8 Aug 2007

The following is footprint of the final PCB design

22424606.cb9b9b.jpg

There are mainly three considerations in the footprint design:

Separation of GNDs and POWERs
There are two voltage regulators to generate 9V and 5V.
9V will be the supply for the heating element, and the 5V is supply for microcontroller, logic buffer and temperature sensor. The same power supply is used for regulating to 5V and 9V.

22398385.8d0abd.jpg

It is recommended that the circuit of temperature sensor is separated from the rest, with power and ground directly connect back to the 5V power supply. Besides this, 5V supply will also drive the logic buffer and the PIC, and both of which are digital components in the circuit. In my circuit, I used "star" grounding to separate ground lines of different components in order to keep current and noise from one component from affecting other components. So in the PCB design, the ground and power of temp sensor, logic buffer and microcontroller all go directly back to the 5V GND or come directly from the 5V POWER. Same as for 9V power. And both 5V and 9V's POWER and GND are directly connected back to the power supply also, which is JP6.

Bypassing

http://www.seattlerobotics.org/encoder/jun97/basics.html article to explain the basics of bypassing capacitor

In my circuit, the one which draws significant switching current is the heating element which would require the circuit to supply power on and off onces it reached the desired state we want, and it draws slighly larger current compare to the rest, ~0.26A. And the component that need to be bypassed from heating element is the 9V generator which is already bypassed.

22398390.da97f1.jpg
The above is the recommended bypassing circuit for the temperature sensor.

Other components to consider would be either PIC and logic buffer, but in our application, they will not be much affected by this consideration, and I could add it in the future design if it is really necessary.

Bypassing capacitor is to be placed as close as possible to the component it is to bypass.

Frequency
As the frequency we tested for the algorithm is 100hz (According to the datasheet of the temp04,
22399162.527491.jpg T1 is nominally 10 ms, A useful worst-case
assumption is that T1 will never exceed 12 ms over the specified temperature range, Substituting this value for T1 in the formula, the maximum value of T2 to be calculated at any maximum operating temperature:T2 (Temp) = (T1max x 400)/(235 – Temp) in seconds, and the temperature range we are using is from 20~40°C. We can get T2 is between 22.33ms ~ 24.62ms, adding up T1 and T2, the frequency of the PWM wouldn't exceed 100hz), it should be ok not to use transimission line techniques which is mainly used in high frequency circuit.

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