Safe USB circuits - what precautions do I need to take.

Safe USB Circuits.

My hobby is making 5v led lamps.

I sometimes use 0.9-5v dc-dc buck circuits from ebay to power them from 1x or 2x 1.5v batteries. Other times I connect them directly to usb using a potted microusb socket.

The circuits are simple parallel (each led has its own 1206 resistor) arrays of leds. They draw less that 350ma.

I have run the usb versions from branded usb chargers / pc usb ports / usb hubs with no problem.

I only have the GND and VBUS connected in the microusb socket, no negotiation but not had any problems with suggested 100ma limit for non negotiated.

I want to start selling these at some point. I wanted to ask about potential dangers to the host device ( such as a £1000+ macbook ) and overload on the device side also (in case the user plugs it into a cheap ebay charger).

I do have a set of smd 1206 - 400ma self healing fuses from china. I also thought of connecting the microusb to the dc-dc buck board, as it seems to contain these fuses already.

I did wonder if there was already a very small board which does this job and is widely used ?

Any advice would be greatly appreciated.

Thanks.

glowlamp said:

1206 - 400ma self healing fuses from china

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I think the computer already has these or a current limit function.

ronsimpson said:

I think the computer already has these or a current limit function.

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True, but I would not rely on such assumptions. By the book you should not exceed 100mA from the port until USB enumeration is finished and you are granted the full 500mA. That rule I don´t think is actually required by anyone for proper functionality, but it may be a good thing to consider regarding the compatibility of most devices with the actual USB standard.

But there are other aspects, I know that some motherboards didn´t have a separate PTC fuse for each USB port, but there actually was a normal 3A fuse shared for the 4 usb ports and some other stuff, so if you exceed it you are screwed. Some designs don´t have any fuse at all and rely on the current limiting of the internal power supply to not burn pcb traces and components, which again is not something to rely upon.

I suggest you use PTC fuse. Either add it to the pcb, or choose a through-hole version and put it inline with the cord, or whatever is feasible.

Or, think about how your circuit is wired, and investigate the possible failure modes. At this voltage I presume a few paralel strings, each with a resistor and a single diode. The diode is much more likely to fail shorted than the resistor, and still an LED most often degrades and stops producing light and acts like a zener, or fails open, rather than going dead short.
If each current limiting resistor has such value and size that with the shorted diode the total current of the device doesn´t exceed 500mA, and with that fault the power dissipation in the resistor is not more than about 1/2 to 2/3 of its rating, then I frankly would not worry about fusing the thing. (unless there is much more than say 10 parallel branches in the device)