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26 January 2010
Author: Giorgos Lazaridis
PC System Health Monitor

Worklog - A huge failure (February 4 2010)

The problems began when i decided to close the PC box and check if the system can operate as good as when the box was open. I was indeed expecting a temperature rise, but i had yet more to discover.

High temperatures

Let's take a look at the temperatures once more, BEFORE i close the box

Everything looks just perfect. And now, AFTER closing the box, within an hour of operation:

I think you've got the point. The temperatures gone MAD. What this means, is that i need to (once more) re-design the air inlet and outlet of the box, as well as the whole air circulation. This is something that it just happens when trying to design something original.

How the resistance of the NTC changes, as the temperature rises

Something else that is not a really good sign, is that when the temperature rises, the PIC A/D converter sometimes halts and the program flow stops. I have not enabled the watchdog for such cases, so i noticed exactly where this error occurs. I put 3 LEDs which turn on and off between specific subroutines. From bigger to smaller subroutines, i found exactly in which line (ALWAYS) the program halts. There was nothing much to debug there, the code was perfectly clear w/o cockroaches.

My first guess was one PIC A/D limitation. The A/D converter must operate with max analog impedance of 10 KOhms. That turned out to be wrong. My NTC may be 10 KOhms in room temperature, but the 40 degrees are NOT room temperature. Actually, the resistance of the NTC (remember that a NTC will reduce the resistance as temperature rises) at 40ï¿½C is about 5 KOhms. On your left hand side you will find a screenshot of an open office spreadsheet that i made to theoretically calculate the resistance of the NTC in various temperatures. This is just FYI.

I opened the box, and discovered that the breadboard along with the PIC was all the time cooked. Also, i had placed the PIC on top of the power supply. This means (i suspect) that when the temperature in the box was 42ï¿½C, the temperature of the PIC was much higher. I took out the breadboard from the box and re-tested the circuit. Right now, it works about 4 hours without any sign of failure. I will let it work a couple of days just to make sure. In the meanwhile, i will re-design and re-arrange the entrails of the box. I am very sure that i can achieve at least 10% better performance with the same fans. The temperature right now is just unacceptable.

 When the breadboard and the PIC was inside the box, it was cooked and the program crashed many times. With temperatures higher then 42ï¿½C, the PIC could operate for less than 10 seconds. So i took out the controller from the box in a more friendly environment... The sensors are still inside the box, the PIC works now perfect, the program does not halt anymore.

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