A week ago I made a circuit based on a PIC16F877a uC and MAX232 ic for RS232 serial communication between PC and device. I used USB port of my laptop to power the circuit as: enter image description here

Image source: bharatbalar.wordpress.com

The circuit ran fine but after almost five minutes the MAX232 ic become very hot, I immediately disconnected the circuit from my laptop. Now the problem is that my usb port is not working although it is giving power to other devices, the area where usb port is located always remains hot even when the system is turned off and the battery of laptop completely drains in off state. I tried shorting the Positive and ground pin of USB port but instead of turning off the system a message appeaser on screen showing that the device connected to USB port is exceeding the power limit of its HUB. I visually inspected the motherboard but did not find any damaged/burned component. There are two USB port and both are not working.

What solution do you suggest for this problem?

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    \$\begingroup\$ Your diagram does not show how you connected the circuit to your USB. MAX232 and F877 chips are generally quite robust, except for excessive things like reverse power or serious overvoltage on the power. Your test of shorting the USB power is not something I would recommend: the USB hardware is required to keep the short circuit current within safe limits, but not all PCs implement that faithfully. If you want my honest opinion: your USB ports are likely to be damaged beyond repair. \$\endgroup\$ Oct 13, 2013 at 19:49

1 Answer 1


There is a distinct possibility that the overload on the USB port 5V from the computer is related to the fact that you only hooked up one set of the VSS and VDD pins of the PIC16F877A. It is never safe to operate a chip in this manner as there is never any assurance that the internal MCU design even connects the various VDD pins together and the various VSS pins together with an internal low impedance connection. Lack of these external connections can cause some parts internal to the chip to not be powered correctly and result in improper forward biasing of chip junction diodes. Massive amounts of current can flow in these cases.

Also be advised if you rebuild the circuit with new parts that you also need to be adding bypass capacitors between the VSS and VDD of each chip connection.

With regard to the USB ports of your computer...It is likely that you have damaged things so bad that they may not see life again. Take this incident to re-think the process of powering experimental circuitry from your computer's USB port. Check out a place like eBay where you can find dozens of choices for very low cost USB power blocks that can supply 5V from a AC power source (or also from an automobile aux power jack) to power your experimental circuits. In the unfortunate case of a short or some oversight in the design of your circuit it is far far better to blow out a 5->10$ power brick than it is to destroy your laptop or desktop computer.


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