Lai Sha:10 June 2007

From the problem of the simple circuit I showed Prof Cheok, there are several parts needed reconsider and design. I summerized them as below with some thoughts and proposed solutions provided:
1. Switching of the circuit shouldn't use transistor

Transistor is a current amplifier, the collector current (in my circuit is the current through the heating element) is some multiple hFE of the base current (the output from PIC), once the heating element is shorted, the Ic would be very high which would cause the IB very high, and therefore destroy the microcontroller. There should need some buffer in the circuit to protect the PIC. However, the buffer will only stop the PIC from being destroyed, when the buffer spoiled, it still a failure of the circuit.
=> MOSFET would be a better choice as a switch as it is controlled by the voltage to the gate which will be connect to the PIC output, and the gate current is very minimum and not controlled by the drain current (the heating element)


2. The single threshold in the programming causes the circuit to switch in a very high speed (on when above 4.7V, and off below 4.7V), which might potentially destroy the chips.

=> there needs to have some band for controlling the on/off state and set a short period of time for each band to make sure the switch will not be very fast. A proper band needs to be found to avoid the above problem at the same time keep the pixel to the color we want it to be.


3. The circuit seems to be noisy as the output from the Vdd or MCLR measured by the oscilloscope are not smooth. One cause might be because of the PIC board's decoupling capacitors are too far away from the circuit they are decoupling.

=> redesign of the PIC board


4. The reading feedback to the PIC analog pin is not stable, causes could be Ground of analog and digital circuit are connected together. The digital circuit tends to be noisy, which if the analog ground is referring to the noisy ground, the reading will not be smooth, so the value we got would not be the exact value also.

=> separate the ground of the analog circuit and digital circuit. Connect the ground of analog parts together, connect the grounds of digital parts together, and connect them at a single point to ensure the reference is the same. While actually I am still not very clear with how should the grounds of analog and digital parts should correctly connected, still to be found out.

I found this article pretty good guide in selecting MOSFET http://homepages.which.net/~paul.hills/SpeedControl/Mosfets.html For MOSFET, the good ones which has on resistance less than 0.02ohm, while the one we bought is ave = 0.35ohm, which might cause power dissipated by the MOSFET.
But one thing to think about whether a resistor is needed between gate and ground, as said in http://brunningsoftware.co.uk/FET.htm


5. Pull-up or pull-down resistor (Is taken into consideration, as without it, the pin of the PIC may be float when it changes from 1->0)


6. Thermistor feedback to the analog input of PIC, if any component is short circuited, the input to the PIC would be 9V which would burn the PIC
=> Better to use some buffer or change the 9V driving voltage to 5V. However, when the driving voltage is small which also means the feedbacks difference would not be obvious enough, probably some voltage amplifer could have the function of amplifying the feedback as well as protect the PIC when the feedback voltage is larger than 5V

sha
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