use a higher rating. Many USB cables are so thin they will catch fire if connected to anything with a higher rating and unprotected on a 10A supply... lo and behold !!!

You might even want to use only 0.75A poly switches depending on the device. Many chargers are limited at 1A or less.
The copper inside those wires is sometimes as thick as 5 strands of hair and might have a propriety plug on the other side so you can't replace them with thicker cables.