In post 2, block diagram for control voltage implementation was given. This control voltage can be actuated in many ways. The standard technique is PWM i.e. longer off time for the control pulse or less duty cycle that keeps the heating coil off for a longer time when the actual temperature is more than the set-point temperature. As DAB pointed out rightly, this may give too many on-offs, an undesirable effect. A smooth control is possible by use of a Voltage Variable Resistor (VVR). VVR may be implemented by a MOSFET, SCR or TRIAC. In case of MOSFET, the device can be kept in saturation region. As current varies in a quadratic manner with overdrive voltage the dc resistance changes very fast in this region. By reducing the resistance more heating of the coil is possible. If the actual temperature is much above the set-point, switch off can be done by applying a control voltage to the gate below the threshold voltage. On-Semi MCH 3481 is a suitable device for this implementation. At port pin of PSOC 4 the control voltage that can be sent is 0 to 5.5 volt. MCH 3481 can accept maximum gate voltage of 9 volts. This is well above 5.5 volts. The threshold voltage is 0.9 volts. The control vector can be scaled to the range 0.9 to 5.5 volts. If sensed temperature or actual temperature goes too high control voltage can be set to zero. This will switch off the circuit.
Update of the week is:
1. Listing of the components.
2. Download of CadSoft Eagle. But board development will be difficult as vendors take only bulk order. I could not find any simulation facility to do a total hardware simulation of PID controller. I am not getting the idea how to utilize the tool for this project.
3. Spending a lot of time to get the kit. The board has reached Bangalore on 6th . The last promise from the courier UPS is: they will deliver it on Monday, anytime between 8.30 am to 3.30 pm. I am not in a hurry. A safe and smooth delivery without any damage of the kit is all that I am trying.
