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Support Forum » nerdkits usb short circuit protection?

December 16, 2011
by victor
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just curious and i think it might be good to be cautious.

the nerdkits usb programming cable directly connect to the PC usb port, but what if the user short circuit those cables by mistake? will this potentially harm the PC ?

December 16, 2011
by missle3944
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Victor,

I think that topic is briefly touched on in the nerdkits guide on page 79. I suppose it could. I don't think it would do much if you short circuted the Tx or Rx cables but possibly the +5 and gnd. Maybe one of the veterans can chime in on this one. But I know that the USB cable supplies 500mA of current so you defiantly don't want to draw too much current. -Dan

December 17, 2011
by Rick_S
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This is copied from a USB power question here on the forum from long ago. We were discussing power for the LCD back light. I also covered USB power output. It seemed apropriate for this thread.


The USB is designed to supply 100mA to 500mA Maximum. This current is defined as the unit load and can be requested by the USB device in 100mA increments to the max. If the higher current requests aren't made by the device (to my understanding anyway) the port is set to 100mA or 1 unit load.

The reason it is not normally recommended to connect the backlight to USB power is because it can draw a substantial amount of current on it's own and combined with the current draw of the USB to serial adapter + your test circuit, it may be enough to overload your port.

While typically the port would just shut down, there is always the remote chance that bad things could happen to your PC. That is why, while in some cases it may not be required, it is usually recommended to use external power such as a "Wall Wart" or battery through a regulator to provide power to your circuit when using the LCD backlight in your project.

Also, if you do opt for external power, keep the ground of your USB adapter connected to your circuits ground. Just disconnect the USB power line from your circuit. This will prevent any possibility of different ground potentials causing problems.

Hope that helped clear it up a bit.


Shorting the USB power wires could cause damage. I'm not saying it WILL cause damage, just that it could. It really depends on the circuitry in your PC. If it is designed to protect itself, the port will just shut down and be OK later (may require a re-boot). If it isn't, it could possibly damage the port.

While I would agree with Dan, that the data lines could probably be OK if shorted, your goal should be to protect your connections against shorts... just in case. Wink

BTW, I really doubt the USB data cable supplied with the NerdKit requests 500mA of current from the port. Most likely it's output is 100mA. Don't know for sure, but I'd be surprised if it was much more. Not Sure

Rick

December 17, 2011
by victor
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thanks guys, that's helpful! :) Merry Xmas!

December 17, 2011
by Rick_S
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Merry Christmas to you!! Christmas_Tree

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